Merge Sort (Segmentation Fault(Xore Dumped)) C++

Question

Somewhere in my program there is loop or declaration problem kindly anyone bug the error in the following program.And say whether this algorithm be a good solution for sorting problem.

#include<iostream>
using namespace std;
void merge(int a[],int start,int mid,int end){
  int p=start,q=mid+1;
  int Arr[end-start+1],k=0;
  for(int i=start;i<=end;i++){
    if(p>mid)
    Arr[k++]=a[q++];
    else if(q>end)
    Arr[k++]=a[p++];
    else if(a[p]<a[q])
    Arr[k++]=a[p++];
    else
    Arr[k++]=a[q++];
    for(int i=0;i<k;i++)
    a[start++]=Arr[i];
  }

}
void merge_sort(int a[],int start,int end){
int mid;
  if(start<end){
   int mid= (start+end) /2;
}
  merge_sort(a,start,mid);
  merge_sort(a,mid+1,end);
  merge(a,start,mid,end);
}
int main(){
  int n;
  cout<<"Enter the n:"<<endl;
  cin>>n;
  int a[n];
  cout<<"Enter the values in the array:"<<endl;
  for(int i=0;i<n;i++){
    cin>>a[i];
  }
  int start=0,end=n-1;
  merge_sort(a,start,end);
  cout<<"After Sorting: "<<endl;
  for(int i=0;i<n;i++){
    cout<<a[i]<<" ";
  }
  return 0;
}

Welcome to Stack Overflow! It sounds like you may need to learn how to use a [debugger](https://en.wikipedia.org/wiki/Debugger) to step through your code. With a good debugger, you can execute your program line by line and see where it is deviating from what you expect. This is an essential tool if you are going to do any programming. Further reading: [How to debug small programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/). — Paul R, Nov 10 '17 at 16:11
[`using namespace std;` is a bad practice](https://stackoverflow.com/q/1452721/2176813), never use it. — tambre, Nov 10 '17 at 16:16
Welcome to StackOverflow. Please read and follow the posting guidelines in the help documentation. [Minimal, complete, verifiable example](http://stackoverflow.com/help/mcve) applies here. We cannot effectively help you until you post your MCVE code and accurately describe the problem. We should be able to paste your posted code into a text file and reproduce the problem you described. — Prune, Nov 10 '17 at 16:36

score 0 · Accepted Answer · answered Nov 10 '17 at 16:16

0

There is something very wrong with your merge_sort function:

void merge_sort(int a[], int start, int end)
{
  int mid;

  if (start < end)
      mid = (start+end) / 2; // Also, don't re-declare mid with int!

  merge_sort(a, start, mid);
  merge_sort(a, mid + 1, end);
  merge(a, start, mid, end);
}

You're unconditionally calling merge_sort inside of merge_sort. Even if your program didn't crash, it would never complete, and probably be killed by your system because of the mess it's causing.

I suggest you to get more comfortable recursive functions before trying to fix your code.

answered Nov 10 '17 at 16:16

Nepho

1,074
1
11
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Note to askers: This is called (incoming search keyword!) Variable Shadowing – user4581301 Nov 10 '17 at 16:31
And Thank U for the reply – Robin Reni Nov 10 '17 at 16:34
1

I can, but it's honestly such a mess that I won't dig into your code anymore. It's probably better to re-code it as a whole. – Nepho Nov 10 '17 at 16:35
Ok buddy thank u but it code it using a reference from hackerearth website – Robin Reni Nov 10 '17 at 16:37
Just move closing bracket after merge – MBo Nov 10 '17 at 17:09

score 0 · Answer 2 · answered Nov 10 '17 at 18:57

The mistake is the displacement of brackets.

#include<iostream>
using namespace std;
void merge(int a[],int start,int mid,int end){
  int p=start,q=mid+1;
  int Arr[end-start+1],k=0;
  for(int i=start;i<=end;i++){
    if(p>mid)
    Arr[k++]=a[q++];
    else if(q>end)
    Arr[k++]=a[p++];
    else if(a[p]<a[q])
    Arr[k++]=a[p++];
    else
    Arr[k++]=a[q++];
  }
    for(int i=0;i<k;i++)
  {
      a[start++]=Arr[i];
  }

}
void merge_sort(int a[],int start,int end){
  if(start<end){
    int mid= (start+end) /2;
  merge_sort(a,start,mid);
  merge_sort(a,mid+1,end);
  merge(a,start,mid,end);
}
}
int main(){
  int n;
  cout<<"Enter the n:"<<endl;
  cin>>n;
  int a[n];
  cout<<"Enter the values in the array:"<<endl;
  for(int i=0;i<n;i++){
    cin>>a[i];
  }
  int start=0,end=n-1;
  merge_sort(a,start,end);
  cout<<"After Sorting: "<<endl;
  for(int i=0;i<n;i++){
    cout<<a[i]<<" ";
  }
  return 0;
}

Merge Sort (Segmentation Fault(Xore Dumped)) C++

2 Answers2