run-time error when resizing vector

Question

When I run this program:

#include <iostream>
#include <vector>
using namespace std;


int main()
{
    vector<vector<char> > screen;
    char ch = 'a';
    unsigned col = 100, row = 100;
    if(screen.size() < (unsigned)row)
        screen.resize(row);
    if(screen[row - 1].size() < (unsigned)col)
        screen[row - 1].resize(col);
    screen[9][9] = ch;
    cout<< "hello";
    cout.flush();
}

cout does not print anything and I get this error:

Segmentation Fault (core dumped)

In linux. Is anything wrong in the program?

If col and row have lower numbers there's no problem.

Answer 1

You're resizing screen to row elements, but then you access element row in it. vector s in C++ are, like arrays, 0-based, so valid indexes are 0...row-1 .

Same goes for the inner vectors and col .

The fact that it works for smaller numbers is an (unfortunate) mishap.

Answer 2

if(screen[row - 1].size() < (unsigned)col)
    screen[row - 1].resize(col);

You're only resizing screen[99] here. screen[9] still has size 0 , which is why you can't access screen[9][9] (you could, however, access screen[99][9] ).