I have a "Contact" object which contains a list called "Linked" containing the "Contact" linked to it.
public class Contact
{
public int Id { get; }
public string FirstName { get; set; }
public string LastName { get; set; }
public int Age { get; set; }
public List<Contact> Linked { get; set; }
}
For example, Contact "A" has 3 linked contacts: B, C and D.
As the links are made in both directions each time, B, C and D all have A in their "Linked" contacts.
B can then have E as a contact, and so on. There is no limit.
I have to make an algo which takes a starting contact and an ending contact as a parameter and which finds the shortest path that links them.
The result must be in the form: A > B > F > H > X, if I have to find the path that goes from A to X. We must therefore find all the steps of the path in the result.
I've tried a lot of stuff (recursion, ...) but it's still getting stuck somewhere.
Do you have an idea?
Dijkstra's algorithm is probably what you are looking for.
http://en.wikipedia.org/wiki/Dijkstra%27s_algorithm
Dijkstra's algorithm (/ˈdaɪkstrəz/ DYKE-strəz) is an algorithm for finding the shortest paths between nodes in a graph, which may represent, for example, road networks. It was conceived by computer scientist Edsger W. Dijkstra in 1956 and published three years later.[4][5][6]
It should be relatively straight forward to find examples in your given language. Here is one in C#, stolen from programmingalgorithms.com
private static int MinimumDistance(int[] distance, bool[] shortestPathTreeSet, int verticesCount)
{
int min = int.MaxValue;
int minIndex = 0;
for (int v = 0; v < verticesCount; ++v)
{
if (shortestPathTreeSet[v] == false && distance[v] <= min)
{
min = distance[v];
minIndex = v;
}
}
return minIndex;
}
private static void Print(int[] distance, int verticesCount)
{
Console.WriteLine("Vertex Distance from source");
for (int i = 0; i < verticesCount; ++i)
Console.WriteLine("{0}\t {1}", i, distance[i]);
}
public static void Dijkstra(int[,] graph, int source, int verticesCount)
{
int[] distance = new int[verticesCount];
bool[] shortestPathTreeSet = new bool[verticesCount];
for (int i = 0; i < verticesCount; ++i)
{
distance[i] = int.MaxValue;
shortestPathTreeSet[i] = false;
}
distance[source] = 0;
for (int count = 0; count < verticesCount - 1; ++count)
{
int u = MinimumDistance(distance, shortestPathTreeSet, verticesCount);
shortestPathTreeSet[u] = true;
for (int v = 0; v < verticesCount; ++v)
if (!shortestPathTreeSet[v] && Convert.ToBoolean(graph[u, v]) && distance[u] != int.MaxValue && distance[u] + graph[u, v] < distance[v])
distance[v] = distance[u] + graph[u, v];
}
Print(distance, verticesCount);
}
It would then be used like so:
int[,] graph = {
{ 0, 4, 0, 0, 0, 0, 0, 8, 0 },
{ 4, 0, 8, 0, 0, 0, 0, 11, 0 },
{ 0, 8, 0, 7, 0, 4, 0, 0, 2 },
{ 0, 0, 7, 0, 9, 14, 0, 0, 0 },
{ 0, 0, 0, 9, 0, 10, 0, 0, 0 },
{ 0, 0, 4, 0, 10, 0, 2, 0, 0 },
{ 0, 0, 0, 14, 0, 2, 0, 1, 6 },
{ 8, 11, 0, 0, 0, 0, 1, 0, 7 },
{ 0, 0, 2, 0, 0, 0, 6, 7, 0 }
};
Dijkstra(graph, 0, 9);
The example above consist of 9 nodes, and the distance to each of the other nodes. If it has 0, there is no connection. In your case, there is no weight - so either there is a connection (1), or there isn't (0).
You have to change the algorithm to take in a list of contacts, instead of a two dimensional array. Try to think of the two dimensional array as a list of lists - very similar to a list of contacts, where each contact has another list of contacts.
Lets for example create a simple contacts list and their contacts:
Peter can contact Mary
Mary can contact Peter and John
John can contact Mary
This would be represented something like this in a two dimensional array:
int[,] contacts = new int[]
{
{ 0, 1, 0 }, //Peter: Peter no, Mary yes, John no
{ 1, 0, 1 }, //Mary: Peter yes, Mary no, John yes
{ 0, 1, 0 } //John: Peter no, Mary yes, John no
}
You would also have to modify the algorithm to keep track of the current path. That should be a relatively straight forward change.
Hope that it helps!
Related
I'm trying to use the Span2D type to "roll" entries in a 2d array, but it's not working as expected.
