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Finding 2 matching sub-lists from list of list

got this little problem in my little C# hobby project that I can't quite work out. I have been stuck in lots of messy and complicated nested loops. Hope someone can give light.
I have a list of list of int, i.e. List<List<int>> . Assume each list of int contains unique items. The minimum size of the list is 5. I need to find exactly two lists of int (List A and List B) that share exactly three common items and another list of int (List X) that contains exactly one of these common items. Another condition must hold: none of the other lists contain any of these three items.
For example:
List<List<int>> allLists = new List<List<int>>();
allLists.Add(new List<int>() {1, 2, 3, 4});
allLists.Add(new List<int>() {1, 2});
allLists.Add(new List<int>() {3, 4});
allLists.Add(new List<int>() {3, 4, 5, 6, 7, 8, 9});
allLists.Add(new List<int>() {4, 6, 8});
allLists.Add(new List<int>() {5, 7, 9, 11});
allLists.Add(new List<int>() {6, 7, 8});
For the above example, I would hope to find a solution as:
ListA and ListB: [3, 5] // indices of allLists
ListX: 6 // index of allLists
The three shared items: [5, 7, 9]
The matching item in ListX: 7
Note: Depending on the content of lists, there may be multiple solutions. There may be also situations that no lists is found matching the above conditions.
I was stuck in some messy nested loops. I was thinking if anyone may come up with a simple and efficient solution (possibly with LINQ?)
Originally I had something stupid like the following:
for (var i = 0; i < allLists.Count - 1; i++)
{
if (allLists[i].Count > 2)
{
for (var j = i + 1; j < allLists.Count; j++)
{
List<int> sharedItems = allLists[i].Intersect(allLists[j]).ToList();
if (sharedItems.Count == 3)
{
foreach (var item in sharedItems)
{
int itemCount = 0;
int? possibleListXIndex = null;
for (var k = 0; k < allLists.Count; k++)
{
if (k != i && k != j && allLists[k].Contains(item))
{
// nested loops getting very ugly here... also not sure what to do....
}
}
}
}
}
}
}
Extended Problem
There is an extended version of this problem in my project. It is in the same fashion:
find exactly three lists of int (List A, List B and List C) that share exactly four common items
find another list of int (List X) that contains exactly one of the above common items
none of the other lists contain these four items.
I was thinking the original algorithm may become scalable to also cover the extended version without having to write another version of algorithm from scratch. With my nested loops above, I think I will have no choice but to add at least two deeper-level loops to cover four items and three lists.
I thank everyone for your contributions in advance! Truly appreciated.

Here's a solution that comes up with your answer. I wouldn't exactly called it efficient, but it's pretty simple to follow.
It breaks the work in two step. First it constructs a list of initial candidates where they have exactly three matches. The second step adds the ListX property and checks to see if the remaining criteria is met.
var matches = allLists.Take(allLists.Count - 1)
.SelectMany((x, xIdx) => allLists
.Skip(xIdx + 1)
.Select(y => new { ListA = x, ListB = y, Shared = x.Intersect(y) })
.Where(y => y.Shared.Count() == 3))
.SelectMany(x => allLists
.Where(y => y != x.ListA && y != x.ListB)
.Select(y => new
{
x.ListA,
x.ListB,
x.Shared,
ListX = y,
SingleShared = x.Shared.Intersect(y)
})
.Where(y => y.SingleShared.Count() == 1
&& !allLists.Any(z => z != y.ListA
&& z != y.ListB
&& z != y.ListX
&& z.Intersect(y.Shared).Any())));
You get the output below after running the following code.
ListA. 3: [3, 4, 5, 6, 7, 8, 9] ListB. 5: [5, 7, 9, 11] => [5, 7, 9], ListX. 6:[6, 7, 10] => 7
matches.ToList().ForEach(x => {
Console.WriteLine("ListA. {0}: [{1}] ListB. {2}: [{3}] => [{4}], ListX. {5}:[{6}] => {7}",
allLists.IndexOf(x.ListA),
string.Join(", ", x.ListA),
allLists.IndexOf(x.ListB),
string.Join(", ", x.ListB),
string.Join(", ", x.Shared),
allLists.IndexOf(x.ListX),
string.Join(", ", x.ListX),
string.Join(", ", x.SingleShared));

