对有序数据表(5，7，9，12，15，18，20，22，25，30，100)，编写程序按折半查找方法查找12和28。C语言代码实现

下面是用 C 语言实现折半查找算法的代码：

#include <stdio.h>

int binary_search(int arr[], int left, int right, int target) {
    if (left > right) {
        return -1;  // 没有找到
    }

    int mid = left + (right - left) / 2;
    if (arr[mid] == target) {
        return mid;  // 找到了，返回下标
    } else if (arr[mid] > target) {
        return binary_search(arr, left, mid - 1, target);  // 在左侧查找
    } else {
        return binary_search(arr, mid + 1, right, target);  // 在右侧查找
    }
}

int main() {
    int arr[] = {5, 7, 9, 12, 15, 18, 20, 22, 25, 30, 100};
    int n = sizeof(arr) / sizeof(arr[0]);
    int target1 = 12;
    int target2 = 28;

    int idx1 = binary_search(arr, 0, n - 1, target1);
    if (idx1 == -1) {
        printf("%d not found in the array.\n", target1);
    } else {
        printf("%d is found at index %d.\n", target1, idx1);
    }

    int idx2 = binary_search(arr, 0, n - 1, target2);
    if (idx2 == -1) {
        printf("%d not found in the array.\n", target2);
    } else {
        printf("%d is found at index %d.\n", target2, idx2);
    }

    return 0;
}

输出结果：

12 is found at index 3.
28 not found in the array.