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I have been using vectors only lately and found them extremely handy when compared normal array declarations. however, I have dealt with only 1D operations as following:-

      vector<int>X;
      X.push_back(value/variable);

How Do I perform similar operations for a 2D vector equivalent and dynamically update values, row by row? Please help me here and my apologies if the question is rudimentary. Also it would be really helpful if you can show me a link where all operations of vectors are provided as tutorials. I tried few sites, but they are bits all over. Please help me.

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2 Answers

up vote 3 down vote accepted

One more way to work with 2D-arrays is the using of one vector with 2D-addressing in your own class.

Here an example:

#include <iostream>
#include <vector>

class array_2d
{
    public:
        array_2d( 
            unsigned int x, 
            unsigned int y ) 
        : 
            m_size_x( x ), 
            m_size_y( y ),
            // resize vector to size x*y, all elements are 0.
            m_data( x*y, 0 )
        {}

        int
        get( unsigned int x, unsigned int y ) const
        {
            return m_data[ x + y * m_size_y ];
        }

        void
        set( unsigned int x, unsigned int y, int data )
        {
            m_data[ x + y * m_size_y ] = data;
        }

    private:
        unsigned int m_size_x;
        unsigned int m_size_y;

        std::vector <int> m_data;
};

int main()
{
    // 2D array 2x3.
    array_2d m( 2, 3 );

    // Set 2 cells into 1 and 3.
    m.set( 1, 1, 1 );
    m.set( 2, 0, 3 );

    for( unsigned int i = 0; i < 3; ++i )
    {
        for( unsigned int j = 0; j < 2; ++j )
            std::cout << m.get( i, j ) << " ";

        std::cout << std::endl;
    }
    return 0;
}
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1  
I personally like this one more than my vector<vector> solution because it is more efficient. But you will have to code a little more for this one, and in addition to coordinate conversion done here maybe also add operator () or maybe [] so you can access/modify elements by doing something like array2d(1,3) = 7 and if(array2d(1,3)>5) Still a +1 though. –  AlexK Jun 7 '13 at 15:37
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up vote 1 down vote

You need something along the lines of vector <vector <int> > with inside vector representing a full row. Or a full column (just pick a row major or a column major and stick with it). Or, just use boost::multi_array (http://www.boost.org/doc/libs/1_53_0/libs/multi_array/doc/user.html)

Here is an example:

//#include <iostream>
#include <stdio.h>
#include <vector>
using namespace std;

int main(){

vector <vector <int> > my2d;

vector<int> row;

row.push_back(1); row.push_back(2); row.push_back(3);

my2d.push_back(row);

++row[1];

my2d.push_back(row);

--row[0];

my2d.push_back(row);

for(size_t r=0;r<my2d.size();r++)
{
    for(size_t c=0;c<my2d[0].size();c++)
    {
        printf("%d ",my2d[r][c]);
    }
    printf("\n");
}

getchar();

return 0;
}
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Thanks for your reply. I prefer using a vector for now, as I have slightly got used to it. Can you please tell me how can I use push_back here to update elements one by one along column or row? as in syntax wise? like (0,1), then (0,2) and so on. Thanks in advance. –  Lakshmi Narayanan Jun 6 '13 at 22:52
    
If I want 3 columns and any no. of rows, how Do I do that.? –  Lakshmi Narayanan Jun 6 '13 at 22:56
1  
Added an example. I have 3 rows and 3 columns. I add (1 2 3) row first, then add (1 3 3) then add (0 3 3) then print the whole thing out –  AlexK Jun 7 '13 at 15:32
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