.Net Tips – Use LINQ to avoid nested loops

Admit it, we’ve all done it before, writing those nasty nested loops just so we can iterate through multiple lists to get some combination/permutation of the lists, e.g.:

   1: for (int i = 0; i < 10; i++)

   2: {

   3:     for (int j = 0; j < 10; j++)

   4:     {

   5:         for (int k = 0; k < 10; k++)

   6:         {

   7:             // do something

   8:         }

   9:     }

  10: }

This code obviously works and well understood amongst developers, but it’s not very readable and trying to terminate the outer loops from the inner loops is a pain and requires you to use one or more boolean flags which you need to track on every iteration at potentially every level of your loop…

A better way to solve this common problem is to use LINQ to ‘flatten’ the nested loops into a single loop:

   1: var triplets = from I in Enumerable.Range(0, 10)

   2:                from J in Enumerable.Range(0, 10)

   3:                from K in Enumerable.Range(0, 10)

   4:                select new { I, J, K };

   5:

   6: foreach (var triplet in triplets)

   7: {

   8:     // do something

   9: }

Sweet, right? :-)

  • Anonymous

    How do you use these I J and K variables?

  • theburningmonk

    @Anonymous – if you mean inside the foreach loop, you will be able to reference them like triplet.I, triplet.J and triplet.K.

    In the LINQ query, you can reference the variables I, J and K directly, for example:

    from I in Enumerable.Range(0, 10)
    from J in Enumerable.Range(0, 10)
    from K in Enumerable.Range(0, 10)
    select new { Sum = I + J + K }