用c语言写一个代码，要求是：用折半查找法在（05 13 19 21 37 56 64 75 80 88 92)表中査找元素21。

好的，以下是代码：

#include <stdio.h>

int binary_search(int arr[], int len, int target)
{
    int low = 0;
    int high = len - 1;
    while (low <= high)
    {
        int mid = (low + high) / 2;
        if (arr[mid] == target)
        {
            return mid;
        }
        else if (arr[mid] < target)
        {
            low = mid + 1;
        }
        else
        {
            high = mid - 1;
        }
    }
    return -1;
}

int main()
{
    int arr[] = {05, 13, 19, 21, 37, 56, 64, 75, 80, 88, 92};
    int len = sizeof(arr) / sizeof(arr[0]);
    int target = 21;
    int index = binary_search(arr, len, target);
    if (index >= 0)
    {
        printf("%d is found at index %d.\n", target, index);
    }
    else
    {
        printf("%d is not found.\n", target);
    }
    return 0;
}

运行结果为：

21 is found at index 3.