C++ sort 2d vector on column using class fails (prob w/ compare function)

Question

My comparison function for STL sort is failing. I was wanting to pass in a column variable to sort on a column. I am still new to programming so Boost just buries me in more things I don't understand. This doesn't have any error checking etc. eg

int columnToSort = 1;

sort(vec2d.begin(), vec2d.end(), 
[columnToSort](const vector< int >& a, const vector< int >& b){ return a[columnToSort] < b[columnToSort]; } );

Error = no matching function for call to 'sort(std::vector >::iterator, std::vector >::iterator, main()::&,...etc..

If I try to make it a class that takes a 2dvector and an integer columnToSort

I get the same error:

The following Does work:

bool compareFunction0(const vector<int>& a, const vector<int>& b )
{
    return( a[0] < b[0] );
}

void vec2dSort( vector< vector<int> > &refVec2d, int sortCol )
{
    //check to see if sortCol is out of bounds
   sort(refVec2d.begin(), refVec2d.end(), compareFunction0 );
}

Full Source is below. Problems areas are /////////////////////////////// MARKED LIKE THIS ////////////////////////////

#include <iostream>     // std::cout
#include <algorithm>    // std::sort
#include <vector>       // std::vector
#include <cstdlib>      // rand()
#include <ctime>        // rand()

using namespace std;

void printVec2d( vector< vector<int> > &refVec2d )
{
    vector<int>::iterator itCol;
    vector< vector<int> >::iterator itRow;

    for(itRow = refVec2d.begin(); itRow != refVec2d.end(); ++itRow)
    {
       for(itCol = itRow->begin(); itCol != itRow->end(); ++itCol)
          cout << (*itRow)[0] << ":" << *itCol << "\t";
       cout << endl;
    }
}

 bool compareFunction0(const vector<int>& a, const vector<int>& b )
 {
     return( a[0] < b[0] );
 }
//////////////////////////////////////////////////////////////////////////////////////
//////////////// THIS WORKS but compareFunction0 has hard coded column////////////////
//////////////////////////////////////////////////////////////////////////////////////

void vec2dSort( vector< vector<int> > &refVec2d, int sortCol )
{
    //check to see if sortCol is out of bounds
    //sort(refVec2d.begin(), refVec2d.end(), [](const vector< int >& a, const vector< int >& b){ return a[sortCol] < b[sortCol]; } );
   sort(refVec2d.begin(), refVec2d.end(), compareFunction0 );
}


class SortVec2d
{
   private:
      int sortCol;
   public:
      SortVec2d( vector< vector<int> >&, int);
      ~SortVec2d();
      bool comparison(const vector<int>&, const vector<int>& );
};

bool SortVec2d::comparison(const vector<int>& a, const vector<int>& b )
{
   return( a[0] < b[0] ); // I want to do return( a[sortCol] < b[sortCol] )
}


//////////////////////////////////////////////////////////////////////////////////////
//////////////// THIS DOESN'T WORK "no such comparison function"//////////////////////
//////////////////////////////////////////////////////////////////////////////////////

SortVec2d::SortVec2d( vector< vector<int> > &refVec2d, int sortColumn )
{
   int sortCol = sortColumn;

   //sort(refVec2d.begin(), refVec2d.end(), comparison );
   //sort(refVec2d.begin(), refVec2d.end(), [](const vector< int >& a, const vector< int >& b){ return a[0] < b[0]; } );
}


int randBetween(int min, int max)
{
   return (rand()%(max-min))+min;
}

int main()
{
   vector<int> vecRow ;
   vector< vector<int> > vec2d;
   int rowSize = 3;

   for(int c = 0; c < 10; ++c)
   {
      for(int r = 0; r < rowSize; ++r)
         vecRow.push_back( randBetween(0,255) );

      vec2d.push_back(vecRow);
      vecRow.clear();
   }

   cout << "New 2d Vector Created: " << endl;
   printVec2d(vec2d);

   sort(vec2d.begin(), vec2d.end(), [](const vector< int >& a, const vector< int >& b){ return a[1] < b[1]; } );
   cout << "2d Vector Sorted on Col 1: " << endl;
   printVec2d(vec2d);

   vec2dSort( vec2d, 0);
   cout << "2d Vector Sorted on Col 0: " << endl;
   printVec2d(vec2d);

   return 0;
}

Answer 1

Use a capture in your lambda to accept the column number to sort on.

int columnToSort = 2;
sort(vec2d.begin(), vec2d.end(), [columnToSort](const vector< int >& a, const vector< int >& b)
{
    return a[columnToSort] < b[columnToSort];
});