Question

code like this,in page 44.

int binsearch(int x, int v[], int n)
{
    int low, high, mid;

    low = 0;
    high = n - 1;
    while(low <= high){
        mid = (high+low)/2;
        if(x < v[mid]){
            high = mid + 1;
        }else if(x > v[mid]){
            low = mid + 1;
        }else{
            return mid;
        }
        printf("mid is %d\n",mid);
    }

    return -1;
}

int main(void)
{
    int v[] = {2,3,4,7,8,23,54,65,76};
    int ret = binsearch(7, v, sizeof(v)/sizeof(int));

    printf("%d,ret is %d\n", sizeof(v),ret);
    return 0;
}

compile and run it,dead loop as a result! so the line “low = mid + 1;” should take as “low = mid;”, is that right?thx.

Answer 1

The problem in the code you posted is not the value of low, but in the test for the setting of the value of high … if x is less than v[mid], then the new index value of high should be one less than, not greater than mid. So you want to change the following:

if(x < v[mid]){
    high = mid + 1;

to:

if(x < v[mid]){
    high = mid - 1;

You can see a working example here: http://ideone.com/E6Ma8