用C++写一个外部排序的代码

外部排序是一种常见的排序方法，它能够处理无法一次性装入内存的大规模数据。下面是一个简单的外部排序的C代码示例：

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

#define MAX_LINE_LENGTH 256

void merge_sort(char **input_files, int num_files, char *output_file) {
    FILE **input_fp = malloc(num_files * sizeof(FILE *));
    FILE *output_fp = fopen(output_file, "w");
    char **lines = malloc(num_files * sizeof(char *));
    int *indices = malloc(num_files * sizeof(int));
    int i;

    // Open input files
    for (i = 0; i < num_files; i++) {
        input_fp[i] = fopen(input_files[i], "r");
        if (!input_fp[i]) {
            perror("Failed to open input file");
            exit(1);
        }

        // Allocate memory for lines
        lines[i] = malloc(MAX_LINE_LENGTH);
        if (!lines[i]) {
            perror("Failed to allocate memory for line");
            exit(1);
        }

        // Read first line
        if (fgets(lines[i], MAX_LINE_LENGTH, input_fp[i])) {
            indices[i] = i;
        } else {
            indices[i] = -1;
        }
    }

    while (1) {
        // Find smallest line
        int min_index = -1;
        for (i = 0; i < num_files; i++) {
            if (indices[i] == -1) {
                continue;
            }
            if (min_index == -1 || strcmp(lines[i], lines[min_index]) < 0) {
                min_index = i;
            }
        }

        if (min_index == -1) {
            break;
        }

        // Write smallest line to output file
        fputs(lines[min_index], output_fp);

        // Read next line from file
        if (fgets(lines[min_index], MAX_LINE_LENGTH, input_fp[min_index])) {
            indices[min_index] = min_index;
        } else {
            indices[min_index] = -1;
        }
    }

    // Close files and free memory
    for (i = 0; i < num_files; i++) {
        fclose(input_fp[i]);
        free(lines[i]);
    }
    free(input_fp);
    free(lines);
    free(indices);

    fclose(output_fp);
}

void external_sort(char *input_file, char *output_file, int max_lines) {
    FILE *input_fp = fopen(input_file, "r");
    char **lines = malloc(max_lines * sizeof(char *));
    int i, j;

    if (!input_fp) {
        perror("Failed to open input file");
        exit(1);
    }

    // Allocate memory for lines
    for (i = 0; i < max_lines; i++) {
        lines[i] = malloc(MAX_LINE_LENGTH);
        if (!lines[i]) {
            perror("Failed to allocate memory for line");
            exit(1);
        }
    }

    // Split input file into smaller files
    i = 0;
    while (!feof(input_fp)) {
        // Read lines
        for (j = 0; j < max_lines && !feof(input_fp); j++) {
            fgets(lines[j], MAX_LINE_LENGTH, input_fp);
        }

        // Sort lines and write to file
        qsort(lines, j, sizeof(char *), strcmp);
        char filename[MAX_LINE_LENGTH];
        sprintf(filename, "temp_%d", i++);
        FILE *temp_fp = fopen(filename, "w");
        if (!temp_fp) {
            perror("Failed to create temporary file");
            exit(1);
        }
        for (int k = 0; k < j; k++) {
            fputs(lines[k], temp_fp);
        }
        fclose(temp_fp);
    }

    // Merge smaller files
    char **input_files = malloc(i * sizeof(char *));
    for (j = 0; j < i; j++) {
        input_files[j] = malloc(MAX_LINE_LENGTH);
        sprintf(input_files[j], "temp_%d", j);
    }
    merge_sort(input_files, i, output_file);

    // Delete temporary files
    for (j = 0; j < i; j++) {
        remove(input_files[j]);
        free(input_files[j]);
    }
    free(input_files);
    for (i = 0; i < max_lines; i++) {
        free(lines[i]);
    }
    free(lines);

    fclose(input_fp);
}

int main(int argc, char *argv[]) {
    if (argc != 4) {
        fprintf(stderr, "Usage: %s input_file output_file max_lines\n", argv[0]);
        exit(1);
    }

    char *input_file = argv[1];
    char *output_file = argv[2];
    int max_lines = atoi(argv[3]);

    external_sort(input_file, output_file, max_lines);

    return 0;
}

这段代码分为两个部分：`external_sort`函数和`merge_sort`函数。`external_sort`函数将输入文件分割成多个小文件，对每个小文件进行内部排序，然后将它们合并为一个输出文件，这里使用了`merge_sort`函数。`merge_sort`函数负责将多个输入文件合并为一个输出文件，基本思路是将每个文件的第一行读入内存，然后找出最小行并写入输出文件，接着将该文件的下一行读入内存，直到所有文件的行都已经处理完毕。

注意，这段代码仅供参考，实际应用中还需要考虑更多的细节问题，例如内存分配、文件打开与关闭、错误处理等等。