合并排序程序不会对具有5个以上元素的数组进行排序

Question

I'm trying to implement the code for merge sort. I use the following code for merge sort, which you can have a look at here . The problem is when I run the program, for example, I asked it to sort the array [1, 2, 3, 4, 6, 5] then print out the sorted array, it return the array [-1560047667 1 2 3 4 5]. From what I observed, it will not print the value of the largest element in the array correctly. My code is below:

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

void merge_sort(int A[], int start, int end){
    My code for merge sort 
    goes here
}

int main(){
    int n, i, *A;
    A = (int *)malloc(n);
    scanf("%d", &n);
    for(i=0; i<n; i++) scanf("%d", A+i);
    merge_sort(A, 0, n);
    for(i=0; i<n; i++) printf("%d", *(A+i));
}

Any suggestion on what is the mistake I've made? I think the problem is at the input/output process, but not quite sure what exactly it is.

Answer 1

Your code is trying to develop psychic powers. You're using n before it gets assigned a value as you use scanf afterwards to find out what n should be. This will lead to undefined behaviour, such as your code only working if there are less than 5 elements.

int n, i, *A;
A = (int *)malloc(n);
scanf("%d", &n);

You're also not allocating enough memory as that will allocate n bytes where as an int will typically be 4 bytes (or more or less depending on your computer's architecture). The best way to ensure you allocate the right amount of memory is to multiple n by the sizeof(*A) - that is the size of whatever A is a pointer to. This ensures if you change A to be a different type, the allocation code will continue to be correct.

scanf("%d", &n);
A = malloc(n*sizeof(*A));