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Recursion instead of multiple nested loops?

I have some problems trying to update a nested for loop to use recursion instead. Is it possible to access variables a, b and c from earlier loops using recursion? Below is a simple example of how I am trying to convert to a recursive call.

for(int a= 0; a < 10; a++) { for(int b = 0; b < 20; b++) { for(int c = 0; c < 10; c++) { int[] indexes = new int[3]{a,b,c} collection.add(indexes); } } }

EDIT: The solution must be configured at runtime, so the user can choose how many levels are required.

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5 answers

Ok try with this

 static void AddToCollectionRecursive( List<int[]> collection, params int[] counts) { AddTo(collection, new List<int>(), counts, counts.Length - 1); } static void AddTo( List<int[]> collection, IEnumerable<int> value, IEnumerable<int> counts, int left) { for (var i = 0; i < counts.First(); i++) { var list = value.ToList(); list.Add(i); if (left == 0) { collection.Add(list.ToArray()); } else { AddTo(collection, list, counts.Skip(1), left - 1); } } }

Usage is similar to AddToCollectionRecursive(collection, 10, 20, 10); .

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Here's a recursive solution (using functional programming ):

 public static IEnumerable<IEnumerable<int>> GetCombinations(IEnumerable<int> limits) { if (limits.Any() == false) { // Base case. yield return Enumerable.Empty<int>(); } else { int first = limits.First(); IEnumerable<int> remaining = limits.Skip(1); IEnumerable<IEnumerable<int>> tails = GetCombinations(remaining); for (int i = 0; i < first; ++i) foreach (IEnumerable<int> tail in tails) yield return Yield(i).Concat(tail); } } // Per http://stackoverflow.com/q/1577822 public static IEnumerable<T> Yield<T>(T item) { yield return item; }

Using an example:

 var sequences = GetCombinations(new [] { 5, 3, 2, 4 /* ... */ }); foreach (var sequence in sequences) Console.WriteLine(string.Join(", ", sequence)); /* Output: 0, 0, 0, 0 0, 0, 0, 1 0, 0, 0, 2 0, 0, 0, 3 0, 0, 1, 0 0, 0, 1, 1 0, 0, 1, 2 0, 0, 1, 3 0, 1, 0, 0 0, 1, 0, 1 0, 1, 0, 2 ... */

For a specific OP scenario (adding arrays to collection ):

 var sequences = GetCombinations(new [] { 10, 20, 10 }); collection.AddRange(sequences.Select(s => s.ToArray()));
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something like this will work:

 public void CreateIndexes(int a, int b, int c, Collection collection) { if(c == 10) {b++; c = 0;} if(b == 20) {a++; b = 0;} if(a == 10) return; int[] indexes = new int[3]{a,b,c} collection.add(indexes); c++; CreateIndexes(a, b, c, collection); }
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From the top of my head, that is, it is not checked, something like this may work:

  List<int[]> collection = new List<int[]>(); private void AddValues(int a, int b, int c) { collection.Add(new[] { a, b, c }); if (c < 10) { c++; AddValues(a, b, c); } if (b < 20) { b++; c = 0; AddValues(a, b, c); } if (a < 10) { a++; b = 0; c = 0; AddValues(a, b, c); } }

Run it by calling:

 AddValues(0, 0, 0);
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Well, I think that if you solve this problem with recursion, it will consume more memory and resources!

But there is my suggestion:

 private void FunctionName(int a, int b, int c, List<int[]> list) { if (a<10) { if (b<20) { if (c<10) { list.Add(new[] { a, b, c }); c++; FunctionName(a,b,c,list); } else { c=0; b++; FunctionName(a,b,c,list); } } else { b=0; a++; FunctionName(a,b,c,list); } } }

You call this: FunctionName (0,0,0, list).

Hope it works! ^^

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