Erratic behaviour of vectors

Hi

The following program was my attempt to implement 2D vectors:

Code:

#include<cstdio>
#include<vector>

int main()
{

    int size,counter=0;
    printf("Enter size:\n");
    scanf("%d",&size);

    std::vector< std::vector<int> > testtwo(size);
    for(int i=0;i<size;i++)
    {
        //for(int j=0;j<size;j++)
        testtwo[i].push_back(0);
    }

    for(int i=0;i<size;i++)
    {
        for(int j=0;j<size;j++)
        {
            testtwo[i][j]=(counter++);
        }
    }

    for(int i=0;i<size;i++)
    {
        for(int j=0;j<size;j++)
        {
            printf("%4d",testtwo[i][j]);
        }
        printf("\n\n");
    }
}

The for loop has been commented on purpose. If it is uncommented, the output would be as expected...
0 1 2

3 4 5

6 7 8

But if the for loop is commented, the output is changed to:

0 1 4 5

4 5 8 9

8 9 12 13

12 13 14 15

Please notice that testtwo[row][col] = testtwo[row-1][col +2]. This pattern is followed throughout the output,,,

Could someone please explain this error?

Thanx in advance...

If you comment it out, you have undefined behavior because your inner vectors will all have a size of 1 (you only call push_back once per outer vector). Later you use j from 0 to size-1 to access the inner vectors, so it is out of bounds. Switch operator[] to at() and you'll get an exception.