合并排序算法问题：不同的输出

Question

I was doing merge sort with printing all the passes when the merging happens. my friend was doing the same. we got two different outputs. and we can't seem to find the problem. Please help!

The problem is I am getting only 5 passes with 6 members in the array and my friend is getting 13 passes for the same 6 digits, our coed is almost identical (according to us). We are calling the same things and have put the printing passes statement at the same place.

I am giving whole codes, and the outputs:

1st code:

#include <stdio.h>

int size;

int merge(int arr[], int l, int m, int r) {
    int n1 = m - l + 1;
    int n2 = r - m;
    int L[n1], R[n2];

    int i, j;
    for (i = 0; i < n1; i++)
        L[i] = arr[l + i];
    for (j = 0; j < n2; j++)
        R[j] = arr[m + 1 + j];

    i = 0, j = 0;
    int k = l;

    while (i < n1 && j < n2) {
        if (L[i] <= R[j]) {
            arr[k] = L[i];
            i++;
        } else {
            arr[k] = R[j];
            j++;
        }
        k++;
    }

    while (i < n1) {           
        arr[k] = L[i];
        i++, k++;
    }
    while (j < n2) {           
        arr[k] = R[j];
        j++, k++;
    }

    printf("\n    pass:  ");                  // PASSES
    for (int x = 0; x < size; x++) {        
        printf("%d  ", arr[x]);
    }
}

int mergesort(int arr[], int l, int r) {
    if (l < r) {
        int m = (l + r) / 2;
        mergesort(arr, l, m);
        mergesort(arr, m + 1, r);
        merge(arr, l, m, r);
    }
}

int main() {
    printf("\n    # MERGE SORT # \n\n");
    int i;
    int arr[20];

    printf("  Enter number of array elements: ");
    scanf("%d", &size);

    printf("  Enter array elements: ");
    for (i = 0; i < size; i++)
        scanf("%d", &arr[i]);

    mergesort(arr, 0, size - 1);

    printf("\n\n  Sorted array: ");
    for (i = 0; i < size; i++)
        printf("%d ", arr[i]);
    
    printf("\n\n\n");
}

Output: OP

2nd code:

#include <stdio.h>

int arr[50];
int n; 
int pass_count = 0;

void m_sort(int start, int end);
void merge(int start, int mid, int end);

int main() {
    int i;

    printf("\n\n______________MERGE SORT____________\n\n");

    printf("\n  HOW MANY ELEMENTS WOULD YOU LIKE TO ENTER? ");
    scanf("%d", &n);
    printf("  Give Array: ");
    for (i = 0; i < n; i++)
        scanf("%d", &arr[i]);

    printf("\nYOUR ARRAY: ");
    for (i = 0; i < n; i++) 
        printf("%d  ", arr[i]);
    
    m_sort(0, n - 1);
    printf("\n\n");
}

void m_sort(int start, int end) {
    if (start < end) {
        int mid = (start + end) / 2;
        m_sort(0, mid);
        m_sort(mid + 1, end);
        merge(start, mid, end);
    }
}

void merge(int start, int mid, int end) {
    int i, j, k;
    int n1, n2;
    n1 = mid - start + 1;
    n2 = end - mid;
    int left_arr[n1], right_arr[n2];

    for (i = 0; i < n1; i++)
        left_arr[i] = arr[start + i];
    for (j = 0; j < n2; j++)
        right_arr[j] = arr[mid + 1 + j];
    
    i = 0;
    j = 0;
    k = start;
    
    while (i < n1 && j < n2) {
        if (left_arr[i] <= right_arr[j]) {
            arr[k] = left_arr[i];
            i++;
        } else {
            arr[k] = right_arr[j];
            j++;
        }
        k++;
    }

    while (i < n1) { 
        arr[k] = left_arr[i];
        i++;
        k++;
    }
    while (j < n2) {
        arr[k] = right_arr[j];
        j++;
        k++;
    }
    
    pass_count = pass_count + 1;
    printf("\nPASS %d: ", pass_count);    // PASSES
    //displaying passes
    for (i = 0; i < n; i++) 
        printf("%d ", arr[i]);
    }

Output: OP

Thanks in advance.

Answer 1

The fist program is correct.

The second program has a bug in the m_sort() function: the recursive calls use incorrect index values. It should be:

void m_sort(int start, int end) {
    if (start < end) {
        int mid = (start + end) / 2;
        m_sort(start, mid);         // instead of m_sort(0, mid)
        m_sort(mid + 1, end);
        merge(start, mid, end);
    }
}

Note also that the first code uses local variables except size that is only used to display the phases. The second program uses too many global variables for no good reason.