Question

I write a simple binary search program and meet a problem, see my three confusions below:

• If I choose icc -g main.c or icc -O0 main.c the code return wrong answer, but if I use icc -O2 main.c I can have the correct answer.

• When the code mid = (beg + end) / 2; be defined before the if(beg <= end) { the answer is right no matter what flag I use to compile it.

• I think the problem is the local var mid in the stack during the recursion.

Help me, thank you!

#include <stdio.h>
#define MAX 6
int a[MAX] = {1,2,3,4,5,6};
int improved_binary_search(int key, int beg, int end) {
    int mid, temp;
    //mid = (beg + end) / 2;
    if(beg <= end) {
             mid = (beg + end ) /2;
             if(a[mid] == key)
                     return mid;
             if(a[mid] < key)
                     return improved_binary_search(key, mid + 1, end);
              if(a[mid] > key)
                      return improved_binary_search(key, beg, mid - 1);
              //printf("test is %d\n", mid);
      }
      if(mid == 0)
              return 0;
      if(mid == MAX - 1)
              return MAX - 1;
      if(a[mid] < key)
              return a[mid+1] - key < key - a[mid] ? mid+1 : mid;
      if(a[mid] > key)
              return a[mid] - key < key - a[mid-1] ? mid : mid-1;
      return -1;
  }

int main(void)
{
     printf("%d\n", improved_binary_search(100, 0, MAX - 1));
     return 0;
}

Answer 1

You just declared mid but did not initialize it, it might happen that if the first if condition(if(beg <= end))is not hit then the value of mid is Indeterminate.

That would result in Undefined Behavior. The behavior you see is because of this UB and not because of compiler configurations.

You should always initialize your variables to meaningful values.