merge_sort 不起作用，输出只是一堆 1 -1 和 0

Question

So I got the task to write merge_sort in a recursive way, and it just returns either an array of 0,-1, 1 of the same length as the original input. Any ideas where I did something wrong? input_merge_sort.h and input_merge_sort.c are given by the task and handle the input and output, so all I have to focus on is the algorithm itself. Some details about the algorithm, to make sure I understood it correctly:

MergeSort sorts lists by splitting them into equally sized lists, splitting them until they're single elements, to then 2 single-element lists together, comparing them and putting the smaller one in front. With the sub-lists you write into the original list by reading from 2 sublists, comparing the value and putting pointer 1 element further, to then compare it with the old element of the other sublist, which was bigger than the other.

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include "input_merge_sort.h"
/*
    array:  Pointer at the start of the array
    first:  Index of the first element
    len  :  Index of the last element
*/

void merge(int a[], int i1, int j1, int j2) {
    int temp[j2 - i1];    //array used for merging
    int i, j, k;
    i = i1;    //beginning of the first list
    int i2 = j1 + 1;
    j = i2;    //beginning of the second list
    k = 0;

    while (i <= j1 && j <= j2) {    //while elements in both lists
        if (a[i] < a[j])
            temp[k++] = a[i++];
        else
            temp[k++] = a[j++];
    }

    while (i <= j1)    //copy remaining elements of the first list
        temp[k++] = a[i++];

    while (j <= j2)    //copy remaining elements of the second list
        temp[k++] = a[j++];

    //Transfer elements from temp[] back to a[]
    for (i = i1, j = 0; i <= j2; i++, j++)
        a[i] = temp[j];
}

void merge_sort(int *array, int first, int last) {
    int middle;
    if (first < last) {
        middle = ((first + last) / 2);
        merge_sort(array, first, middle);
        merge_sort(array, middle + 1, last);
        merge(array, first, middle, last);
    }
}

/*
Reads integers from files and outputs them into the stdout after mergesorting them.

How to run: ./introprog_merge_sort_rekursiv <max_amount>  <filepath>
*/
int main(int argc, char *argv[]) {
    if (argc != 3) {
        printf("usage: %s <max_amount>  <filepath>\n", argv[0]);
        exit(2);
    }

    char *filename = argv[2];

    // Initialize array
    int *array = (int*)malloc(atoi(argv[1]) * sizeof(int));        //MINE
    int len = read_array_from_file(array, atoi(argv[1]), filename);

    printf("Input:\n");
    print_array(array, len);

    // Call of "merge_sort()"
    merge_sort(array, array[0], array[len - 1]); //MINE

    printf("Sorted:\n");
    print_array(array, len);
    free(array);
    return 0;
}

Answer 1

The function merge_sort takes an array and the indices of its first and last elements as arguments, but you pass the elements themselves. Change:

merge_sort(array, array[0],array[len-1]);

to:

merge_sort(array, 0, len - 1);

In merge you crate a temporary array on the stack, but it is one element short. It should be:

int temp[j2 - i1 + 1];

I recommend that you change the functions so that they don't take the last element as upper bound but the first element outside the range, as is usual in C arrays and loops. In my opinion, that makes the code simpler. The two halves of the array are then [low, mid) and [mid, high) . The length of the whole array is high - low .