binary_search.c

#include <stdlib.h>
#include <stdio.h>
#include <time.h>

#define NOT_FOUND (-1)
#define N (10)

int binary_search(int x, int *a, int left, int right)
{
    int mid;

    while ( left < right ) {
        mid = (left + right) / 2;
        if (a[mid] < x) 
            left  = mid + 1;
        else
            right = mid;
    }

    if ( a[left] ==x )
        return left;

    return NOT_FOUND;
}

int main(void)
{
    int i, found;
    int array[N];

    srand((unsigned int) time(NULL));
    array[0] = rand() % 3;
    for ( i = 1; i < N; i++ ) {
        array[i] = array[i - 1] + rand() % 3;
    }

    for (i = 0; i < N; i++ ) {
        printf("%d ", array[i]);
    }

    scanf("%d", &i);
    found = binary_search(i, array, 0, N);

    if ( found > -1 )
        printf("index: %d\n", found);

    return EXIT_SUCCESS;
}