int binary_search(int arr[], int k, int left, int right)
{
    //int right = sizeof(arr) / sizeof(arr[0]) - 1;放在这里错误，arr是指针，长度为4个字节，right=0
    while (left <= right)
    {
        int mid = left + (right - left) / 2;//每次都要改变，所以要循环
        if (arr[mid] > k)
        {
            right = mid - 1;
        }
        else if (arr[mid] < k)
        {
            left = mid + 1;
        }
        else
        {
            return mid;
        }
    }
    return -1;
}

int main()
{
    int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };//初始化数组可以替换
    int k = 0;
    int left = 0;
    int right = sizeof(arr) / sizeof(arr[0]) - 1;//在哪定义在哪使用
    printf("输入需要找到数字\n");
    scanf("%d",&k);
    int ret = binary_search(arr, k, left, right);
    if (ret == -1)
    {
        printf("找不到\n");
    }
    else
    {
        printf("找到了,下标是:%d\n", ret);
    }
    //int mid = (left+right)/2;
    return 0;
}

标题名称：c语言二分法查找数组中一个数字的下标位置
标题来源：http://scyanting.com/article/piesos.html