I will leave the exercise of further work such as which list matches which other one given your fairly generic requirement. So here, I find those that match 3 other values in a given array, processing all arrays - so there are duplicates here where an A matches a B and a B matches an A for example.
This should give you something you can work from:
using System;
using System.Collections.Generic;
using System.Linq;
public class Program
{
public static void Main()
{
Console.WriteLine("Hello World");
var s = new List<int>();
List<List<int>> allLists = new List<List<int>>();
allLists.Add(new List<int>()
{1, 2, 3, 4});
allLists.Add(new List<int>()
{1, 2});
allLists.Add(new List<int>()
{3, 4});
allLists.Add(new List<int>()
{3, 4, 5, 6, 7, 8, 9});
allLists.Add(new List<int>()
{4, 6, 8});
allLists.Add(new List<int>()
{5, 7, 9, 11});
allLists.Add(new List<int>()
{6, 7, 8});
/*
// To iterate over it.
foreach (List<int> subList in allLists)
{
foreach (int item in subList)
{
Console.WriteLine(item);
}
}
*/
var countMatch = 3;
/* iterate over our lists */
foreach (var sub in allLists)
{
/* not the sub list */
var ns = allLists.Where(g => g != sub);
Console.WriteLine("Check:{0}", ns.Count()); // 6 of the 7 lists so 6 to check against
//foreach (var glist in ns) - all of them, now refactor to filter them:
foreach (var glist in ns.Where(n=> n.Intersect(sub).Count() == countMatch))
{
var r = sub.Intersect(glist); // get all the matches of glist and sub
Console.WriteLine("Matches:{0} in {1}", r.Count(), glist.Count());
foreach (int item in r)
{
Console.WriteLine(item);
}
}
}
}
}
This will output this:
Hello World
Check:6
Check:6
Check:6
Check:6
Matches:3 in 3
4
6
8
Matches:3 in 4
5
7
9
Matches:3 in 3
6
7
8
Check:6
Matches:3 in 7
4
6
8
Check:6
Matches:3 in 7
5
7
9
Check:6
Matches:3 in 7
6
7
8

I think it would be better to break this functionality into several methods, then it will look easier to read.
var allLists = new List<List<int>>();
allLists.Add(new List<int>() {1, 2, 3, 4});
allLists.Add(new List<int>() {1, 2});
allLists.Add(new List<int>() {3, 4});
allLists.Add(new List<int>() {3, 4, 5, 6, 7, 8, 9});
allLists.Add(new List<int>() {4, 6, 8});
allLists.Add(new List<int>() {5, 7, 9, 11});
allLists.Add(new List<int>() {6, 7, 8});
var count = allLists.Count;
for (var i = 0; i < count - 1; i++)
{
var left = allLists[i];
if (left.Count > 2)
{
for (var j = i + 1; j < count; j++)
{
var right = allLists[j];
var sharedItems = left.Intersect(right).ToList();
if (sharedItems.Count == 3)
{
for (int k = 0; k < count; k++)
{
if (k == i || k == j)
continue;
var intersected = allLists[k].Intersect(sharedItems).ToList();
if (intersected.Count == 1)
{
Console.WriteLine($"Found index k:{k},i:{i},j:{j}, Intersected numbers:{string.Join(",",intersected)}");
}
}
}
}
}
}