By rolling I mean the following - given an array such as:
{
{ 1, 1, 1, 1, 1 },
{ 2, 2, 2, 2, 2 },
{ 0, 0, 0, 0, 0 },
}
I would like to copy the first two rows down one row, so the top row can be repopulated. After the roll operation the array should look like this:
{
{ 1, 1, 1, 1, 1 },
{ 1, 1, 1, 1, 1 },
{ 2, 2, 2, 2, 2 },
}
The Span2d CopyTo method seems perfect for this - I create a Slice of the top two rows, and a slice of the bottom two rows, copy the first slice to the second slice. But instead of the expected result above, I get:
{
{ 1, 1, 1, 1, 1 },
{ 1, 1, 1, 1, 1 },
{ 1, 1, 1, 1, 1 },
}
Here's a runnable class that shows the problem:
public class SpanTest
{
public static void Main()
{
int[,] array =
{
{ 1, 2, 3, 4, 5 },
{ 0, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0 },
};
var h = array.GetLength(0) - 1;
var w = array.GetLength(1);
Console.WriteLine($"slice height:{h} width: {w}\n-----------");
Span2D<int> span = array;
Console.WriteLine($"{span.ToStringMatrix()}-----------");
var sourceSlice = span.Slice(0, 0, h, w);
Console.WriteLine($"{sourceSlice.ToStringMatrix()}-----------");
var targetSlice = span.Slice(1, 0, h, w);
Console.WriteLine($"{targetSlice.ToStringMatrix()}-----------");
sourceSlice.CopyTo(targetSlice);
Console.WriteLine($"{span.ToStringMatrix()}-----------");
}
}
with a helper for printing the Span2Ds:
public static class Utils
{
public static string ToStringMatrix<T>(this Span2D<T> arr)
{
var sb = new StringBuilder();
for (var i = 0; i < arr.Height; i++)
{
for (var j = 0; j < arr.Width; j++)
{
sb.Append($"{arr[i, j]} ");
}
sb.Append(Environment.NewLine);
}
return sb.ToString();
}
}
How can I make the copy operation behave as expected? Thanks
Well, the answer is quite obvious actually - at least it was when it occurred to me at 5am this morning!
Span2D wraps an array, copying to itself alters the backing array during the copy process. By the time the second row is copied, it already contains the contents of the first row. And hence, the first row gets propagated throughout the 2d array.
Let's say I have a 2d array.
int[,] rooms = new int[3,5];
How would I find the middle most position?
var middle = rooms[rooms.GetLength(0) / 2, rooms.GetLength(1) / 2]
But that's only going to work where they're odd. Define further behaviour required...
In the example, this would return 1:
int[,] rooms = new int[3,5] { { 0, 0, 0, 0, 0 }, { 0, 0, 1, 0, 0 }, { 0, 0, 0, 0, 0 } };
int len0 = rooms.GetLength(0);
int len1 = rooms.GetLength(1);
return rooms[len0 / 2, len1 / 2];
Use rooms.GetLength(0) / 2 and rooms.GetLength(1) / 2 to get the first and second index of the middle position.
This was answered here.
To find the row length
int rowLength = arr.length;
To find the column length
int columnLength = arr[0].length;
To find the middle element
int mid = arr[arr.length/2][arr[0].length/2];
I have an application that has a guid variable which needs to be unique (of course). I know that statistically any guid should just be assumed to be unique, but due to dev/test environment reasons, the same value may be seen multiple times. So when that happens, I want to "increment" the value of the Guid, rather than just creating a whole new one. There does not seem to be a simple way to do this. I found a hack, which I will post as a possible answer, but want a cleaner solution.
You can get the byte components of the guid, so you can just work on that:
static class GuidExtensions
{
private static readonly int[] _guidByteOrder =
new[] { 15, 14, 13, 12, 11, 10, 9, 8, 6, 7, 4, 5, 0, 1, 2, 3 };
public static Guid Increment(this Guid guid)
{
var bytes = guid.ToByteArray();
bool carry = true;
for (int i = 0; i < _guidByteOrder.Length && carry; i++)
{
int index = _guidByteOrder[i];
byte oldValue = bytes[index]++;
carry = oldValue > bytes[index];
}
return new Guid(bytes);
}
}
EDIT: now with correct byte order
Thanks to Thomas Levesque's byte order, here's a nifty LINQ implementation:
static int[] byteOrder = { 15, 14, 13, 12, 11, 10, 9, 8, 6, 7, 4, 5, 0, 1, 2, 3 };
static Guid NextGuid(Guid guid)
{
var bytes = guid.ToByteArray();
var canIncrement = byteOrder.Any(i => ++bytes[i] != 0);
return new Guid(canIncrement ? bytes : new byte[16]);
}
Note it wraps around to Guid.Empty if you manage to increment it that far.
