How to create a sequence of integers in C#? - c#

F# has sequences that allows to create sequences:
seq { 0 .. 10 }
Create sequence of numbers from 0 to 10.
Is there something similar in C#?

You can use Enumerable.Range(0, 10);. Example:
var seq = Enumerable.Range(0, 10);
MSDN page here.

Enumerable.Range(0, 11);
Generates a sequence of integral numbers within a specified range.
http://msdn.microsoft.com/en-us/library/system.linq.enumerable.range.aspx

You could create a simple function. This would work for a more complicated sequence. Otherwise the Enumerable.Range should do.
IEnumerable<int> Sequence(int n1, int n2)
{
while (n1 <= n2)
{
yield return n1++;
}
}

Linq projection with the rarely used indexer overload (i):
(new int[11]).Select((o,i) => i)
I prefer this method for its flexibilty.
For example, if I want evens:
(new int[11]).Select((item,i) => i*2)
Or if I want 5 minute increments of an hour:
(new int[12]).Select((item,i) => i*5)
Or strings:
(new int[12]).Select((item,i) => "Minute:" + i*5)

In C# 8.0 you can use Indices and ranges
For example:
var seq = 0..2;
var array = new string[]
{
"First",
"Second",
"Third",
};
foreach(var s in array[seq])
{
System.Console.WriteLine(s);
}
// Output: First, Second
Or if you want create IEnumerable<int> then you can use extension:
public static IEnumerable<int> ToEnumerable(this Range range)
{
for (var i = range.Start.Value; i < range.End.Value; i++)
{
yield return i;
}
}
...
var seq = 0..2;
foreach (var s in seq.ToEnumerable())
{
System.Console.WriteLine(s);
}
// Output: 0, 1
P.S. But be careful with 'indexes from end'. For example, ToEnumerable extension method is not working with var seq = ^2..^0.

My implementation:
private static IEnumerable<int> Sequence(int start, int end)
{
switch (Math.Sign(end - start))
{
case -1:
while (start >= end)
{
yield return start--;
}
break;
case 1:
while (start <= end)
{
yield return start++;
}
break;
default:
yield break;
}
}

I have these functions in my code
private static IEnumerable<int> FromZero(this int count)
{
if (count <= 0)
yield break;
for (var i = 0; i < count; i++)
{
yield return i;
}
}
private static IEnumerable<int> FromOne(this int count)
{
if (count <= 0)
yield break;
for (var i = 1; i <= count; i++)
{
yield return i;
}
}
This helps to reduce some for(i) code.

In case you wish to also save the generated sequence in a variable:
using System.Collections.Generic;
using System.Linq;
IEnumerable<int> numbersToPrint = Enumerable.Range(1, 11);
This is implicit in other solutions shown above, but I am also explicitly including the needed namespaces for this to work as expected.

Originally answered here.
If you want to enumerate a sequence of numbers (IEnumerable<int>) from 0 to a 10, then try
Enumerable.Range(0, ++10);
In explanation, to get a sequence of numbers from 0 to 10, you want the sequence to start at 0 (remembering that there are 11 numbers between 0 and 10, inclusive).
If you want an unlimited linear series, you could write a function like
IEnumerable<int> Series(int k = 0, int n = 1, int c = 1)
{
while (true)
{
yield return k;
k = (c * k) + n;
}
}
which you could use like
var ZeroTo10 = Series().Take(11);
If you want a function you can call repeatedly to generate incrementing numbers, perhaps you want somthing like.
using System.Threading;
private static int orderNumber = 0;
int Seq()
{
return Interlocked.Increment(ref orderNumber);
}
When you call Seq() it will return the next order number and increment the counter.

Related

Filter over IEnumerable ... .Where?

I made a function to generate the odd numbers:
static IEnumerable<int> OddNumbers()
{
int n = 1;
while (true)
yield return 1 + 2 * (n++ - 1);
}
How do I go through and filter this list? I am trying to remove all the multiples of a certain number factor, I wrote this:
using (var oddNumbers = OddNumbers().GetEnumerator())
{
oddNumbers.MoveNext();
int factor = oddNumbers.Current;
yield return factor;
oddNumbers = oddNumbers.Where(x => x % factor != 0);
}
but I get told that
The type arguments for method `System.Linq.Enumerable.Where<TSource>(
this System.Collections.Generic.IEnumerable<TSource>,
System.Func<TSource,bool>)' cannot be inferred from the usage.
Try specifying the type arguments explicitly`
To my understanding there is no need to access the enumerator directly, it can be done using Linq alone as follows.
var FilteredNumbers = OddNumbers().Where(x => x % factor != 0);
You can either use Linq:
// Initial generator
static IEnumerable<int> OddNumbers() {
for (int n = 1; ; n += 2) // for loop is far better than while here
yield return n;
}
...
var result = OddNumbers()
.Where(x => x % factor ! = 0);
Or modify the generator itself:
static IEnumerable<int> OddNumbersAdvanced(int factorToExclude = int.MinValue) {
for (int n = 1; ; n += 2)
if (n % factorToExclude != 0)
yield return n;
}
...
var result = OddNumbersAdvanced(factor);
To go through use foreach loop:
foreach (int item in result) {
//TODO: put relevant code here; do not forget to break the loop
}
Since you need that to generate sequence of lucky numbers, here is some code I wrote to do this with iterators. Note that it's very inefficient to do that this way, but hopefully you are doing this just for fun or for learning
static IEnumerable<int> LuckyNumbers(IEnumerable<int> all = null, int n = 2, int step = 0) {
if (step == 0) {
all = Enumerable.Range(1, int.MaxValue); // start with all numbers
yield return 1;
step++;
}
// apply a filter for current "n" (starting with 2)
var filtered = Filtered(all, n);
// get next item from the sequence (skip items first, because this sequence represents whole lucky number sequence, starting from 1)
var current = filtered.Skip(step).First();
yield return current;
step++;
// now recursive call back into LuckyNumber
foreach (var other in LuckyNumbers(filtered, current, step)) {
yield return other;
}
}
static IEnumerable<int> Filtered(IEnumerable<int> previous, int n) {
// filter out each n-th item
return previous.Where((x, i) => (i + 1)%n != 0);
}
Use like this:
foreach (var next in LuckyNumbers().Take(10)) {
Console.WriteLine(next);
}

LINQ get x amount of elements from a list

I have a query which I get as:
var query = Data.Items
.Where(x => criteria.IsMatch(x))
.ToList<Item>();
This works fine.
However now I want to break up this list into x number of lists, for example 3. Each list will therefore contain 1/3 the amount of elements from query.
Can it be done using LINQ?
You can use PLINQ partitioners to break the results into separate enumerables.
var partitioner = Partitioner.Create<Item>(query);
var partitions = partitioner.GetPartitions(3);
You'll need to reference the System.Collections.Concurrent namespace. partitions will be a list of IEnumerable<Item> where each enumerable returns a portion of the query.
I think something like this could work, splitting the list into IGroupings.
const int numberOfGroups = 3;
var groups = query
.Select((item, i) => new { item, i })
.GroupBy(e => e.i % numberOfGroups);
You can use Skip and Take in a simple for to accomplish what you want
var groupSize = (int)Math.Ceiling(query.Count() / 3d);
var result = new List<List<Item>>();
for (var j = 0; j < 3; j++)
result.Add(query.Skip(j * groupSize).Take(groupSize).ToList());
If the order of the elements doesn't matter using an IGrouping as suggested by Daniel Imms is probably the most elegant way (add .Select(gr => gr.Select(e => e.item)) to get an IEnumerable<IEnumerable<T>>).
If however you want to preserve the order you need to know the total number of elements. Otherwise you wouldn't know when to start the next group. You can do this with LINQ but it requires two enumerations: one for counting and another for returning the data (as suggested by Esteban Elverdin).
If enumerating the query is expensive you can avoid the second enumeration by turning the query into a list and then use the GetRange method:
public static IEnumerable<List<T>> SplitList<T>(List<T> list, int numberOfRanges)
{
int sizeOfRanges = list.Count / numberOfRanges;
int remainder = list.Count % numberOfRanges;
int startIndex = 0;
for (int i = 0; i < numberOfRanges; i++)
{
int size = sizeOfRanges + (remainder > 0 ? 1 : 0);
yield return list.GetRange(startIndex, size);
if (remainder > 0)
{
remainder--;
}
startIndex += size;
}
}
static void Main()
{
List<int> list = Enumerable.Range(0, 10).ToList();
IEnumerable<List<int>> result = SplitList(list, 3);
foreach (List<int> values in result)
{
string s = string.Join(", ", values);
Console.WriteLine("{{ {0} }}", s);
}
}
The output is:
{ 0, 1, 2, 3 }
{ 4, 5, 6 }
{ 7, 8, 9 }
You can create an extension method:
public static IList<List<T>> GetChunks<T>(this IList<T> items, int numOfChunks)
{
if (items.Count < numOfChunks)
throw new ArgumentException("The number of elements is lower than the number of chunks");
int div = items.Count / numOfChunks;
int rem = items.Count % numOfChunks;
var listOfLists = new List<T>[numOfChunks];
for (int i = 0; i < numOfChunks; i++)
listOfLists[i] = new List<T>();
int currentGrp = 0;
int currRemainder = rem;
foreach (var el in items)
{
int currentElementsInGrp = listOfLists[currentGrp].Count;
if (currentElementsInGrp == div && currRemainder > 0)
{
currRemainder--;
}
else if (currentElementsInGrp >= div)
{
currentGrp++;
}
listOfLists[currentGrp].Add(el);
}
return listOfLists;
}
then use it like this :
var chunks = query.GetChunks(3);
N.B.
in case of number of elements not divisible by the number of groups, the first groups will be bigger. e.g. [0,1,2,3,4] --> [0,1] - [2,3] - [4]

