Question

There are lots of recursion questions and I basically understand some simple recursion algorithm such as sum of array elements. However, my friend gave me this code which reverses an array:

void r(int a[], int s)
{
     if(s <=2 ) return;
     int t = a[0];
     a[0] = a[s-1];
     a[s-1] = t;

     r(&a[1], s-2); //  this line confused me, why &a[1]
}

I know how to reverse an array using a normal for loop. But this code really confused me about recursion.

Can anyone explain the above line of code?

Answer 1

It is equvalent to

void r(int *arr, size_t len)
{
     for ( ; len >= 2; arr+=1,len-=2 ) {
       int t = arr[0];
       arr[0] = arr[len-1];
       arr[len-1] = t;
       }

}

, where the recursive call is replaced by the loop. Ihe “increment” part of the loop (arr+=1,len-=2) is exactly the same as the parameters for the recursive call; the end condition (len >= 2) is equivalent to the recursion stopper (which was wrong in the original).