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c++vectoroperator-precedencecomma-operator

Resizing a vector of vectors

Below is chunk of code for resizing a vector of vectors. The output produced for size of each row is printed 0? Why is this happening even though i resized each row to W;

int main() {
    int H,W,i;
    cin >> H,W; // H=3,W=5;
    vector<vector<int> >v;
    v.resize(H);

    for(i=0;i<H;i++)
        v[i].resize(W);

    cout << v[1].size(); // Output is printed 0 
}
Solution

  • cin >> H,W; doesn't do what you expect. According to the C++ Operator Precedence, it's the same as (cin >> H), W;, the 2nd expression W does nothing in fact, so W is not initialized at all. Any usage of it would lead to undefined behavior.

    Change it to cin >> H >> W;.

    BTW: Change cout << v[1].size(); to cout << v[i].size();.