It would be more efficient if you were to keep incrementing a single copy of bytes rather than calling ToByteArray on each GUID in turn.
Possible solution -- I think this works (not really tested), but want a better solution.
public static Guid Increment(this Guid value)
{
var bytes = value.ToByteArray();
// Note that the order of bytes in the returned byte array is different from the string representation of a Guid value.
// Guid: 00112233-4455-6677-8899-aabbccddeeff
// byte array: 33 22 11 00 55 44 77 66 88 99 AA BB CC DD EE FF
// So the byte order of the following indexes indicates the true low-to-high sequence
if (++bytes[15] == 0) if (++bytes[14] == 0) if (++bytes[13] == 0) if (++bytes[12] == 0) if (++bytes[11] == 0) if (++bytes[10] == 0) // normal order
if (++bytes[9] == 0) if (++bytes[8] == 0) // normal order
if (++bytes[6] == 0) if (++bytes[7] == 0) // reverse order
if (++bytes[5] == 0) if (++bytes[4] == 0) // reverse order
if (++bytes[3] == 0) if (++bytes[2] == 0) if (++bytes[1] == 0) { ++bytes[0]; } // reverse order
return new Guid(bytes);
}
Edit: here is the code I ended up using; props to the answers above for the general technique, although without the "unchecked" clause they both would throw exceptions in some cases. But I also tried to make the below as readable as possible.
private static int[] _guidByteOrder = { 15, 14, 13, 12, 11, 10, 9, 8, 6, 7, 4, 5, 0, 1, 2, 3 };
public static Guid NextGuid(this Guid guid)
{
var bytes = guid.ToByteArray();
for (int i = 0; i < 16; i++)
{
var iByte = _guidByteOrder[i];
unchecked { bytes[iByte] += 1; }
if (bytes[iByte] != 0)
return new Guid(bytes);
}
return Guid.Empty;
}
Verified Solution for Ordered Strings:
private static Guid Increment(Guid guid)
{
byte[] bytes = guid.ToByteArray();
byte[] order = { 15, 14, 13, 12, 11, 10, 9, 8, 6, 7, 4, 5, 0, 1, 2, 3 };
for (int i = 0; i < 16; i++)
{
if (bytes[order[i]] == byte.MaxValue)
{
bytes[order[i]] = 0;
}
else
{
bytes[order[i]]++;
return new Guid(bytes);
}
}
throw new OverflowException("Congratulations you are one in a billion billion billion billion etc...");
}
Verification:
private static Guid IncrementProof(Guid guid, int start, int end)
{
byte[] bytes = guid.ToByteArray();
byte[] order = { 15, 14, 13, 12, 11, 10, 9, 8, 6, 7, 4, 5, 0, 1, 2, 3 };
for (int i = start; i < end; i++)
{
if (bytes[order[i]] == byte.MaxValue)
{
bytes[order[i]] = 0;
}
else
{
bytes[order[i]]++;
return new Guid(bytes);
}
}
throw new OverflowException("Congratulations you are one in a billion billion billion billion etc...");
}
static void Main(string[] args)
{
Guid temp = new Guid();
for (int j = 0; j < 16; j++)
{
for (int i = 0; i < 255; i++)
{
Console.WriteLine(temp.ToString());
temp = IncrementProof(temp, j, j + 1);
}
}
}
I am trying to find the path of a maze, below is the code, it suppose to goes inside the recursiveSolve loop but it keep exiting, after second if condition what i am doing wrong here can someone help me please?
im setting Washere and correctpath array as false by default.
recursiveSolve(0, 0);
public static int[,] maze = {{0, 0, 0, 0, 0, 1},
{1, 1, 0, 0, 0, 1},
{0, 0, 0, 1, 0, 0},
{0, 1, 1, 0, 0, 1},
{0, 1, 0, 0, 1, 0},
{0, 1, 0, 0, 0, 1}};
public static Boolean recursiveSolve(int row, int col) {
Boolean[,] wasHere = new Boolean[6, 6];
Boolean[,] correctPath = new Boolean[6, 6]; // The solution to the maze
if (maze[row, col] == 1 || wasHere[row, col]) {
return false;
}
else if (row == 0 || row == 6 - 1 || col == 0 || col ==6 - 1) {
correctPath[row, col] = true;
return true;
}
else {
wasHere[row, col] = true;
if (recursiveSolve(row - 1, col) || recursiveSolve(row + 1, col) ||
recursiveSolve(row, col - 1) ||
recursiveSolve(row, col +1)) {
correctPath[row, col] = true;
return true; // successfully escaped; this square is on path
}
else {
return false;
}
}
}
Your wasHere and correctPath arrays are local to the recursiveSolve function, which means each time you enter in this function, the arrays will be inited to false (or random value).