Generating permutations of a set (most efficiently)

I would like to generate all permutations of a set (a collection), like so:
Collection: 1, 2, 3
Permutations: {1, 2, 3}
{1, 3, 2}
{2, 1, 3}
{2, 3, 1}
{3, 1, 2}
{3, 2, 1}
This isn't a question of "how", in general, but more about how most efficiently.
Also, I wouldn't want to generate ALL permutations and return them, but only generating a single permutation, at a time, and continuing only if necessary (much like Iterators - which I've tried as well, but turned out to be less efficient).
I've tested many algorithms and approaches and came up with this code, which is most efficient of those I tried:
public static bool NextPermutation<T>(T[] elements) where T : IComparable<T>
{
// More efficient to have a variable instead of accessing a property
var count = elements.Length;
// Indicates whether this is the last lexicographic permutation
var done = true;
// Go through the array from last to first
for (var i = count - 1; i > 0; i--)
{
var curr = elements[i];
// Check if the current element is less than the one before it
if (curr.CompareTo(elements[i - 1]) < 0)
{
continue;
}
// An element bigger than the one before it has been found,
// so this isn't the last lexicographic permutation.
done = false;
// Save the previous (bigger) element in a variable for more efficiency.
var prev = elements[i - 1];
// Have a variable to hold the index of the element to swap
// with the previous element (the to-swap element would be
// the smallest element that comes after the previous element
// and is bigger than the previous element), initializing it
// as the current index of the current item (curr).
var currIndex = i;
// Go through the array from the element after the current one to last
for (var j = i + 1; j < count; j++)
{
// Save into variable for more efficiency
var tmp = elements[j];
// Check if tmp suits the "next swap" conditions:
// Smallest, but bigger than the "prev" element
if (tmp.CompareTo(curr) < 0 && tmp.CompareTo(prev) > 0)
{
curr = tmp;
currIndex = j;
}
}
// Swap the "prev" with the new "curr" (the swap-with element)
elements[currIndex] = prev;
elements[i - 1] = curr;
// Reverse the order of the tail, in order to reset it's lexicographic order
for (var j = count - 1; j > i; j--, i++)
{
var tmp = elements[j];
elements[j] = elements[i];
elements[i] = tmp;
}
// Break since we have got the next permutation
// The reason to have all the logic inside the loop is
// to prevent the need of an extra variable indicating "i" when
// the next needed swap is found (moving "i" outside the loop is a
// bad practice, and isn't very readable, so I preferred not doing
// that as well).
break;
}
// Return whether this has been the last lexicographic permutation.
return done;
}
It's usage would be sending an array of elements, and getting back a boolean indicating whether this was the last lexicographical permutation or not, as well as having the array altered to the next permutation.
Usage example:
var arr = new[] {1, 2, 3};
PrintArray(arr);
while (!NextPermutation(arr))
{
PrintArray(arr);
}
The thing is that I'm not happy with the speed of the code.
Iterating over all permutations of an array of size 11 takes about 4 seconds.
Although it could be considered impressive, since the amount of possible permutations of a set of size 11 is 11! which is nearly 40 million.
Logically, with an array of size 12 it will take about 12 times more time, since 12! is 11! * 12, and with an array of size 13 it will take about 13 times more time than the time it took with size 12, and so on.
So you can easily understand how with an array of size 12 and more, it really takes a very long time to go through all permutations.
And I have a strong hunch that I can somehow cut that time by a lot (without switching to a language other than C# - because compiler optimization really does optimize pretty nicely, and I doubt I could optimize as good, manually, in Assembly).
Does anyone know any other way to get that done faster?
Do you have any idea as to how to make the current algorithm faster?
Note that I don't want to use an external library or service in order to do that - I want to have the code itself and I want it to be as efficient as humanly possible.
This might be what you're looking for.
private static bool NextPermutation(int[] numList)
{
/*
Knuths
1. Find the largest index j such that a[j] < a[j + 1]. If no such index exists, the permutation is the last permutation.
2. Find the largest index l such that a[j] < a[l]. Since j + 1 is such an index, l is well defined and satisfies j < l.
3. Swap a[j] with a[l].
4. Reverse the sequence from a[j + 1] up to and including the final element a[n].
*/
var largestIndex = -1;
for (var i = numList.Length - 2; i >= 0; i--)
{
if (numList[i] < numList[i + 1]) {
largestIndex = i;
break;
}
}
if (largestIndex < 0) return false;
var largestIndex2 = -1;
for (var i = numList.Length - 1 ; i >= 0; i--) {
if (numList[largestIndex] < numList[i]) {
largestIndex2 = i;
break;
}
}
var tmp = numList[largestIndex];
numList[largestIndex] = numList[largestIndex2];
numList[largestIndex2] = tmp;
for (int i = largestIndex + 1, j = numList.Length - 1; i < j; i++, j--) {
tmp = numList[i];
numList[i] = numList[j];
numList[j] = tmp;
}
return true;
}
Update 2018-05-28:
A new multithreaded version (lot faster) is available below as another answer.
Also an article about permutation: Permutations: Fast implementations and a new indexing algorithm allowing multithreading
A little bit too late...
According to recent tests (updated 2018-05-22)
Fastest is mine BUT not in lexicographic order
For fastest lexicographic order, Sani Singh Huttunen solution seems to be the way to go.
Performance test results for 10 items (10!) in release on my machine (millisecs):
Ouellet : 29
SimpleVar: 95
Erez Robinson : 156
Sani Singh Huttunen : 37
Pengyang : 45047
Performance test results for 13 items (13!) in release on my machine (seconds):
Ouellet : 48.437
SimpleVar: 159.869
Erez Robinson : 327.781
Sani Singh Huttunen : 64.839
Advantages of my solution:
Heap's algorithm (Single swap per permutation)
No multiplication (like some implementations seen on the web)
Inlined swap
Generic
No unsafe code
In place (very low memory usage)
No modulo (only first bit compare)
My implementation of Heap's algorithm:
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Runtime.CompilerServices;
namespace WpfPermutations
{
/// <summary>
/// EO: 2016-04-14
/// Generator of all permutations of an array of anything.
/// Base on Heap's Algorithm. See: https://en.wikipedia.org/wiki/Heap%27s_algorithm#cite_note-3
/// </summary>
public static class Permutations
{
/// <summary>
/// Heap's algorithm to find all pmermutations. Non recursive, more efficient.
/// </summary>
/// <param name="items">Items to permute in each possible ways</param>
/// <param name="funcExecuteAndTellIfShouldStop"></param>
/// <returns>Return true if cancelled</returns>
public static bool ForAllPermutation<T>(T[] items, Func<T[], bool> funcExecuteAndTellIfShouldStop)
{
int countOfItem = items.Length;
if (countOfItem <= 1)
{
return funcExecuteAndTellIfShouldStop(items);
}
var indexes = new int[countOfItem];
// Unecessary. Thanks to NetManage for the advise
// for (int i = 0; i < countOfItem; i++)
// {
// indexes[i] = 0;
// }
if (funcExecuteAndTellIfShouldStop(items))
{
return true;
}
for (int i = 1; i < countOfItem;)
{
if (indexes[i] < i)
{ // On the web there is an implementation with a multiplication which should be less efficient.
if ((i & 1) == 1) // if (i % 2 == 1) ... more efficient ??? At least the same.
{
Swap(ref items[i], ref items[indexes[i]]);
}
else
{
Swap(ref items[i], ref items[0]);
}
if (funcExecuteAndTellIfShouldStop(items))
{
return true;
}
indexes[i]++;
i = 1;
}
else
{
indexes[i++] = 0;
}
}
return false;
}
/// <summary>
