Merge Sort on Array of Int C++

Question

I'm trying to create a program merge-sort on an array of int butI keep having troubles running this merge sort, it gives me a segment fault but I couldn't find anything wrong with it. In void mergesort when I put first <= last then the segment fault appears if not, then 5 5 5 5 is being print.

#include <iostream>

 using namespace std;


void merge(int *arr, int size, int first, int middle, int last)
{
    int temp[size];
    for(int i = first; i<=last; i++)
    {
       temp[i] = arr[i];
    }
    int i=first, j=middle+1, k=0;
    while(i<=middle && j<=last)
    {
       if(temp[i] <= temp[j])
       {
          arr[k] = temp[i];
          i++;
       }
       else
       {
          arr[k]=temp[i];
          j++;
       }
       k++;
    }
    while(i<=middle)
    {
       arr[k]=temp[i];
       k++;
       i++;
    }
}

void mergesort(int *arr, int size, int first, int last)
{
    if(first<last)
    {
       int middle = ( first + last )/2;
       mergesort(arr,size,first,middle);
       mergesort(arr,size,middle+1,last);
       merge(arr,size,first,middle,last);
    }
}
int main()
{
    cout <<"Him";
    const int size = 10;
    int numbers [] = {5,10,1,6,2,9,3,8,7,4};
    mergesort(numbers,size,0,9);
    for( int i= 0; i<size; ++i)
    {
        cout << numbers[i] << " ";
    }
    return 0;
}

Answer 1

There are (at least) two bugs. This:

else
{
   arr[k]=temp[i];                                          
   j++;
}

should be this:

else
{
   arr[k]=temp[j];                                          
   j++;
}

and this:

int i=first, j=middle+1, k=0;

should be this:

int i=first, j=middle+1, k=first;

In general, you ought to learn to step through the code, at least by putting diagnostic output statements here and there. Once you have the hang of that you can move up to a good debugger.

Answer 2

The standard library already implements a function that merges correctly: std::inplace_merge . Implementation adapted from this more general post

void mergesort(int * first, int * last)
{
    std::ptrdiff_t N = std::distance(first, last);
    if (N <= 1) return;                   
    int * middle = std::next(first, N / 2);
    mergesort(first, middle); 
    mergesort(middle, last);  
    std::inplace_merge(first, middle, last); 
}

int main()
{
    cout <<"Him";
    const int size = 10;
    int numbers [] = {5,10,1,6,2,9,3,8,7,4};
    mergesort(numbers, numbers+size);
    for( int i= 0; i<size; ++i)
    {
        cout << numbers[i] << " ";
    }
    return 0;
}

Answer 3

Suggestion 1:

Instead of that line:

int temp[size];

If you need a dynamic size array use:

int temp = new int[size];

Then once you are done with it

delete[] temp;

Edit: As Neil suggested using std::vector is may be more useful than arrays in such situations (if you are allowed to use it).

Answer 4

Your code has 3 bugs , Also you can reduce your code length too if required.

void merge(int *arr, int size, int first, int middle, int last)
{
    int temp[size];
    for(int i = first; i<=last; i++)
      temp[i] = arr[i];
    int i=first, j=middle+1, k=first; // 1st Change, Set k to first instead of 0
    while(i<=middle && j<=last)
    {
       if(temp[i] <= temp[j])
          arr[k++] = temp[i++];
       else
          arr[k++]=temp[j++]; // 2nd Change, use j instead of i
    }
    while(i<=middle)
       arr[k++]=temp[i++];
    while(j<=last)    // 3rd Change you missed this case
        arr[k++]=temp[j++];
}

Live Code