CODEWITHSUNDEEP
c++sortingvectorimplementationmergesort
Serillan
Serillan

Reputation: 193

MergeSort vectors using vector iterators

I need to implement MergeSort to sort vector of integer using vector iterators. I implemented it but I'm getting this error: Vector iterator not dereferencable.

As parameters for mergesort function I use (A.begin(),A.end()) where A is my vector which contain n elements.

#include <iostream>
#include <vector>

using namespace std;

void swap(vector<int>::iterator first,vector<int>::iterator last)
{
    int temp;
    temp=*first;
    *first=*last;
    *last=temp;
    return;
}


void mergesort(vector<int>::iterator first,vector<int>::iterator last)
{
    if(first==last) return; 
    if(first+1==last)  
    {
        if(*last>*first) swap(first,last);
        return;
    }

    vector<int>::iterator middle;
    middle=first+(last-first)/2;

    mergesort(first,middle);
    mergesort(middle+1,last);

    int a,b,pa,pb;
    vector<int> h;
        //the number of elements in a
    pa=middle-first+1; 
        //the number of elements in b
    pb=last-middle;    
         h.resize(pa+pb);
    a=0; b=0;

    while(a<pa && b<pb)
        if(*(first+a)<*(middle+1+b)) 
          {
              h[a+b]=*(first+a); a++;
          }
        else
          {
              h[a+b]=*(middle+1+b); b++;
          }

    while(a<pa) 
       {
           h[a+b]=*(first+a); a++;
       }
    while(b<pb)
       {
           h[a+b]=*(middle+1+b); b++;
       }

    for(int i=0;i<((a+b)-1);i++)
    *(first+i)=h[i];

    return;
}




int main(){

    vector<int>A;

    for(int i=1000;i>0;i--)
    {
        A.push_back(i);               
           //vector of integer: 1000,999,998 ... 3,2,1
    }

    mergesort(A.begin(),A.end());     
 //sort vector elements from smallest to biggest: 1,2,3...998,999,1000


    system("pause");
    return 0;
}

Upvotes: 0

Views: 1994

Answers (2)

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275896

Iterator ranges should be expressed as an iterator referring to the first element, and an iterator referring to one-past-the-end. If they are equal, this is an empty range. You aren't doing that.

To fix this, the if(first+1==last) clause should just return. The second mergesort(middle+1,last) call should be mergesort(middle,last). pa should just equal middle-last. pb is calculated correctly. And I think the -1 in the last for loop should be removed.

Having a range defined by iterator to first and iterator to last is a bad idea.

Upvotes: 2

alestanis
alestanis

Reputation: 21863

You should do

mergesort(A.begin(),A.end()-1);

Actually, the end() iterator points an element after the end of the vector. It doesn't point to the last element of the vector.

Upvotes: 1

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