/// This function is to show a linq way but is far less efficient
/// From: StackOverflow user: Pengyang : http://stackoverflow.com/questions/756055/listing-all-permutations-of-a-string-integer
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="list"></param>
/// <param name="length"></param>
/// <returns></returns>
static IEnumerable<IEnumerable<T>> GetPermutations<T>(IEnumerable<T> list, int length)
{
if (length == 1) return list.Select(t => new T[] { t });
return GetPermutations(list, length - 1)
.SelectMany(t => list.Where(e => !t.Contains(e)),
(t1, t2) => t1.Concat(new T[] { t2 }));
}
/// <summary>
/// Swap 2 elements of same type
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="a"></param>
/// <param name="b"></param>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
static void Swap<T>(ref T a, ref T b)
{
T temp = a;
a = b;
b = temp;
}
/// <summary>
/// Func to show how to call. It does a little test for an array of 4 items.
/// </summary>
public static void Test()
{
ForAllPermutation("123".ToCharArray(), (vals) =>
{
Console.WriteLine(String.Join("", vals));
return false;
});
int[] values = new int[] { 0, 1, 2, 4 };
Console.WriteLine("Ouellet heap's algorithm implementation");
ForAllPermutation(values, (vals) =>
{
Console.WriteLine(String.Join("", vals));
return false;
});
Console.WriteLine("Linq algorithm");
foreach (var v in GetPermutations(values, values.Length))
{
Console.WriteLine(String.Join("", v));
}
// Performance Heap's against Linq version : huge differences
int count = 0;
values = new int[10];
for (int n = 0; n < values.Length; n++)
{
values[n] = n;
}
Stopwatch stopWatch = new Stopwatch();
ForAllPermutation(values, (vals) =>
{
foreach (var v in vals)
{
count++;
}
return false;
});
stopWatch.Stop();
Console.WriteLine($"Ouellet heap's algorithm implementation {count} items in {stopWatch.ElapsedMilliseconds} millisecs");
count = 0;
stopWatch.Reset();
stopWatch.Start();
foreach (var vals in GetPermutations(values, values.Length))
{
foreach (var v in vals)
{
count++;
}
}
stopWatch.Stop();
Console.WriteLine($"Linq {count} items in {stopWatch.ElapsedMilliseconds} millisecs");
}
}
}
An this is my test code:
Task.Run(() =>
{
int[] values = new int[12];
for (int n = 0; n < values.Length; n++)
{
values[n] = n;
}
// Eric Ouellet Algorithm
int count = 0;
var stopwatch = new Stopwatch();
stopwatch.Reset();
stopwatch.Start();
Permutations.ForAllPermutation(values, (vals) =>
{
foreach (var v in vals)
{
count++;
}
return false;
});
stopwatch.Stop();
Console.WriteLine($"This {count} items in {stopwatch.ElapsedMilliseconds} millisecs");
// Simple Plan Algorithm
count = 0;
stopwatch.Reset();
stopwatch.Start();
PermutationsSimpleVar permutations2 = new PermutationsSimpleVar();
permutations2.Permutate(1, values.Length, (int[] vals) =>
{
foreach (var v in vals)
{
count++;
}
});
stopwatch.Stop();
Console.WriteLine($"Simple Plan {count} items in {stopwatch.ElapsedMilliseconds} millisecs");
// ErezRobinson Algorithm
count = 0;
stopwatch.Reset();
stopwatch.Start();
foreach(var vals in PermutationsErezRobinson.QuickPerm(values))
{
foreach (var v in vals)
{
count++;
}
};
stopwatch.Stop();
Console.WriteLine($"Erez Robinson {count} items in {stopwatch.ElapsedMilliseconds} millisecs");
});
Usage examples:
ForAllPermutation("123".ToCharArray(), (vals) =>
{
Console.WriteLine(String.Join("", vals));
return false;
});
int[] values = new int[] { 0, 1, 2, 4 };
ForAllPermutation(values, (vals) =>
{
Console.WriteLine(String.Join("", vals));
return false;
});
Well, if you can handle it in C and then translate to your language of choice, you can't really go much faster than this, because the time will be dominated by print:
void perm(char* s, int n, int i){
if (i >= n-1) print(s);
else {
perm(s, n, i+1);
for (int j = i+1; j<n; j++){
swap(s[i], s[j]);
perm(s, n, i+1);
swap(s[i], s[j]);
}
}
}
perm("ABC", 3, 0);
Update 2018-05-28, a new version, the fastest ... (multi-threaded)
Time taken for fastest algorithms
Need: Sani Singh Huttunen (fastest lexico) solution and my new OuelletLexico3 which support indexing
Indexing has 2 main advantages:
allows to get anyone permutation directly
allows multi-threading (derived from the first advantage)
Article: Permutations: Fast implementations and a new indexing algorithm allowing multithreading
On my machine (6 hyperthread cores : 12 threads) Xeon E5-1660 0 # 3.30Ghz, tests algorithms running with empty stuff to do for 13! items (time in millisecs):
53071: Ouellet (implementation of Heap)
65366: Sani Singh Huttunen (Fastest lexico)
11377: Mix OuelletLexico3 - Sani Singh Huttunen
A side note: using shares properties/variables between threads for permutation action will strongly impact performance if their usage is modification (read / write). Doing so will generate "false sharing" between threads. You will not get expected performance. I got this behavior while testing. My experience showed problems when I try to increase the global variable for the total count of permutation.
Usage:
PermutationMixOuelletSaniSinghHuttunen.ExecuteForEachPermutationMT(
new int[] {1, 2, 3, 4},
p =>
{
Console.WriteLine($"Values: {p[0]}, {p[1]}, p[2]}, {p[3]}");
});
Code:
using System;
using System.Runtime.CompilerServices;
namespace WpfPermutations
{
public class Factorial
{
// ************************************************************************
protected static long[] FactorialTable = new long[21];
// ************************************************************************
static Factorial()
{
FactorialTable[0] = 1; // To prevent divide by 0
long f = 1;
for (int i = 1; i <= 20; i++)
{
f = f * i;
FactorialTable[i] = f;
}
}
// ************************************************************************
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static long GetFactorial(int val) // a long can only support up to 20!
{
if (val > 20)
{
throw new OverflowException($"{nameof(Factorial)} only support a factorial value <= 20");
}
return FactorialTable[val];
}
// ************************************************************************
}
}
namespace WpfPermutations
{
public class PermutationSaniSinghHuttunen
{
public static bool NextPermutation(int[] numList)
{
/*
Knuths
1. Find the largest index j such that a[j] < a[j + 1]. If no such index exists, the permutation is the last permutation.
2. Find the largest index l such that a[j] < a[l]. Since j + 1 is such an index, l is well defined and satisfies j < l.
3. Swap a[j] with a[l].
4. Reverse the sequence from a[j + 1] up to and including the final element a[n].
*/
var largestIndex = -1;
for (var i = numList.Length - 2; i >= 0; i--)
{
if (numList[i] < numList[i + 1])
{
largestIndex = i;
break;
}
}
if (largestIndex < 0) return false;
var largestIndex2 = -1;
for (var i = numList.Length - 1; i >= 0; i--)
{
if (numList[largestIndex] < numList[i])
{
largestIndex2 = i;
break;
}
}
var tmp = numList[largestIndex];
numList[largestIndex] = numList[largestIndex2];
numList[largestIndex2] = tmp;
for (int i = largestIndex + 1, j = numList.Length - 1; i < j; i++, j--)
{
tmp = numList[i];
numList[i] = numList[j];
numList[j] = tmp;
}
return true;
}
}
}
using System;
namespace WpfPermutations
{
public class PermutationOuelletLexico3<T> // Enable indexing
{
// ************************************************************************
private T[] _sortedValues;
private bool[] _valueUsed;
public readonly long MaxIndex; // long to support 20! or less
// ************************************************************************
public PermutationOuelletLexico3(T[] sortedValues)
{
_sortedValues = sortedValues;
Result = new T[_sortedValues.Length];
_valueUsed = new bool[_sortedValues.Length];
MaxIndex = Factorial.GetFactorial(_sortedValues.Length);
}
// ************************************************************************
public T[] Result { get; private set; }
// ************************************************************************
