C 語言 qsort 函數完整使用指南與比較函數詳解

qsort 原型

void qsort(void *base, size_t nmemb, size_t size,
           int (*compar)(const void *, const void *));

• base：陣列起始位址
• nmemb：元素個數
• size：單個元素大小
• compar：比較函數指標，回傳負值/0/正值決定順序

結構體排序範例

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

typedef struct {
    unsigned long id;
    int score;
} Record;

int cmp_id(const void *a, const void *b) {
    Record *p1 = (Record *)a;
    Record *p2 = (Record *)b;
    return (p1->id > p2->id) - (p1->id < p2->id); // 避免溢位
}

int cmp_score(const void *a, const void *b) {
    Record *p1 = (Record *)a;
    Record *p2 = (Record *)b;
    return (p1->score > p2->score) - (p1->score < p2->score);
}

int main() {
    Record arr[] = {{3,99},{5,87},{4,56},{2,100},{1,17}};
    int n = sizeof(arr)/sizeof(arr[0]);

    qsort(arr, n, sizeof(Record), cmp_id);
    printf("按 ID 排序：\n");
    for (int i=0;i