Erratic behaviour of vectors

[sajanphilip]

Hi

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

#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...

[Daved]

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.