/// <summary>
/// Sort Index is 0 based and should be less than MaxIndex. Otherwise you get an exception.
/// </summary>
/// <param name="sortIndex"></param>
/// <param name="result">Value is not used as inpu, only as output. Re-use buffer in order to save memory</param>
/// <returns></returns>
public void GetSortedValuesFor(long sortIndex)
{
int size = _sortedValues.Length;
if (sortIndex < 0)
{
throw new ArgumentException("sortIndex should greater or equal to 0.");
}
if (sortIndex >= MaxIndex)
{
throw new ArgumentException("sortIndex should less than factorial(the lenght of items)");
}
for (int n = 0; n < _valueUsed.Length; n++)
{
_valueUsed[n] = false;
}
long factorielLower = MaxIndex;
for (int index = 0; index < size; index++)
{
long factorielBigger = factorielLower;
factorielLower = Factorial.GetFactorial(size - index - 1); // factorielBigger / inverseIndex;
int resultItemIndex = (int)(sortIndex % factorielBigger / factorielLower);
int correctedResultItemIndex = 0;
for(;;)
{
if (! _valueUsed[correctedResultItemIndex])
{
resultItemIndex--;
if (resultItemIndex < 0)
{
break;
}
}
correctedResultItemIndex++;
}
Result[index] = _sortedValues[correctedResultItemIndex];
_valueUsed[correctedResultItemIndex] = true;
}
}
// ************************************************************************
}
}
using System;
using System.Collections.Generic;
using System.Threading.Tasks;
namespace WpfPermutations
{
public class PermutationMixOuelletSaniSinghHuttunen
{
// ************************************************************************
private long _indexFirst;
private long _indexLastExclusive;
private int[] _sortedValues;
// ************************************************************************
public PermutationMixOuelletSaniSinghHuttunen(int[] sortedValues, long indexFirst = -1, long indexLastExclusive = -1)
{
if (indexFirst == -1)
{
indexFirst = 0;
}
if (indexLastExclusive == -1)
{
indexLastExclusive = Factorial.GetFactorial(sortedValues.Length);
}
if (indexFirst >= indexLastExclusive)
{
throw new ArgumentException($"{nameof(indexFirst)} should be less than {nameof(indexLastExclusive)}");
}
_indexFirst = indexFirst;
_indexLastExclusive = indexLastExclusive;
_sortedValues = sortedValues;
}
// ************************************************************************
public void ExecuteForEachPermutation(Action<int[]> action)
{
// Console.WriteLine($"Thread {System.Threading.Thread.CurrentThread.ManagedThreadId} started: {_indexFirst} {_indexLastExclusive}");
long index = _indexFirst;
PermutationOuelletLexico3<int> permutationOuellet = new PermutationOuelletLexico3<int>(_sortedValues);
permutationOuellet.GetSortedValuesFor(index);
action(permutationOuellet.Result);
index++;
int[] values = permutationOuellet.Result;
while (index < _indexLastExclusive)
{
PermutationSaniSinghHuttunen.NextPermutation(values);
action(values);
index++;
}
// Console.WriteLine($"Thread {System.Threading.Thread.CurrentThread.ManagedThreadId} ended: {DateTime.Now.ToString("yyyyMMdd_HHmmss_ffffff")}");
}
// ************************************************************************
public static void ExecuteForEachPermutationMT(int[] sortedValues, Action<int[]> action)
{
int coreCount = Environment.ProcessorCount; // Hyper treading are taken into account (ex: on a 4 cores hyperthreaded = 8)
long itemsFactorial = Factorial.GetFactorial(sortedValues.Length);
long partCount = (long)Math.Ceiling((double)itemsFactorial / (double)coreCount);
long startIndex = 0;
var tasks = new List<Task>();
for (int coreIndex = 0; coreIndex < coreCount; coreIndex++)
{
long stopIndex = Math.Min(startIndex + partCount, itemsFactorial);
PermutationMixOuelletSaniSinghHuttunen mix = new PermutationMixOuelletSaniSinghHuttunen(sortedValues, startIndex, stopIndex);
Task task = Task.Run(() => mix.ExecuteForEachPermutation(action));
tasks.Add(task);
if (stopIndex == itemsFactorial)
{
break;
}
startIndex = startIndex + partCount;
}
Task.WaitAll(tasks.ToArray());
}
// ************************************************************************
}
}
The fastest permutation algorithm that i know of is the QuickPerm algorithm.
Here is the implementation, it uses yield return so you can iterate one at a time like required.
Code:
public static IEnumerable<IEnumerable<T>> QuickPerm<T>(this IEnumerable<T> set)
{
int N = set.Count();
int[] a = new int[N];
int[] p = new int[N];
var yieldRet = new T[N];
List<T> list = new List<T>(set);
int i, j, tmp; // Upper Index i; Lower Index j
for (i = 0; i < N; i++)
{
// initialize arrays; a[N] can be any type
a[i] = i + 1; // a[i] value is not revealed and can be arbitrary
p[i] = 0; // p[i] == i controls iteration and index boundaries for i
}
yield return list;
//display(a, 0, 0); // remove comment to display array a[]
i = 1; // setup first swap points to be 1 and 0 respectively (i & j)
while (i < N)
{
if (p[i] < i)
{
j = i%2*p[i]; // IF i is odd then j = p[i] otherwise j = 0
tmp = a[j]; // swap(a[j], a[i])
a[j] = a[i];
a[i] = tmp;
//MAIN!
for (int x = 0; x < N; x++)
{
yieldRet[x] = list[a[x]-1];
}
yield return yieldRet;
//display(a, j, i); // remove comment to display target array a[]
// MAIN!
p[i]++; // increase index "weight" for i by one
i = 1; // reset index i to 1 (assumed)
}
else
{
// otherwise p[i] == i
p[i] = 0; // reset p[i] to zero
i++; // set new index value for i (increase by one)
} // if (p[i] < i)
} // while(i < N)
}
Here is the fastest implementation I ended up with:
public class Permutations
{
private readonly Mutex _mutex = new Mutex();
private Action<int[]> _action;
private Action<IntPtr> _actionUnsafe;
private unsafe int* _arr;
private IntPtr _arrIntPtr;
private unsafe int* _last;
private unsafe int* _lastPrev;
private unsafe int* _lastPrevPrev;
public int Size { get; private set; }
public bool IsRunning()
{
return this._mutex.SafeWaitHandle.IsClosed;
}
public bool Permutate(int start, int count, Action<int[]> action, bool async = false)
{
return this.Permutate(start, count, action, null, async);
}
public bool Permutate(int start, int count, Action<IntPtr> actionUnsafe, bool async = false)
{
return this.Permutate(start, count, null, actionUnsafe, async);
}
private unsafe bool Permutate(int start, int count, Action<int[]> action, Action<IntPtr> actionUnsafe, bool async = false)
{
if (!this._mutex.WaitOne(0))
{
return false;
}
var x = (Action)(() =>
{
this._actionUnsafe = actionUnsafe;
this._action = action;
this.Size = count;
this._arr = (int*)Marshal.AllocHGlobal(count * sizeof(int));
this._arrIntPtr = new IntPtr(this._arr);
for (var i = 0; i < count - 3; i++)
{
this._arr[i] = start + i;
}
this._last = this._arr + count - 1;
this._lastPrev = this._last - 1;
this._lastPrevPrev = this._lastPrev - 1;
*this._last = count - 1;
*this._lastPrev = count - 2;
*this._lastPrevPrev = count - 3;
this.Permutate(count, this._arr);
});
if (!async)
{
x();
}
else
{
new Thread(() => x()).Start();
}
return true;
}
private unsafe void Permutate(int size, int* start)
{
if (size == 3)
{
this.DoAction();
Swap(this._last, this._lastPrev);
this.DoAction();
Swap(this._last, this._lastPrevPrev);
this.DoAction();
Swap(this._last, this._lastPrev);
this.DoAction();
Swap(this._last, this._lastPrevPrev);
this.DoAction();
Swap(this._last, this._lastPrev);
this.DoAction();
return;
}
var sizeDec = size - 1;
var startNext = start + 1;
var usedStarters = 0;
for (var i = 0; i < sizeDec; i++)
{
this.Permutate(sizeDec, startNext);
usedStarters |= 1 << *start;
for (var j = startNext; j <= this._last; j++)
{
var mask = 1 << *j;
if ((usedStarters & mask) != mask)
{
Swap(start, j);
break;
}
}
}
this.Permutate(sizeDec, startNext);
if (size == this.Size)
{
this._mutex.ReleaseMutex();
}
}
private unsafe void DoAction()
{
if (this._action == null)
{
if (this._actionUnsafe != null)
{
this._actionUnsafe(this._arrIntPtr);
}
return;
}
var result = new int[this.Size];
fixed (int* pt = result)