I just thought if I try to find the item in List X first, I might need fewer loops in practice. Correct me if I am wrong.
public static void match(List<List<int>> allLists, int numberOfMainListsToExistIn, int numberOfCommonItems)
{
var possibilitiesToCheck = allLists.SelectMany(i => i).GroupBy(e => e).Where(e => (e.Count() == numberOfMainListsToExistIn + 1));
foreach (var pGroup in possibilitiesToCheck)
{
int p = pGroup.Key;
List<int> matchingListIndices = allLists.Select((l, i) => l.Contains(p) ? i : -1).Where(i => i > -1).ToList();
for (int i = 0; i < matchingListIndices.Count; i++)
{
int aIndex = matchingListIndices[i];
int bIndex = matchingListIndices[(i + 1) % matchingListIndices.Count];
int indexOfListXIndex = (i - 1 + matchingListIndices.Count) % matchingListIndices.Count;
int xIndex = matchingListIndices[indexOfListXIndex];
IEnumerable<int> shared = allLists[aIndex].Intersect(allLists[bIndex]).OrderBy(e => e);
IEnumerable<int> xSingle = shared.Intersect(allLists[xIndex]);
bool conditionsHold = false;
if (shared.Count() == numberOfCommonItems && xSingle.Count() == 1 && xSingle.Contains(p))
{
conditionsHold = true;
for (int j = 2; j < matchingListIndices.Count - 1; j++)
{
int cIndex = matchingListIndices[(i + j) % matchingListIndices.Count];
if (!Enumerable.SequenceEqual(shared, allLists[aIndex].Intersect(allLists[cIndex]).OrderBy(e => e)))
{
conditionsHold = false;
break;
}
}
if (conditionsHold)
{
List<int> theOtherListIndices = Enumerable.Range(0, allLists.Count - 1).Except(matchingListIndices).ToList();
if (theOtherListIndices.Any(x => shared.Intersect(allLists[x]).Count() > 0))
{
conditionsHold = false;
}
}
}
if (conditionsHold)
{
matchingListIndices.RemoveAt(indexOfListXIndex);
Console.Write("List A and B: {0}. ", String.Join(", ", matchingListIndices));
Console.Write("Common items: {0}. ", String.Join(", ", shared));
Console.Write("List X: {0}.", xIndex);
Console.WriteLine("Common item in list X: {0}. ", p);
}
}
}
}
For the above example, I will just call the method like this:
match(allLists, 2, 3);
This method will also work with the extended problem:
match(allLists, 3, 4);
... and even more if the problem is even further more extended to (4, 5) and so on...

How to Zip two Lists of different size to create a new list that is same as the size of the longest amongst the original lists?

I have two C# Lists of different sizes e.g.
List<int> list1 = new List<int>{1,2,3,4,5,6,7};
List<int> list2 = new List<int>{4,5,6,7,8,9};
I want to use the linq Zip method to combine these two into a list of tuples that is of the size list1. Here is the resulting list I am looking for
{(1,4), (2,5), (3,6), (4,7), (5,8), (6,9), (7,0)} //this is of type List<(int,int)
Since the last item of list1 does not has a counterpart in list2, I fill up my last item of the resulting list with a default value (in this case 0 as in my case it will never appear in any of the original lists).
Is there a way I can use the linq Zip method alone to achieve this?

You can use Concat to make them both the same size, and then zip it:
var zipped = list1.Concat(Enumerable.Repeat(0,Math.Max(list2.Count-list1.Count,0)))
.Zip(list2.Concat(Enumerable.Repeat(0,Math.Max(list1.Count-list2.Count,0))),
(a,b)=>(a,b));
Or create an extension method:
public static class ZipExtension{
public static IEnumerable<TResult> Zip<TFirst,TSecond,TResult>(
this IEnumerable<TFirst> first,
IEnumerable<TSecond> second,
Func<TFirst,TSecond,TResult> func,
TFirst padder1,
TSecond padder2)
{
var firstExp = first.Concat(
Enumerable.Repeat(
padder1,
Math.Max(second.Count()-first.Count(),0)
)
);
var secExp = second.Concat(
Enumerable.Repeat(
padder2,
Math.Max(first.Count()-second.Count(),0)
)
);
return firstExp.Zip(secExp, (a,b) => func(a,b));
}
}
So you can use like this:
//last 2 arguments are the padder values for list1 and list2
var zipped = list1.Zip(list2, (a,b) => (a,b), 0, 0);

There is a useful and popular MoreLinq library. Install it and use.
using MoreLinq;
var result = list1.ZipLongest(list2, (x, y) => (x, y));

Try this using Zip function-
static void Main(string[] args)
{
List<int> firstList = new List<int>() { 1, 2, 3, 4, 5, 6, 0, 34, 56, 23 };
List<int> secondList = new List<int>() { 4, 5, 6, 7, 8, 9, 1 };
int a = firstList.Count;
int b = secondList.Count;
for (int k = 0; k < (a - b); k++)
{
if(a>b)
secondList.Add(0);
else
firstList.Add(0);
}
var zipArray = firstList.Zip(secondList, (c, d) => c + " " + d);
foreach(var item in zipArray)
{
Console.WriteLine(item);
}
Console.Read();
}
Or you can try this using ZipLongest Function by installing MoreLinq nuget package-
static void Main(string[] args)
{
List<int> firstList = new List<int>() { 1, 2, 3, 4, 5, 6, 0, 34, 56, 23 };
List<int> secondList = new List<int>() { 4, 5, 6, 7, 8, 9, 1 };
var zipArray = firstList.ZipLongest(secondList, (c, d) => (c,d));
foreach (var item in zipArray)
{
Console.WriteLine(item);
}
Console.Read();
}

