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c++sortingmultidimensional-arraystl2d
Anmol Sood
Anmol Sood

Reputation: 73

How to sort a 2d array according to the second column using stl sort function?

How to sort a 2d array according to the second column using stl sort function ?

For eg

If we have an array a[5][2] and we want to to sort according to the ar[i][1] entry , how do we do it using the stl sort function. I understand we have to use a boolean function to pass as the third parameter but I am not able to design the appropriate boolean function ?

Upvotes: 1

Views: 8486

Answers (2)

deerishi
deerishi

Reputation: 647

The stl sort requires the rvalue of the iterator being passed as the arguments. If you wanna use the sort function, you will have to compile in c++11 and use the array stl to store the array. The code is as follows

#include "bits/stdc++.h"
using namespace std;
bool compare( array<int,2> a, array<int,2> b)
{
    return a[0]<b[0];
}
int main()
{
    int i,j;
    array<array<int,2>, 5> ar1;
    for(i=0;i<5;i++)
    {
        for(j=0;j<2;j++)
        {
            cin>>ar1[i][j];
        }
    }
    cout<<"\n earlier it is \n";
    for(i=0;i<5;i++)
    {
        for(j=0;j<2;j++)
        {
            cout<<ar1[i][j]<<" ";
        }
        cout<<"\n";
    }
    sort(ar1.begin(),ar1.end(),compare);
    cout<<"\n after sorting \n";
    for(i=0;i<5;i++)
    {
        for(j=0;j<2;j++)
        {
            cout<<ar1[i][j]<<" ";
        }
        cout<<"\n";
    }
    return 0;
}

Compiling in c++11 can be done by g++ -std=c++11 filename.cpp -o out. In case you do not want to use c++11 or use the "array" stl, use the std::qsort function. With this you can use the traditional way to define the arrays like int a[10][2]. The code is as follows

#include "bits/stdc++.h"
using namespace std;
int compare( const void *aa, const void  *bb)
{
    int *a=(int *)aa;
    int *b=(int *)bb;
    if (a[0]<b[0])
     return -1;
    else if (a[0]==b[0]) 
    return 0;
    else  
     return 1;

}
int main() 
{
    int a[5][2];
    cout<<"enter\n";
    for(int i=0;i<5;i++)
    {
        for(int j=0;j<2;j++)
        {
            cin>>a[i][j];
        }
        //cout<<"\n";
    }
    cout<<"\n\n";
    qsort(a,5,sizeof(a[0]),compare);
    for(int i=0;i<5;i++)
    {
        for(int j=0;j<2;j++)
        {
            cout<<a[i][j]<<" ";
        }
        cout<<"\n";
    }
    return 0;
   }

Upvotes: 3

Michael
Michael

Reputation: 2250

Make your own compare function.

See the Beginners guide to std::sort(). http://www.cplusplus.com/articles/NhA0RXSz/

Upvotes: 0

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