C语言中的快速排序算法实现 [英] quicksort algorithm implementaion in c

问题描述

这是c中quicksort算法的实现，对于给定的输入，我的程序以4 5 -343534 1的形式输出，我是编程新手，如果您能指出我的错误，那将非常有帮助

This is the implementation of quicksort algorithm in c,here for the given input my program is giving output as 4 5 -343534 1,i am new to programming ,Please it would be really helpful if you can point out my error

    #include<stdio.h>
    #include<stdlib.h>
    int partition(int* A, int start, int end) {
        int pivot = A[end];
        int pindex = start;
        for (int i = 0; i < end; i++) {
            if (A[i] <= pivot) {
                int temp = A[i];
                A[i] = A[pindex];
                A[pindex] = temp;
                pindex = pindex + 1;
            }
        }
        int temp = A[end];
        A[end] = A[pindex];
        A[pindex] = temp;
        return pindex;
    }
    void quicksort(int* A, int start, int end) {
        if (start < end) {
            int pindex = partition(A, start, end);
            quicksort(A, start, pindex - 1);
            quicksort(A, pindex+1, end);
        }
    }
    int main() {
        int* a = (int*)malloc(sizeof(int) * 4);
        a[0] = 1;
        a[1] = 5;
        a[2] = 4;
        a[3] = 2;
        quicksort(a, 0, 3);
        printf("%d %d %d %d", a[0], a[1], a[2], a[3]);
        system("pause");
    }

推荐答案

您的问题来自partition函数:

i变量应以不为0的start开头:

The i variable should start a start not at 0:

int partition(int* A, int start, int end) 
{
    int pivot = A[end];
    int pindex = start;
    for (int i = start; i < end; i++) {
        if (A[i] <= pivot) {
            int temp = A[i];
            A[i] = A[pindex];
            A[pindex] = temp;
            pindex = pindex + 1;
        }
    }
    int temp = A[end];
    A[end] = A[pindex];
    A[pindex] = temp;
    return pindex;
}

可以通过以下方式改进代码:定义swap函数以避免重复:

Your code can be improved in a way: define a swap function to avoid repeating yourself:

#include<stdio.h>
#include<stdlib.h>
void print_array(int *A, int size)
{
    for (int i = 0; i < size; ++i)
        printf("%d ", A[i]);

    puts("");
}
void swap(int *A, int i, int j)
{
    if (i == j)
        return ;

    int temp = A[i];
    A[i] = A[j];
    A[j] = temp;
}

int partition(int* A, int start, int end) {
    int pivot = A[end];
    int pindex = start;
    for (int i = start; i < end; i++) {
        if (A[i] <= pivot) {
            swap(A, i, pindex);
            pindex = pindex + 1;
        }
    }
    swap(A, end, pindex);

    return pindex;
}
void quicksort(int* A, int start, int end) {
    if (start < end) {
        int pindex = partition(A, start, end);
        quicksort(A, start, pindex - 1);
        quicksort(A, pindex+1, end);
    }

}
int main() {
    int i, size = 32;
    int* a = malloc(sizeof * a * size);
    for (i = 0; i < size; ++i)
        a[i] = rand()%100;

    print_array(a, size);

    quicksort(a, 0, size-1);

    print_array(a, size);

    return 0;
}

赠予:

83 86 77 15 93 35 86 92 49 21 62 27 90 59 63 26 40 26 72 36 11 68 67 29 82 30 62 23 67 35 29 2 
2 11 15 21 23 26 26 27 29 29 30 35 35 36 40 49 59 62 62 63 67 67 68 72 77 82 83 86 86 90 92 93     

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