Try this code-
static void Main(string[] args)
{
List<int> firstList=new List<int>() { 1, 2, 3, 4, 5, 6,0,34,56,23};
List<int> secondList=new List<int>() { 4, 5, 6, 7, 8, 9,1};
int a = firstList.Count;
int b = secondList.Count;
if (a > b)
{
for(int k=0;k<(a-b);k++)
secondList.Add(0);
}
else
{
for (int k = 0; k < (b-a); k++)
firstList.Add(0);
}
for(int i=0;i<firstList.Count;i++)
{
for(int j=0;j<=secondList.Count;j++)
{
if(i==j)
Console.Write($"({Convert.ToInt32(firstList[i])},{ Convert.ToInt32(secondList[j])})" + "");
}
}
Console.Read();
}

How to iterate through all subcollections of size K, in ascending order of their sums.

Suppose I have a sorted array such as
int[] sarr = new int[] { 0, 1, 3, 5 };
and I want to iterate through all combinations of size K in ascending order of their sum. For example, if K=2 then the combinations in order are
{0, 1} (sum = 1)
{1, 0} (sum = 1)
{0, 3} (sum = 3)
{3, 0} (sum = 3)
{3, 1} (sum = 4)
{1, 3} (sum = 4)
{5, 0} (sum = 5)
.
.
.
I want to do this without first getting all combinations because I want to stop as soon as I've found one that satisfies a condition Func<int[],bool> cond.
Is there a known way of doing this?

I would use yield return to describe all combinations, arrangements or whatever subcollections you want to generate and then use FirstOrDefault on the result.
That way you will only generate all subcollections if there isn't one you are looking for or the one you are looking for is the very last.
On about getting them ascending by the sum of the elements, sort the initial collection and then pick k elements from start to end. You can even generate combinations and from these to generate all possible permutations and thus you will get all your arrangements.
A quick way to get all combinations:
class Program
{
static void Main(string[] args)
{
var initialArray = new[] { 0, 1, 3, 5 };
var subArrayLength = 2;
foreach (var subArray in GetSubArrays(initialArray, subArrayLength))
Console.WriteLine($"[{string.Join(", ", subArray)}]");
Console.WriteLine("Searching for array that contains both 1 and 5.");
var arrayFulfillingCriteria = GetSubArrays(initialArray, subArrayLength).FirstOrDefault(array => array.Contains(1) && array.Contains(5));
if (arrayFulfillingCriteria != null)
Console.WriteLine($"[{string.Join(", ", arrayFulfillingCriteria)}]");
else
Console.WriteLine("No array found.");
}
static IEnumerable<int[]> GetSubArrays(int[] initialArray, int subArrayLength)
{
var indexStack = new Stack<int>(Enumerable.Range(0, subArrayLength));
do
{
var subArray = indexStack.Select(i => initialArray[i]).Reverse().ToArray();
yield return subArray;
var index = indexStack.Pop();
while (indexStack.Count != 0 && indexStack.Count < subArrayLength && index == initialArray.Length - (subArrayLength - indexStack.Count))
index = indexStack.Pop();
while (indexStack.Count < subArrayLength && index < initialArray.Length - (subArrayLength - indexStack.Count))
{
index++;
indexStack.Push(index);
}
}
while (indexStack.Count != 0);
}
}
The only reason I can think of where you would need arrangements (seeing as you order by sum) is that the items within the sub array need to be in a particular order.

Does this work for you?
Func<IEnumerable<int>, IEnumerable<IEnumerable<int>>> getAllSubsets = null;
getAllSubsets = xs =>
(xs == null || !xs.Any())
? Enumerable.Empty<IEnumerable<int>>()
: xs.Skip(1).Any()
? getAllSubsets(xs.Skip(1))
.SelectMany(ys => new [] { ys, xs.Take(1).Concat(ys) })
: new [] { Enumerable.Empty<int>(), xs.Take(1) };
Now, given this:
Func<int[],bool> cond = xs => true;
int[] sarr = new int[] { 0, 1, 3, 5, };
var result =
getAllSubsets(sarr)
.Where(xs => xs.Count() == 2)
.Where(xs => cond(xs.ToArray()));
I get this as the result:
{0, 1}
{0, 3}
{1, 3}
{0, 5}
{1, 5}
{3, 5}

