Created a 3x2 matrix dynamically; printing it displays a 2x2 matrix - c++

Question

I am getting issues. I want to create 2D array rows=3 cols=2
My code is as follows
int **ptr;
int row=3;
int col=2;
ptr=new int *[col];
for (int i=0;i<col;i++)
{
ptr[i]=new int [row];
}
for (int i=0;i<row;i++)
{
for (int j=0;j<col;j++)
{
cout<<ptr[i][j]<<" ";
}
cout<<endl;
}
But i am getting output (2*2)
0 0
0 0

Answer 1

When you work with 2D arrays you have the following:
HOLDS YOUR ROWS
|
|
[x0] ([x0_1][x0_2][x0_3]...[x0_n]) <- ROW ARRAY
[x1] ([x1_1][x1_2][x1_3]...[x1_n]) <- ROW ARRAY
[x2] ([x2_1][x2_2][x2_3]...[x2_n]) <- ROW ARRAY
. .
. .
. .
[xm] ([xm_1][xm_2][xm_3]...[xm_n]) <- ROW ARRAY
Which means that first you have to create each row:
for (int i=0;i<row;i++)
{
ptr[i]=new int[col]; // Each row has col number of cells
}
From the table at the beginning of my post this gets you every ([xP_1][xP_2][xP_3]...[xP_n]).
Next part of your code has to actually initialize the cells in each row so in your outer loop you have to iterate over your rows and then in the inner loop you have to iterate over you columns because each row has COL cells from ptr[i]=new int[COL];. So we get:
for (int i=0;i<row;i++)
{
for (int j=0;j<col;j++)
{
cout<<ptr[i][j]<<" ";
}
cout<<endl;
}
So at the end we have (I've replaced row with rows and col with cols in order to make things more readable for you...I hope :D):
#include <iostream>
#include <string>
using namespace std;
int main(int argc, char *argv[])
{
int **ptr;
int rows = 3;
int cols = 2;
ptr = new int *[rows];
for (int row=0; row<rows; row++)
{
ptr[row]=new int [cols];
}
for(int row=0; row<rows; row++)
{
for(int col=0; col<cols; col++)
{
ptr[row][col] = 0;
cout << ptr[row][col] << " ";
}
cout << endl;
}
// Do something with the 2D array
// ...
// Delete all
for(int row = 0; row < rows; row++) delete[] ptr[row];
delete[] ptr;
return 0;
}
The output is:
0 0
0 0
0 0
Hope this helps. Also have in mind that you need to initialize your array's cells with some values. It's just not considered a good practice to leave it like you do - create and then simply go right to the displaying part without adding any values.