{
var limit = pt + this.Size;
var resultPtr = pt;
var arrayPtr = this._arr;
while (resultPtr < limit)
{
*resultPtr = *arrayPtr;
resultPtr++;
arrayPtr++;
}
}
this._action(result);
}
private static unsafe void Swap(int* a, int* b)
{
var tmp = *a;
*a = *b;
*b = tmp;
}
}
Usage and testing performance:
var perms = new Permutations();
var sw1 = Stopwatch.StartNew();
perms.Permutate(0,
11,
(Action<int[]>)null); // Comment this line and...
//PrintArr); // Uncomment this line, to print permutations
sw1.Stop();
Console.WriteLine(sw1.Elapsed);
Printing method:
private static void PrintArr(int[] arr)
{
Console.WriteLine(string.Join(",", arr));
}
Going deeper:
I did not even think about this for a very long time, so I can only explain my code so much, but here's the general idea:
Permutations aren't lexicographic - this allows me to practically perform less operations between permutations.
The implementation is recursive, and when the "view" size is 3, it skips the complex logic and just performs 6 swaps to get the 6 permutations (or sub-permutations, if you will).
Because the permutations aren't in a lexicographic order, how can I decide which element to bring to the start of the current "view" (sub permutation)? I keep record of elements that were already used as "starters" in the current sub-permutation recursive call and simply search linearly for one that wasn't used in the tail of my array.
The implementation is for integers only, so to permute over a generic collection of elements you simply use the Permutations class to permute indices instead of your actual collection.
The Mutex is there just to ensure things don't get screwed when the execution is asynchronous (notice that you can pass an UnsafeAction parameter that will in turn get a pointer to the permuted array. You must not change the order of elements in that array (pointer)! If you want to, you should copy the array to a tmp array or just use the safe action parameter which takes care of that for you - the passed array is already a copy).
Note:
I have no idea how good this implementation really is - I haven't touched it in so long.
Test and compare to other implementations on your own, and let me know if you have any feedback!
Enjoy.
Here is a generic permutation finder that will iterate through every permutation of a collection and call an evalution function. If the evalution function returns true (it found the answer it was looking for), the permutation finder stops processing.
public class PermutationFinder<T>
{
private T[] items;
private Predicate<T[]> SuccessFunc;
private bool success = false;
private int itemsCount;
public void Evaluate(T[] items, Predicate<T[]> SuccessFunc)
{
this.items = items;
this.SuccessFunc = SuccessFunc;
this.itemsCount = items.Count();
Recurse(0);
}
private void Recurse(int index)
{
T tmp;
if (index == itemsCount)
success = SuccessFunc(items);
else
{
for (int i = index; i < itemsCount; i++)
{
tmp = items[index];
items[index] = items[i];
items[i] = tmp;
Recurse(index + 1);
if (success)
break;
tmp = items[index];
items[index] = items[i];
items[i] = tmp;
}
}
}
}
Here is a simple implementation:
class Program
{
static void Main(string[] args)
{
new Program().Start();
}
void Start()
{
string[] items = new string[5];
items[0] = "A";
items[1] = "B";
items[2] = "C";
items[3] = "D";
items[4] = "E";
new PermutationFinder<string>().Evaluate(items, Evaluate);
Console.ReadLine();
}
public bool Evaluate(string[] items)
{
Console.WriteLine(string.Format("{0},{1},{2},{3},{4}", items[0], items[1], items[2], items[3], items[4]));
bool someCondition = false;
if (someCondition)
return true; // Tell the permutation finder to stop.
return false;
}
}
Here is a recursive implementation with complexity O(n * n!)1 based on swapping of the elements of an array. The array is initialised with values from 1, 2, ..., n.
using System;
namespace Exercise
{
class Permutations
{
static void Main(string[] args)
{
int setSize = 3;
FindPermutations(setSize);
}
//-----------------------------------------------------------------------------
/* Method: FindPermutations(n) */
private static void FindPermutations(int n)
{
int[] arr = new int[n];
for (int i = 0; i < n; i++)
{
arr[i] = i + 1;
}
int iEnd = arr.Length - 1;
Permute(arr, iEnd);
}
//-----------------------------------------------------------------------------
/* Method: Permute(arr) */
private static void Permute(int[] arr, int iEnd)
{
if (iEnd == 0)
{
PrintArray(arr);
return;
}
Permute(arr, iEnd - 1);
for (int i = 0; i < iEnd; i++)
{
swap(ref arr[i], ref arr[iEnd]);
Permute(arr, iEnd - 1);
swap(ref arr[i], ref arr[iEnd]);
}
}
}
}
On each recursive step we swap the last element with the current element pointed to by the local variable in the for loop and then we indicate the uniqueness of the swapping by: incrementing the local variable of the for loop and decrementing the termination condition of the for loop, which is initially set to the number of the elements in the array, when the latter becomes zero we terminate the recursion.
Here are the helper functions:
//-----------------------------------------------------------------------------
/*
Method: PrintArray()
*/
private static void PrintArray(int[] arr, string label = "")
{
Console.WriteLine(label);
Console.Write("{");
for (int i = 0; i < arr.Length; i++)
{
Console.Write(arr[i]);
if (i < arr.Length - 1)
{
Console.Write(", ");
}
}
Console.WriteLine("}");
}
//-----------------------------------------------------------------------------
/*
Method: swap(ref int a, ref int b)
*/
private static void swap(ref int a, ref int b)
{
int temp = a;
a = b;
b = temp;
}
1. There are n! permutations of n elements to be printed.
I would be surprised if there are really order of magnitude improvements to be found. If there are, then C# needs fundamental improvement. Furthermore doing anything interesting with your permutation will generally take more work than generating it. So the cost of generating is going to be insignificant in the overall scheme of things.
That said, I would suggest trying the following things. You have already tried iterators. But have you tried having a function that takes a closure as input, then then calls that closure for each permutation found? Depending on internal mechanics of C#, this may be faster.
Similarly, have you tried having a function that returns a closure that will iterate over a specific permutation?
With either approach, there are a number of micro-optimizations you can experiment with. For instance you can sort your input array, and after that you always know what order it is in. For example you can have an array of bools indicating whether that element is less than the next one, and rather than do comparisons, you can just look at that array.
There's an accessible introduction to the algorithms and survey of implementations in Steven Skiena's Algorithm Design Manual (chapter 14.4 in the second edition)
Skiena references D. Knuth. The Art of Computer Programming, Volume 4 Fascicle 2: Generating All Tuples and Permutations. Addison Wesley, 2005.
I created an algorithm slightly faster than Knuth's one:
11 elements:
mine: 0.39 seconds
Knuth's: 0.624 seconds
13 elements:
mine: 56.615 seconds
Knuth's: 98.681 seconds
Here's my code in Java:
public static void main(String[] args)
{
int n=11;
int a,b,c,i,tmp;
int end=(int)Math.floor(n/2);
int[][] pos = new int[end+1][2];
int[] perm = new int[n];
for(i=0;i<n;i++) perm[i]=i;
while(true)
{
//this is where you can use the permutations (perm)
i=0;
c=n;
while(pos[i][1]==c-2 && pos[i][0]==c-1)
{
pos[i][0]=0;
pos[i][1]=0;
i++;
c-=2;
}
if(i==end) System.exit(0);
a=(pos[i][0]+1)%c+i;
b=pos[i][0]+i;
tmp=perm[b];
perm[b]=perm[a];
perm[a]=tmp;
if(pos[i][0]==c-1)
{
pos[i][0]=0;
pos[i][1]++;
}
else
{
pos[i][0]++;
}
}
}
