Hi, this question is not specifically D related but I'll just ask anyway. Consider the following snippet:

void mergeSort(int[] arr, int l, int r)
{
   if (l < r)                       // 1
   {
      int m = l+(r-l)/2;            // 2
      mergeSort(arr, l, m);         // 3
      mergeSort(arr, m+1, r);       // 4
      merge(arr, l, m, r);          // 5
   }                                // 6
}                                   // 7

mergeSort(arr, 0, 4);

When I see this, I visualize the recursion to perform this way:

mergeSort(arr, 0, 4):
    0 < 4 ? true: mergeSort(0, 2):
        0 < 2 ? true: mergeSort(0, 1):
            0 < 1 ? true: mergeSort(0, 0):
                0 < 0 ? false: //reach the end of mergeSort / reach line 6 and then 7

I don't see the computer ever reaching line 4 and 5? Obviously I'm wrong but where is my mistake?

Thanks.

On 06/25/2018 07:45 PM, zbr wrote:

> void mergeSort(int[] arr, int l, int r)
> {
>     if (l < r)                       // 1
>     {
>        int m = l+(r-l)/2;            // 2
>        mergeSort(arr, l, m);         // 3
>        mergeSort(arr, m+1, r);       // 4
>        merge(arr, l, m, r);          // 5
>     }                                // 6
> }                                   // 7
> 
> mergeSort(arr, 0, 4);
> 
> When I see this, I visualize the recursion to perform this way:
> 
> mergeSort(arr, 0, 4):
>      0 < 4 ? true: mergeSort(0, 2):
>          0 < 2 ? true: mergeSort(0, 1):
>              0 < 1 ? true: mergeSort(0, 0):
>                  0 < 0 ? false: //reach the end of mergeSort / reach line 6 and then 7
> 
> I don't see the computer ever reaching line 4 and 5? Obviously I'm wrong but where is my mistake?

You seem to think that a recursive call takes over completely, and that the caller ceases to exist. That's not so. mergeSort does call "itself", but that means there's two active calls now. And when it calls "itself" again, there's three. And so on. When an inner call returns, the outer one resumes with the next line as usual.

It's not just a list of recursive calls, it's a tree:

mergeSort(0, 3)
    mergeSort(0, 1) // line 3
        mergeSort(0, 0) // line 3
        mergeSort(1, 1) // line 4
        merge // line 5
    mergeSort(2, 3) // line 4
        mergesort(2, 2) // line 3
        mergesort(3, 3) // line 4
        merge // line 5
    merge // line 5

On Monday, 25 June 2018 at 17:45:01 UTC, zbr wrote:
> Hi, this question is not specifically D related but I'll just ask anyway. Consider the following snippet:
>
> [...]

Your mistake is in your visualization :-)

But... more like:

0 < 4 ? true : mergeSort(0,2) && mergeSort(3, 4)
And so on.

I.e, the it's not either or to run the second mergeSort, they both happen.

On Monday, 25 June 2018 at 17:45:01 UTC, zbr wrote:
> Hi, this question is not specifically D related but I'll just ask anyway. Consider the following snippet:
>
> void mergeSort(int[] arr, int l, int r)
> {
>    if (l < r)                       // 1
>    {
>       int m = l+(r-l)/2;            // 2
>       mergeSort(arr, l, m);         // 3
>       mergeSort(arr, m+1, r);       // 4
>       merge(arr, l, m, r);          // 5
>    }                                // 6
> }                                   // 7
>
> mergeSort(arr, 0, 4);
>
> When I see this, I visualize the recursion to perform this way:
>
> mergeSort(arr, 0, 4):
>     0 < 4 ? true: mergeSort(0, 2):
>         0 < 2 ? true: mergeSort(0, 1):
>             0 < 1 ? true: mergeSort(0, 0):
>                 0 < 0 ? false: //reach the end of mergeSort / reach line 6 and then 7
>
> I don't see the computer ever reaching line 4 and 5? Obviously I'm wrong but where is my mistake?
>
> Thanks.

It's a stack (https://en.wikipedia.org/wiki/Call_stack).

When the program calls a function it is pushed onto the stack. If that function returns it pops from the stack and the previous function gets to continue execution from where it stopped before.