How to display how many times an array element appears

I am new to C# and hope I can get some help on this topic. I have an array with elements and I need to display how many times every item appears.
For instance, in [1, 2, 3, 4, 4, 4, 3], 1 appears one time, 4 appears three times, and so on.
I have done the following but don`t know how to put it in the foreach/if statement...
int[] List = new int[]{1,2,3,4,5,4,4,3};
foreach(int d in List)
{
if("here I want to check for the elements")
}
Thanks you, and sorry if this is a very basic one...

You can handle this via Enumerable.GroupBy. I recommend looking at the C# LINQ samples section on Count and GroupBy for guidance.
In your case, this can be:
int[] values = new []{1,2,3,4,5,4,4,3};
var groups = values.GroupBy(v => v);
foreach(var group in groups)
Console.WriteLine("Value {0} has {1} items", group.Key, group.Count());

You can keep a Dictionary of items found as well as their associated counts. In the example below, dict[d] refers to an element by its value. For example d = 4.
int[] List = new int[]{1,2,3,4,5,4,4,3};
var dict = new Dictionary<int, int>();
foreach(int d in List)
{
if (dict.ContainsKey(d))
dict[d]++;
else
dict.Add(d, 1);
}
When the foreach loop terminates you'll have one entry per unique value in dict. You can get the count of each item by accessing dict[d], where d is some integer value from your original list.

The LINQ answers are nice, but if you're trying to do it yourself:
int[] numberFound = new int[6];
int[] List = new int[] { 1, 2, 3, 4, 5, 4, 4, 3 };
foreach (int d in List)
{
numberFound[d]++;
}

var list = new int[] { 1, 2, 3, 4, 5, 4, 4, 3 };
var groups = list.GroupBy(i => i).Select(i => new { Number = i.Key, Count = i.Count() });

private static void CalculateNumberOfOccurenceSingleLoop()
{
int[] intergernumberArrays = { 1, 2, 3, 4, 1, 2, 4, 1, 2, 3, 5, 6, 1, 2, 1, 1, 2 };
Dictionary<int, int> NumberOccurence = new Dictionary<int, int>();
for (int i = 0; i < intergernumberArrays.Length; i++)
{
if (NumberOccurence.ContainsKey(intergernumberArrays[i]))
{
var KeyValue = NumberOccurence.Where(j => j.Key == intergernumberArrays[i]).FirstOrDefault().Value;
NumberOccurence[intergernumberArrays[i]] = KeyValue + 1;
}
else
{
NumberOccurence.Add(intergernumberArrays[i], 1);
}
}
foreach (KeyValuePair<int, int> item in NumberOccurence)
{
Console.WriteLine(item.Key + " " + item.Value);
}
Console.ReadLine();
}

Possible to group by Count in LINQ?

This might be either impossible or so obvious I keep passing over it.
I have a list of objects(let's say ints for this example):
List<int> list = new List<int>() { 1, 2, 3, 4, 5, 6 };
I'd like to be able to group by pairs with no regard to order or any other comparison, returning a new IGrouping object.
ie,
list.GroupBy(i => someLogicToProductPairs);
There's the very real possibility I may be approaching this problem from the wrong angle, however, the goal is to group a set of objects by a constant capacity. Any help is greatly appreciated.

Do you mean like this:
List<int> list = new List<int>() { 1, 2, 3, 4, 5, 6 };
IEnumerable<IGrouping<int,int>> groups =
list
.Select((n, i) => new { Group = i / 2, Value = n })
.GroupBy(g => g.Group, g => g.Value);
foreach (IGrouping<int, int> group in groups) {
Console.WriteLine(String.Join(", ", group.Select(n=>n.ToString()).ToArray()));
}
Output
1, 2
3, 4
5, 6

you can do something like this...
List<int> integers = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
var p = integers.Select((x, index) => new { Num = index / 2, Val = x })
.GroupBy(y => y.Num);

int counter = 0;
// this function returns the keys for our groups.
Func<int> keyGenerator =
() =>
{
int keyValue = counter / 2;
counter += 1;
return keyValue;
};
var groups = list.GroupBy(i => {return keyGenerator()});