The problem is my algorithm only works for odd numbers of elements. I wrote this code quickly so I'm pretty sure there's a better way to implement my idea to get better performance, but I don't really have the time to work on it right now to optimize it and solve the issue when the number of elements is even.
It's one swap for every permutation and it uses a really simple way to know which elements to swap.
I wrote an explanation of the method behind the code on my blog: http://antoinecomeau.blogspot.ca/2015/01/fast-generation-of-all-permutations.html
As the author of this question was asking about an algorithm:
[...] generating a single permutation, at a time, and continuing only if necessary
I would suggest considering Steinhaus–Johnson–Trotter algorithm.
Steinhaus–Johnson–Trotter algorithm on Wikipedia
Beautifully explained here
It's 1 am and I was watching TV and thought of this same question, but with string values.
Given a word find all permutations. You can easily modify this to handle an array, sets, etc.
Took me a bit to work it out, but the solution I came up was this:
string word = "abcd";
List<string> combinations = new List<string>();
for(int i=0; i<word.Length; i++)
{
for (int j = 0; j < word.Length; j++)
{
if (i < j)
combinations.Add(word[i] + word.Substring(j) + word.Substring(0, i) + word.Substring(i + 1, j - (i + 1)));
else if (i > j)
{
if(i== word.Length -1)
combinations.Add(word[i] + word.Substring(0, i));
else
combinations.Add(word[i] + word.Substring(0, i) + word.Substring(i + 1));
}
}
}
Here's the same code as above, but with some comments
string word = "abcd";
List<string> combinations = new List<string>();
//i is the first letter of the new word combination
for(int i=0; i<word.Length; i++)
{
for (int j = 0; j < word.Length; j++)
{
//add the first letter of the word, j is past i so we can get all the letters from j to the end
//then add all the letters from the front to i, then skip over i (since we already added that as the beginning of the word)
//and get the remaining letters from i+1 to right before j.
if (i < j)
combinations.Add(word[i] + word.Substring(j) + word.Substring(0, i) + word.Substring(i + 1, j - (i + 1)));
else if (i > j)
{
//if we're at the very last word no need to get the letters after i
if(i== word.Length -1)
combinations.Add(word[i] + word.Substring(0, i));
//add i as the first letter of the word, then get all the letters up to i, skip i, and then add all the lettes after i
else
combinations.Add(word[i] + word.Substring(0, i) + word.Substring(i + 1));
}
}
}
//+------------------------------------------------------------------+
//| |
//+------------------------------------------------------------------+
/**
* http://marknelson.us/2002/03/01/next-permutation/
* Rearranges the elements into the lexicographically next greater permutation and returns true.
* When there are no more greater permutations left, the function eventually returns false.
*/
// next lexicographical permutation
template <typename T>
bool next_permutation(T &arr[], int firstIndex, int lastIndex)
{
int i = lastIndex;
while (i > firstIndex)
{
int ii = i--;
T curr = arr[i];
if (curr < arr[ii])
{
int j = lastIndex;
while (arr[j] <= curr) j--;
Swap(arr[i], arr[j]);
while (ii < lastIndex)
Swap(arr[ii++], arr[lastIndex--]);
return true;
}
}
return false;
}
//+------------------------------------------------------------------+
//| |
//+------------------------------------------------------------------+
/**
* Swaps two variables or two array elements.
* using references/pointers to speed up swapping.
*/
template<typename T>
void Swap(T &var1, T &var2)
{
T temp;
temp = var1;
var1 = var2;
var2 = temp;
}
//+------------------------------------------------------------------+
//| |
//+------------------------------------------------------------------+
// driver program to test above function
#define N 3
void OnStart()
{
int i, x[N];
for (i = 0; i < N; i++) x[i] = i + 1;
printf("The %i! possible permutations with %i elements:", N, N);
do
{
printf("%s", ArrayToString(x));
} while (next_permutation(x, 0, N - 1));
}
// Output:
// The 3! possible permutations with 3 elements:
// "1,2,3"
// "1,3,2"
// "2,1,3"
// "2,3,1"
// "3,1,2"
// "3,2,1"
// Permutations are the different ordered arrangements of an n-element
// array. An n-element array has exactly n! full-length permutations.
// This iterator object allows to iterate all full length permutations
// one by one of an array of n distinct elements.
// The iterator changes the given array in-place.
// Permutations('ABCD') => ABCD DBAC ACDB DCBA
// BACD BDAC CADB CDBA
// CABD ADBC DACB BDCA
// ACBD DABC ADCB DBCA
// BCAD BADC CDAB CBDA
// CBAD ABDC DCAB BCDA
// count of permutations = n!
// Heap's algorithm (Single swap per permutation)
// http://www.quickperm.org/quickperm.php
// https://stackoverflow.com/a/36634935/4208440
// https://en.wikipedia.org/wiki/Heap%27s_algorithm
// My implementation of Heap's algorithm:
template<typename T>
class PermutationsIterator
{
int b, e, n;
int c[32]; /* control array: mixed radix number in rising factorial base.
the i-th digit has base i, which means that the digit must be
strictly less than i. The first digit is always 0, the second
can be 0 or 1, the third 0, 1 or 2, and so on.
ArrayResize isn't strictly necessary, int c[32] would suffice
for most practical purposes. Also, it is much faster */
public:
PermutationsIterator(T &arr[], int firstIndex, int lastIndex)
{
this.b = firstIndex; // v.begin()
this.e = lastIndex; // v.end()
this.n = e - b + 1;
ArrayInitialize(c, 0);
}
// Rearranges the input array into the next permutation and returns true.
// When there are no more permutations left, the function returns false.
bool next(T &arr[])
{
// find index to update
int i = 1;
// reset all the previous indices that reached the maximum possible values
while (c[i] == i)
{
c[i] = 0;
++i;
}
// no more permutations left
if (i == n)
return false;
// generate next permutation
int j = (i & 1) == 1 ? c[i] : 0; // IF i is odd then j = c[i] otherwise j = 0.
swap(arr[b + j], arr[b + i]); // generate a new permutation from previous permutation using a single swap
// Increment that index
++c[i];
return true;
}
};
I found this algo on rosetta code and it is really the fastest one I tried. http://rosettacode.org/wiki/Permutations#C
/* Boothroyd method; exactly N! swaps, about as fast as it gets */
void boothroyd(int *x, int n, int nn, int callback(int *, int))
{
int c = 0, i, t;
while (1) {
if (n > 2) boothroyd(x, n - 1, nn, callback);
if (c >= n - 1) return;
i = (n & 1) ? 0 : c;
c++;
t = x[n - 1], x[n - 1] = x[i], x[i] = t;
if (callback) callback(x, nn);
}
}
/* entry for Boothroyd method */
void perm2(int *x, int n, int callback(int*, int))
{
if (callback) callback(x, n);
boothroyd(x, n, n, callback);
}
If you just want to calculate the number of possible permutations you can avoid all that hard work above and use something like this (contrived in c#):
public static class ContrivedUtils
{
public static Int64 Permutations(char[] array)
{
if (null == array || array.Length == 0) return 0;
Int64 permutations = array.Length;
for (var pos = permutations; pos > 1; pos--)
permutations *= pos - 1;
return permutations;
}
}
You call it like this:
var permutations = ContrivedUtils.Permutations("1234".ToCharArray());
// output is: 24
var permutations = ContrivedUtils.Permutations("123456789".ToCharArray());
// output is: 362880
Simple C# recursive solution by swapping, for the initial call the index must be 0
static public void Permute<T>(List<T> input, List<List<T>> permutations, int index)
{
if (index == input.Count - 1)
{
permutations.Add(new List<T>(input));
return;
}
Permute(input, permutations, index + 1);
for (int i = index+1 ; i < input.Count; i++)
{
//swap
T temp = input[index];
input[index] = input[i];
input[i] = temp;
Permute(input, permutations, index + 1);
//swap back
temp = input[index];
input[index] = input[i];
input[i] = temp;
}
}

