4. 函数bisearch 的作用是应用折半查找法从存有N个整数的升序数组a中对关键字key进行查找。请将程序补充完整。 #include<stdio.h> #define N 15 int bisearch(int a[N], int key) { int low=0, high=N-1, mid; while(______(1)_________) { mid=(low+high)/2; if(key<a[mid]) ______(2)________; else if(key>a[mid]) low=mid+1; else return mid; } return _____(3)________ ; } void main() { int b[N],n,i,j; printf("Please input a %d element increasing sequence: ", N); for(i=0;i<N;i++) { printf("b[%d]= ",i); scanf("%d",&b[i]); } printf("Please input a searching key: "); scanf("%d",&n); j=bisearch(b,n); if(j==1) printf("Don’t find %d\n",n); else printf("b[%d]=%d\n",j,n); }

下面是完整的程序，包括了需要填空的部分：

#include <stdio.h>
#define N 15

int bisearch(int a[N], int key) {
    int low = 0, high = N - 1, mid;
    while (low <= high) {
        mid = (low + high) / 2;
        if (key < a[mid])
            high = mid - 1;
        else if (key > a[mid])
            low = mid + 1;
        else
            return mid;
    }
    return -1;
}

int main() {
    int b[N], n, i, j;
    printf("Please input a %d element increasing sequence: ", N);
    for (i = 0; i < N; i++) {
        printf("b[%d]= ", i);
        scanf("%d", &b[i]);
    }
    printf("Please input a searching key: ");
    scanf("%d", &n);
    j = bisearch(b, n);
    if (j == -1)
        printf("Don’t find %d\n", n);
    else
        printf("b[%d]=%d\n", j, n);
    return 0;
}

需要填空的部分如下：

1. `while (low <= high)`
2. `high = mid - 1;`
3. `-1;`

这个程序实现了一个折半查找（二分查找）算法，用于在一个有序数组中查找指定元素的位置。算法的核心思想是将数组分成两半，然后判断目标元素在左半边还是右半边，进而逐步缩小查找范围，直到找到目标元素或者确定不存在目标元素。