17 外排序-编程知识

17 外排序

排序分为内排序和外排序，内排序是在内存中的排序。外排序指在磁盘中文件的排序，因为在磁盘中，不能进行下标访问，归并排序经常用于磁盘中文件的排序

假如有10亿个整形数据在磁盘中，要对它排序，内存中只有1G空间，10亿需要4G。可以将这个文件划分成好几块1G大小的排序，再对这些1G文件两两归并，得到4G的排序后文件

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方便演示外排序，手动生成100的数据量，命名为data.txt文件每10个数据读到内存中一排序，可以用快速排序，排完序写到新文件，sub+编号.txt，不断循环对这些小文件归并汇总并生成新文件
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#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <string.h>
#include <stdlib.h>void Swap(int* a, int* b)
{int temp = *a;*a = *b;*b = temp;
}//三数取中
int GetIndex(int ary[], int left, int right)
{//int mid = (left + right) / 2;int mid = left + rand() % (right - left);if (ary[mid] > ary[left]){if (ary[mid] < ary[right]){return mid;}else if (ary[left] > ary[right]){return left;}else{return right;}}//ary[mid] < ary[left]else{if (ary[mid] > ary[right]){return mid;}else if (ary[left] > ary[right]){return right;}else{return left;}}
}int QSplit(int ary[], int left, int right)
{//三数取中int mid = GetIndex(ary, left, right);Swap(&ary[left], &ary[mid]);int keyi = left;while (left < right){//右边找小while (left < right && ary[right] >= ary[keyi]){right--;}//左边找大while (left < right && ary[left] <= ary[keyi]){left++;}Swap(&ary[left], &ary[right]);}Swap(&ary[left], &ary[keyi]);return left;
}//升序排列 区间[left,right]
void Qsort(int ary[], int left, int right)
{//区间错误,返回if (left >= right){return;}int keyi = QSplit(ary, left, right);//递归左右区间,keyi处除外Qsort(ary, left, keyi - 1);Qsort(ary, keyi + 1, right);}void MergeFile(const char* file1, const char* file2, const char* mfile)
{FILE* fout1;fout1 = fopen(file1, "r");FILE* fout2;fout2 = fopen(file2, "r");FILE* fin;fin = fopen(mfile, "w");int num1 = 0;int num2 = 0;int ret1 = fscanf(fout1, "%d", &num1);int ret2 = fscanf(fout2, "%d", &num2);while (ret1 != EOF && ret2 != EOF){if (num1 <= num2){fprintf(fin, "%d\n", num1);ret1 = fscanf(fout1, "%d", &num1);}else{fprintf(fin, "%d\n", num2);ret2 = fscanf(fout2, "%d", &num2);}}while (ret1 != EOF){fprintf(fin, "%d\n", num1);ret1 = fscanf(fout1, "%d", &num1);}while (ret2 != EOF){fprintf(fin, "%d\n", num2);ret2 = fscanf(fout2, "%d", &num2);}fclose(fout1);fclose(fout2);fclose(fin);
}int main()
{FILE* fdata;fdata = fopen("data.txt", "r");//分割一段段数据,内存排序写到小文件int ary[10];int n = 10; //每10个排序int num = 0; //临时读取内容char subfile[20]; //小文件名字int filei = 1;  //小文件编号int i = 0;while (fscanf(fdata, "%d", &num) != EOF){if (i < n){ary[i++] = num;}//排序if (i == 10){Qsort(ary, 0, n - 1);sprintf(subfile, "sub%d.txt", filei++);FILE* fsub;fsub = fopen(subfile, "w");for (int i = 0; i < n; i++){fprintf(fsub, "%d\n", ary[i]);}fclose(fsub);i = 0;}}//互相归并到文件,实现整体有序char file1[100] = "sub1.txt";char file2[100] = "sub2.txt";char mfile[100] = "merge.txt";for (int i = 2; i <= filei - 1; i++){//读取file1和file2,归并出mfileMergeFile(file1, file2, mfile);remove(file1);remove(file2);strcpy(file1, mfile);sprintf(file2, "sub%d.txt", i + 1);sprintf(mfile, "merge%d.txt",i + 1);}printf("hello world\r\n");return 0;
}