Find the first occurrence/starting index of the sub-array in C#

Given two arrays as parameters (x and y) and find the starting index where the first occurrence of y in x. I am wondering what the simplest or the fastest implementation would be.
Example:
when x = {1,2,4,2,3,4,5,6}
y = {2,3}
result
starting index should be 3
Update: Since my code is wrong I removed it from the question.
Simplest to write?
return (from i in Enumerable.Range(0, 1 + x.Length - y.Length)
where x.Skip(i).Take(y.Length).SequenceEqual(y)
select (int?)i).FirstOrDefault().GetValueOrDefault(-1);
Not quite as efficient, of course... a bit more like it:
private static bool IsSubArrayEqual(int[] x, int[] y, int start) {
for (int i = 0; i < y.Length; i++) {
if (x[start++] != y[i]) return false;
}
return true;
}
public static int StartingIndex(this int[] x, int[] y) {
int max = 1 + x.Length - y.Length;
for(int i = 0 ; i < max ; i++) {
if(IsSubArrayEqual(x,y,i)) return i;
}
return -1;
}
Here is a simple (yet fairly efficient) implementation that finds all occurances of the array, not just the first one:
static class ArrayExtensions {
public static IEnumerable<int> StartingIndex(this int[] x, int[] y) {
IEnumerable<int> index = Enumerable.Range(0, x.Length - y.Length + 1);
for (int i = 0; i < y.Length; i++) {
index = index.Where(n => x[n + i] == y[i]).ToArray();
}
return index;
}
}
Example:
int[] x = { 1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4 };
int[] y = { 2, 3 };
foreach (int i in x.StartingIndex(y)) {
Console.WriteLine(i);
}
Output:
1
5
9
The method first loops through the x array to find all occurances of the first item in the y array, and place the index of those in the index array. Then it goes on to reduce the matches by checking which of those also match the second item in the y array. When all items in the y array is checked, the index array contains only the full matches.
Edit:
An alternative implementation would be to remove the ToArray call from the statement in the loop, making it just:
index = index.Where(n => x[n + i] == y[i]);
This would totally change how the method works. Instead of looping through the items level by level, it would return an enumerator with nested expressions, deferring the search to the time when the enumerator was iterated. That means that you could get only the first match if you wanted:
int index = x.StartingIndex(y).First();
This would not find all matches and then return the first, it would just search until the first was found and then return it.
The simplest way is probably this:
public static class ArrayExtensions
{
private static bool isMatch(int[] x, int[] y, int index)
{
for (int j = 0; j < y.Length; ++j)
if (x[j + index] != y[j]) return false;
return true;
}
public static int IndexOf(this int[] x, int[] y)
{
for (int i = 0; i < x.Length - y.Length + 1; ++i)
if (isMatch(x, y, i)) return i;
return -1;
}
}
But it's definitely not the fastest way.
This is based off of Mark Gravell's answer but I made it generic and added some simple bounds checking to keep exceptions from being thrown
private static bool IsSubArrayEqual<T>(T[] source, T[] compare, int start) where T:IEquatable<T>
{
if (compare.Length > source.Length - start)
{
//If the compare string is shorter than the test area it is not a match.
return false;
}
for (int i = 0; i < compare.Length; i++)
{
if (source[start++].Equals(compare[i]) == false) return false;
}
return true;
}
Could be improved further by implementing Boyer-Moore but for short patterns it works fine.
"Simplest" and "fastest" are opposites in this case, and besides, in order to describe fast algorithms we need to know lots of things about how the source array and the search array are related to each other.
This is essentially the same problem as finding a substring inside a string. Suppose you are looking for "fox" in "the quick brown fox jumps over the lazy dog". The naive string matching algorithm is extremely good in this case. If you are searching for "bananananananananananananananana" inside a million-character string that is of the form "banananananabanananabananabananabanananananbananana..." then the naive substring matching algorithm is terrible -- far faster results can be obtained by using more complex and sophisticated string matching algorithms. Basically, the naive algorithm is O(nm) where n and m are the lengths of the source and search strings. There are O(n+m) algorithms but they are far more complex.
Can you tell us more about the data you're searching? How big is it, how redundant is it, how long are the search arrays, and what is the likelihood of a bad match?
I find something along the following lines more intuitive, but that may be a matter of taste.
public static class ArrayExtensions
{
public static int StartingIndex(this int[] x, int[] y)
{
var xIndex = 0;
while(xIndex < x.length)
{
var found = xIndex;
var yIndex = 0;
while(yIndex < y.length && xIndex < x.length && x[xIndex] == y[yIndex])
{
xIndex++;
yIndex++;
}
if(yIndex == y.length-1)
{
return found;
}
xIndex = found + 1;
}
return -1;
}
}
This code also addresses an issue I believe your implementation may have in cases like x = {3, 3, 7}, y = {3, 7}. I think what would happen with your code is that it matches the first number, then resets itself on the second, but starts matching again on the third, rather than stepping back to the index just after where it started matching. May be missing something, but it's definitely something to consider and should be easily fixable in your code.
//this is the best in C#
//bool contains(array,subarray)
// when find (subarray[0])
// while subarray[next] IS OK
// subarray.end then Return True
public static bool ContainSubArray<T>(T[] findIn, out int found_index,
params T[]toFind)
{
found_index = -1;
if (toFind.Length < findIn.Length)
{
int index = 0;
Func<int, bool> NextOk = (i) =>
{
if(index < findIn.Length-1)
return findIn[++index].Equals(toFind[i]);
return false;
};
//----------
int n=0;
for (; index < findIn.Length; index++)
{
if (findIn[index].Equals(toFind[0]))
{
found_index=index;n=1;
while (n < toFind.Length && NextOk(n))
n++;
}
if (n == toFind.Length)
{
return true;
}
}
}
return false;
}
using System;
using System.Linq;
public class Test
{
public static void Main()
{
int[] x = {1,2,4,2,3,4,5,6};
int[] y = {2,3};
int? index = null;
for(int i=0; i<x.Length; ++i)
{
if (y.SequenceEqual(x.Skip(i).Take(y.Length)))
{
index = i;
break;
}
}
Console.WriteLine($"{index}");
}
}
Output
3

