为什么这个字符串二进制搜索程序不能搜索一半的数据？

Question

一开始我以为程序运行良好，但是当我测试搜索“马”时，搜索失败。而且我注意到该程序最多只能搜索一半的数据。有谁知道为什么？ 我已经添加了 qsort，但它仍然不会搜索下半部分数据。

#include <stdio.h>
#include<stdbool.h>
#include<string.h>
#include<stdlib.h>

int compare(const void *a, const void *b){

    const char *ia = (const char *)a;
    const char *ib = (const char *)b;

    return strcmp(ia, ib);
}

int main(int argc, char argv){

    char data[10][50]={"cow", "goat", "dog", "sheep", "chicken", "duck", "bird","fish", "bee", "horse"};
    int size1 = sizeof(data[0]);
    int n = sizeof(data) / size1;
    printf("%d\n\n", n);
    int i, j;
    bool check = false;
    char key[10];
    int low = 0;
    int high = n - 1;
    int mid;
    char temp;

    for(i = 0; i<n; i++){
        printf("%s\n", data[i]);
    }

    qsort(data, n, size1, compare);
    printf("=============\n\n");

    for(i = 0; i<n; i++){
        printf("%s\n", data[i]);
    }
    system("pause");
    printf("Search: "); scanf("%s", &key);
    fflush(stdin);

    while(low<=high){
        mid = (low + high) / 2;
        if(strcmp(key,data[mid])==0){
            printf("Data Found ! on index - %d\n", mid);
            break;
        } else if(strcmp(key,data[mid])<0){
            low = mid + 1;
        }else {
            high = mid - 1;
        }
    }

    if(low > high){
        printf("Data not found");
    }
    return 0;
}

Answer 1

使用qsort，您可以对字符串数组进行排序，这里的else if (strcmp(key, data[mid]) < 0) 应该是>，如下面的代码所示。否则您的搜索将无法进行。

static int myCompare(const void* a, const void* b)
{
    return strcmp(*(const char**)a, *(const char**)b);
}

int main()
{
    char* data [] = { 
        "cow", "goat", "dog", "sheep", "chicken", "duck", "bird","fish", "bee", "horse" 
    };
    int n = sizeof(data) / sizeof(data[0]);
    int i;

    qsort(data, n, sizeof(const char*), myCompare);
    char key[10];
    int low = 0;
    int high = n - 1;
    int mid;
    char temp;

    scanf("%s", key);
    fflush(stdin);

    while (low <= high) {
        mid = (low + high) / 2;
        int a = strcmp(key, data[mid]);
        if (a == 0) {
            printf("Data Found ! on index - %d\n", mid);
            break;
        }
        else if (a > 0) {
            low = mid + 1;
        }
        else { // or only else(no special need for else if
            high = mid - 1;
        }
    }

    if (low > high) {
        printf("Data not found");
    }
    return 0;
}

Answer 2

和上面的 cmets 一样，二分查找是针对已排序的数据。你的显然没有排序。因此，首先，您必须对数据进行排序。排序数据的算法有很多，但是下面的代码，我使用了快速排序，这也可以帮助你理解你的搜索算法。

#include <stdio.h> 
#include <stdlib.h> 
#include <string.h>

void swap(char ** a, char ** b) { 
    char * t = *a;
    *a = *b;
    *b = t;
}


/* This function takes last element as pivot, places 
   the pivot element at its correct position in sorted 
    array, and places all smaller (smaller than pivot) 
   to left of pivot and all greater elements to right 
   of pivot */
int partition (char ** arr, int low, int high) { 
    char * pivot = arr[high];    // pivot 
    int i = (low - 1);  // Index of smaller element 

    for (int j = low; j <= high- 1; j++) { 
        // If current element is smaller than the pivot 
        if (strcmp(arr[j],pivot) < 0) { 
            i++;    // increment index of smaller element 
            swap(&arr[i], &arr[j]); 
        } 
    } 
    swap(&arr[i + 1], &arr[high]); 
    return (i + 1); 
}

/* arr[] --> Array to be sorted, 
   low  --> Starting index, 
   high  --> Ending index */
void quickSort(char **arr, int low, int high) { 
    if (low < high) { 
        /* pi is partitioning index, arr[p] is now 
           at right place */
        int pi = partition(arr, low, high); 

        // Separately sort elements before 
        // partition and after partition 
        quickSort(arr, low, pi - 1); 
        quickSort(arr, pi + 1, high); 
    } 
} 

int main(){

   char *a = "abc";
   char *b = "cdf";
   swap(&a, &b);
   printf("a = %s, b= %s\n", a, b);
   char * data[10]={"cow", "goat", "dog", "sheep", "chicken", "duck", "bird","fish", "bee", "horse"};
   quickSort(data, 0, 9);
   for(int i = 0; i < 10; i++) {
    printf("%s, ", data[i]);
   }

   return 0;
}

排序后，您可以使用您的搜索算法。