First of all try to make these arrays static as well, and see if that solves your problem with the always false.
Also, you should start your search from somewhere inside the maze instead of from an edge (0,0 means you already exiting the maze).
If you want to start from 0,0 ,mark that as a starting point, and don't allow it as a valid solution.
If you're actually doing path finding and this is not an excercise which requires this particular solution, then you might also want to look into the A* algorithm which is probably more efficient and robust.
Wikipedia
I have 2 byte arrays:
Dim A() As Byte = {1, 2, 3, 4, 5, 6, 7, 8, 9}
Dim B() As Byte = {5, 6, 7}
Now I want to find the occurance of the full B in A. I tried Array.IndexOf(A, B) with no luck. Is there a simple way to search an array by array without the need to use any loops?
It should find the index (position) of 5,6,7 in the same order as in B().
If A() contains {1,2,3,4,7,6,5,9} it should return false or -1 because they are not in the same order.
The following Linq statement will give an IEnumerable<int> containing the positions of b in a (or an empty set if none occur):
Enumerable
.Range( 0, 1 + a.Length - b.Length )
.Where( i => a.Skip(i).Take(b.Length).SequenceEqual(b) );
I have no idea how to translate to VB.NET.
This might work, but it's C# and uses a loop:
private static int[] GetIndicesOf(byte[] needle, byte[] haystack)
{
int[] foundIndices = new int[needle.Length];
int found = 0;
for (int i = 0; i < haystack.Length; i++)
{
if (needle[found] == haystack[i])
{
foundIndices[found++] = i;
if (found == needle.Length)
return foundIndices;
}
else
{
i -= found; // Re-evaluate from the start of the found sentence + 1
found = 0; // Gap found, reset, maybe later in the haystack another occurrance of needle[0] is found
continue;
}
}
return null;
}
Tested with input:
Byte[] haystack = { 5, 6, 7, 8, 9, 0, 5, 6, 7 };
Byte[] needle = { 5, 6, 7 };
// Returns {0, 1, 2}
Byte[] haystack = { 5, 6, 0, 8, 9, 0, 5, 6, 7 };
Byte[] needle = { 5, 6, 7 };
// Returns {6, 7, 8}
Byte[] haystack = { 5, 6, 0, 7, 9, 0, 5, 6, 8 };
Byte[] needle = { 5, 6, 7 };
// Returns null
Byte[] haystack = { 1, 2, 1, 2, 2 };
Byte[] needle = { 1, 2, 2 };
// Returns {2, 3, 4}
Byte[] haystack = { 1, 2, 1, 2, 1, 2, 3 };
Byte[] needle = { 1, 2, 1, 2, 3 };
// Returns {2, 3, 4, 5, 6}
Byte[] haystack = { 1, 1, 1, 1, 2 };
Byte[] needle = { 1, 2 };
// Returns {3, 4}
But the Linq implementation of #spender looks nicer. :-P
How about creating a method that:
Concatinates the elements of the searched list to one string
Concatinates the elements of the list to search for to one string
Looks in the first string for the precense of the second string
Like so:
public bool IsSubSetOf(IList<int> list1, IList<int> list2){
var string1 = string.Join("", list1);
var string2 = string.Join("", list2);
return string1.Contains(string2);
}
Not tested...
An efficient way of solving this problem in general is the KMP algorithm. Quick googling suggest that a .NET implementation may be found here. It's implementational pseudocode is availible from Wikipedia.
An inefficient, but harmlessly easy to code way is presented in one of the links above as follows:
int[] T = new[]{1, 2, 3, 4, 5};
int[] P = new[]{3, 4};
for (int i = 0; i != T.Length; i++)
{
int j = 0
for (;i+j != T.Length && j != P.Length && T[i+j]==P[j]; j++);
if (j == P.Length) return i;
}
My take would be:
public static int Search<T>(T[] space, T[] searched) {
foreach (var e in Array.FindAll(space, e => e.Equals(searched[0]))) {
var idx = Array.IndexOf(space, e);
if (space.ArraySkip(idx).Take(searched.Length).SequenceEqual(searched))
return idx;
}
return -1;
}
public static class Linqy {
public static IEnumerable<T> ArraySkip<T>(this T[] array, int index) {
for (int i = index; i < array.Length; i++) {
yield return array[i];
}
}
}
As always, it depends on your data whether this is "good enough" or you will have to resort to more complex yet efficient algorithms. I introduced an arrayskip as the Linq skip does indeed only assume the IEnumerable interface and would enumerate up to the index.