Array slices in C#

How do you do it? Given a byte array:
byte[] foo = new byte[4096];
How would I get the first x bytes of the array as a separate array? (Specifically, I need it as an IEnumerable<byte>)
This is for working with Sockets. I figure the easiest way would be array slicing, similar to Perls syntax:
#bar = #foo[0..40];
Which would return the first 41 elements into the #bar array. Is there something in C# that I'm just missing, or is there some other thing I should be doing?
LINQ is an option for me (.NET 3.5), if that helps any.
You could use ArraySegment<T>. It's very light-weight as it doesn't copy the array:
string[] a = { "one", "two", "three", "four", "five" };
var segment = new ArraySegment<string>( a, 1, 2 );
Arrays are enumerable, so your foo already is an IEnumerable<byte> itself.
Simply use LINQ sequence methods like Take() to get what you want out of it (don't forget to include the Linq namespace with using System.Linq;):
byte[] foo = new byte[4096];
var bar = foo.Take(41);
If you really need an array from any IEnumerable<byte> value, you could use the ToArray() method for that. That does not seem to be the case here.
You could use the arrays CopyTo() method.
Or with LINQ you can use Skip() and Take()...
byte[] arr = {1, 2, 3, 4, 5, 6, 7, 8};
var subset = arr.Skip(2).Take(2);
Starting from C# 8.0/.Net Core 3.0
Array slicing will be supported, along with the new types Index and Range being added.
Range Struct docs
Index Struct docs
Index i1 = 3; // number 3 from beginning
Index i2 = ^4; // number 4 from end
int[] a = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
Console.WriteLine($"{a[i1]}, {a[i2]}"); // "3, 6"
var slice = a[i1..i2]; // { 3, 4, 5 }
Above code sample taken from the C# 8.0 blog.
note that the ^ prefix indicates counting from the end of the array. As shown in the docs example
var words = new string[]
{
// index from start index from end
"The", // 0 ^9
"quick", // 1 ^8
"brown", // 2 ^7
"fox", // 3 ^6
"jumped", // 4 ^5
"over", // 5 ^4
"the", // 6 ^3
"lazy", // 7 ^2
"dog" // 8 ^1
}; // 9 (or words.Length) ^0
Range and Index also work outside of slicing arrays, for example with loops
Range range = 1..4;
foreach (var name in names[range])
Will loop through the entries 1 through 4
note that at the time of writing this answer, C# 8.0 is not yet officially released
C# 8.x and .Net Core 3.x are now available in Visual Studio 2019 and onwards
static byte[] SliceMe(byte[] source, int length)
{
byte[] destfoo = new byte[length];
Array.Copy(source, 0, destfoo, 0, length);
return destfoo;
}
//
var myslice = SliceMe(sourcearray,41);
In C# 7.2, you can use Span<T>. The benefit of the new System.Memory system is that it doesn't need copying around data.
The method you need is Slice:
Span<byte> slice = foo.Slice(0, 40);
A lot of methods now support Span and IReadOnlySpan, so it will be very straightforward to use this new type.
Note that at the time of writing the Span<T> type is not defined in the the most recent version of .NET yet (4.7.1) so to use it you need to install the System.Memory package from NuGet.
C# 8 now (since 2019) supports Ranges which allows you to achieve Slice much easier (similar to JS syntax):
var array = new int[] { 1, 2, 3, 4, 5 };
var slice1 = array[2..^3]; // array[new Range(2, new Index(3, fromEnd: true))]
var slice2 = array[..^3]; // array[Range.EndAt(new Index(3, fromEnd: true))]
var slice3 = array[2..]; // array[Range.StartAt(2)]
var slice4 = array[..]; // array[Range.All]
You can use ranges instead of the well known LINQ functions: Skip(), Take(), Count().
Another possibility I haven't seen mentioned here: Buffer.BlockCopy() is slightly faster than Array.Copy(), and it has the added benefit of being able to convert on-the-fly from an array of primitives (say, short[]) to an array of bytes, which can be handy when you've got numeric arrays that you need to transmit over Sockets.
If you want IEnumerable<byte>, then just
IEnumerable<byte> data = foo.Take(x);
Here's a simple extension method that returns a slice as a new array:
public static T[] Slice<T>(this T[] arr, uint indexFrom, uint indexTo) {
if (indexFrom > indexTo) {
throw new ArgumentOutOfRangeException("indexFrom is bigger than indexTo!");
}
uint length = indexTo - indexFrom;
T[] result = new T[length];
Array.Copy(arr, indexFrom, result, 0, length);
return result;
}
Then you can use it as:
byte[] slice = foo.Slice(0, 40);
If you don't want to add LINQ or other extensions just do:
float[] subArray = new List<float>(myArray).GetRange(0, 8).ToArray();
byte[] foo = new byte[4096];
byte[] bar = foo.Take(40).ToArray();
For byte arrays System.Buffer.BlockCopy will give you the very best performance.
You could use a wrapper around the original array (which is IList), like in this (untested) piece of code.
public class SubList<T> : IList<T>
{
#region Fields
private readonly int startIndex;
private readonly int endIndex;
private readonly int count;
private readonly IList<T> source;
#endregion
public SubList(IList<T> source, int startIndex, int count)
{
this.source = source;
this.startIndex = startIndex;
this.count = count;
this.endIndex = this.startIndex + this.count - 1;
}
#region IList<T> Members
public int IndexOf(T item)
{
if (item != null)
{
for (int i = this.startIndex; i <= this.endIndex; i++)
{
if (item.Equals(this.source[i]))
return i;
}
}
else
{
for (int i = this.startIndex; i <= this.endIndex; i++)
{
if (this.source[i] == null)
return i;
}
}
return -1;
}
public void Insert(int index, T item)
{
throw new NotSupportedException();
}
public void RemoveAt(int index)
{
throw new NotSupportedException();
}
public T this[int index]
{
get
{
if (index >= 0 && index < this.count)
return this.source[index + this.startIndex];
else
throw new IndexOutOfRangeException("index");
}
set
{
if (index >= 0 && index < this.count)
this.source[index + this.startIndex] = value;
else
throw new IndexOutOfRangeException("index");
}
}
#endregion
#region ICollection<T> Members
public void Add(T item)
{
throw new NotSupportedException();
}
public void Clear()
{
throw new NotSupportedException();
}
public bool Contains(T item)
{
return this.IndexOf(item) >= 0;
}
public void CopyTo(T[] array, int arrayIndex)
{
for (int i=0; i<this.count; i++)
{
array[arrayIndex + i] = this.source[i + this.startIndex];
}
}
public int Count
{
get { return this.count; }
}
public bool IsReadOnly
{
get { return true; }
}
public bool Remove(T item)
{
throw new NotSupportedException();
}
#endregion
#region IEnumerable<T> Members
public IEnumerator<T> GetEnumerator()
{
for (int i = this.startIndex; i < this.endIndex; i++)
{
yield return this.source[i];
}
}
#endregion
#region IEnumerable Members
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
#endregion
}
You can use Take extension method
var array = new byte[] {1, 2, 3, 4};
var firstTwoItems = array.Take(2);
This may be a solution that:
var result = foo.Slice(40, int.MaxValue);
Then the result is an IEnumerable< IEnumerable< byte>> with a first IEnumerable< byte> contains the first 40 bytes of foo, and a second IEnumerable< byte> holds the rest.
I wrote a wrapper class, the whole iteration is lazy, hope it could help:
public static class CollectionSlicer
{
public static IEnumerable<IEnumerable<T>> Slice<T>(this IEnumerable<T> source, params int[] steps)
{
if (!steps.Any(step => step != 0))
{
throw new InvalidOperationException("Can't slice a collection with step length 0.");
}
return new Slicer<T>(source.GetEnumerator(), steps).Slice();
}
}
public sealed class Slicer<T>
{
public Slicer(IEnumerator<T> iterator, int[] steps)
{
_iterator = iterator;
_steps = steps;
_index = 0;
_currentStep = 0;
_isHasNext = true;
}
public int Index
{
get { return _index; }
}
public IEnumerable<IEnumerable<T>> Slice()
{
var length = _steps.Length;
var index = 1;
var step = 0;
for (var i = 0; _isHasNext; ++i)
{
if (i < length)
{
step = _steps[i];
_currentStep = step - 1;
}
while (_index < index && _isHasNext)
{
_isHasNext = MoveNext();
}
if (_isHasNext)
{
yield return SliceInternal();
index += step;
}
}
}
private IEnumerable<T> SliceInternal()
{
if (_currentStep == -1) yield break;
yield return _iterator.Current;
for (var count = 0; count < _currentStep && _isHasNext; ++count)
{
_isHasNext = MoveNext();
if (_isHasNext)
{
yield return _iterator.Current;
}
}
}
private bool MoveNext()
{
++_index;
return _iterator.MoveNext();
}
private readonly IEnumerator<T> _iterator;
private readonly int[] _steps;
private volatile bool _isHasNext;
private volatile int _currentStep;
private volatile int _index;
}
I do not think C# supports the Range semantics. You could write an extension method though, like:
public static IEnumerator<Byte> Range(this byte[] array, int start, int end);
But like others have said if you do not need to set a start index then Take is all you need.
Here is an extension function that uses a generic and behaves like the PHP function array_slice. Negative offset and length are allowed.
public static class Extensions
{
public static T[] Slice<T>(this T[] arr, int offset, int length)
{
int start, end;
// Determine start index, handling negative offset.
if (offset < 0)
start = arr.Length + offset;
else
start = offset;
// Clamp start index to the bounds of the input array.
if (start < 0)
start = 0;
else if (start > arr.Length)
start = arr.Length;
// Determine end index, handling negative length.
if (length < 0)
end = arr.Length + length;
else
end = start + length;
// Clamp end index to the bounds of the input array.
if (end < 0)
end = 0;
if (end > arr.Length)
end = arr.Length;
// Get the array slice.
int len = end - start;
T[] result = new T[len];
for (int i = 0; i < len; i++)
{
result[i] = arr[start + i];
}
return result;
}
}
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace data_seniens
{
class Program
{
static void Main(string[] args)
{
//new list
float [] x=new float[]{11.25f,18.0f,20.0f,10.75f,9.50f, 11.25f, 18.0f, 20.0f, 10.75f, 9.50f };
//variable
float eat_sleep_area=x[1]+x[3];
//print
foreach (var VARIABLE in x)
{
if (VARIABLE < x[7])
{
Console.WriteLine(VARIABLE);
}
}
//keep app run
Console.ReadLine();
}
}
}

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