Linux下归并排序(Merge Sort)下归并排序算法的C语言实现
Linux下归并排序(Merge Sort)下归并排序算法的C语言实现
在Linux下实现了一个归并排序的算法,分成多个文件,这里记录三点:归并排序的算法、makefile的使用、gdb调试心得
一、归并排序算法
算法的递推关系:一个大的数列需要排序,把它从中间分成两部分,每一部分归并排序,然后把排好序的这两个部分再合并起来(合并的时候要按顺序合并)。
算法的Base Case:如果分成的这部分只有一个数,那么这个部分就不用再排序(看做已经排好序的)。
实现这个算法用了三个函数,每个函数在一个文件中,分别为:merge.c sort.c 和 main.c,其中merge.c实现的是合并的方法,sort.c实现的是排序的方法,main.c是一个测试实例。还有三个头文件,分别指出了函数原型。
merge.c:
/*This is a merge program.
* Given an integer ARRAY and three numbers which indicate the begain
*and the end of two subarrays, merge the two subarrays to a bigger
*one. The two subarrays are alrealy sorted from small to big.
* For example, given an array a[10] and three numbers 0, 3 and 5. The
*first array is from a[0] to a[2], the seconde array is from a[3] to
*a[4]. The number 3 and 5 are the upper side. This program merge the
*two arrays together.
*
*/
#include <stdio.h>
#include <stdlib.h>
#include "main.h"
void merge(int *a, int idxa, int idxb, int idxc)
{
int i = idxa, j = idxb, k = 0;
int total = idxc-idxa;
//int temp[total] = {0};
int *temp = (int *)malloc(sizeof(int) * total);
if(temp == NULL)
{
fprintf(stderr, "malloc error in merge function\n");
return;
}
while(i < idxb && j < idxc)
{
if(a[i] < a[j])
temp[k++] = a[i++];
else
temp[k++] = a[j++];
}
if(i == idxb)
{
while(j < idxc)
temp[k++] = a[j++];
}
else if(j == idxc)
{
while(i < idxb)
temp[k++] = a[i++];
}
/*Copy the temp to the sorce array*/
for(i = 0, k = idxa; i < total; k++, i++)
a[k] = temp[i];
free(temp);
}
#ifndef MAIN
/*For test*/
int main()
{
int a[10];
int i = 0;
int idxa=1, idxb=5, idxc=8;
printf("Please input 10 numbers to the array:");
for(i = 0; i < 10; i++)
scanf("%d", &a[i]);
printf("Three indexes are %d, %d and %d.\nThe first subarray is:", idxa, idxb, idxc);
for(i = idxa; i < idxb; i++)
printf(" %d", a[i]);
printf("\nThe second subarray is:");
for(i = idxb; i < idxc; i++)
printf(" %d", a[i]);
printf("\n");
merge(a, idxa, idxb, idxc);
printf("The merged array is:");
for(i = idxa; i < idxc; i++)
printf(" %d", a[i]);
printf("\n");
return 0;
}
#endif
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