Question

 void merge(int *a1, int *a2, int *a3, int s1, int s2, int s3){
     s2 -=1;
     int track =0;
     s3-=1;
     int p2=0;
     int p3=0;
     while(p2<s2 && p3 <s3){
         if(a2[p2]<a3[p3])
             a1[track++]=a2[p2++];
         else
             a1[track++]=a3[p3++];
     }
     while(p2<s2) a1[track++]=a2[p2++];
     while(p3<s3) a1[track++]=a3[p3++];
 }


 void msort(int *array, int n){
     if(n<=1) return;
     int a1_size = n/2;
     int a2_size = n-n/2;
     int a1[n/2];
     int a2[n-n/2];
     for(int i=0;i<n/2;i++){
    a1[i] = array[i];
     }
     for(int i=0,j=n/2;j<n;i++,j++){
         a2[i] = array[j];
     }
     msort(a1, n/2);
     msort(a2, n-n/2);
     merge(array, a1, a2, n, a1_size, a2_size);
 }

 int main(){
     int a[]={1,2,1,2,3,94,5,67};
     msort(a,8);
     for(int i=0;i<8;i++)
         cout << a[i] << endl;
}

Here is my code, and i expect it will return a sorted array.
I know the problem is about passing the address of array to merge will ammend data inside array and hence affect the result. I tried for a long time but still cant figure out how to ammend it. Would someone can give me a hint or help?

Answer 1

The problem is in s2 -= 1 and s3 -= 1 statements. Why are you doing that? A standard idiom for iterating array from zero-based index is to count while index < length, not while index < length-1.

A couple of other notes:

• you allocate a lot of additional memory (n*log(n) ints) which stays unused most of the time. It is only necessary to allocate inside merge, that way at most n ints will be allocated at any time.
• learn to use std::vector and/or iterator ranges instead of C-style arrays