2d vector

[cash]

How would I create a vector matrix of size 3*4 using vector and with nested loop?


I dont want to use this

  std::vector<std::vector<int> > sparseArray(3,std::vector<int>(4));

[gunnerfunner]

I dont want to use this

why not?

[JLBorges]

> How would I create a vector matrix of size 3*4 using vector and with nested loop?

Something along these lines, perhaps:

#include <iostream>
#include <vector>
#include <cassert>

template < typename T > struct vec_2d
{
    vec_2d( std::size_t nrows, std::size_t ncols )
       : nrows(nrows), ncols(ncols), vec(nrows*ncols) {}

    T* operator[] ( std::size_t row_num )
    {
        assert( !vec.empty() && row_num < nrows ) ;
        return std::addressof( vec.front() ) + row_num*ncols ;
    }

    std::size_t nrows ;
    std::size_t ncols ;
    std::vector<T> vec ;
};

int main()
{
    vec_2d<int> vec(5,7) ;

    for( std::size_t i = 0 ; i < vec.nrows ; ++i )
        for( std::size_t j = 0 ; j < vec.ncols ; ++j ) vec[i][j] = i*10 + j + 11 ;

    for( std::size_t i = 0 ; i < vec.nrows ; ++i )
    {
        for( std::size_t j = 0 ; j < vec.ncols ; ++j ) std::cout << vec[i][j] << ' ' ;
        std::cout << '\n' ;
    }
}

http://coliru.stacked-crooked.com/a/26a2aea9e588d870

Note that there would be a significant performance improvement (due to better locality of reference) only when the number of rows is somewhat large and the number of elements in each row is not very huge.