How to add items to a 2-dimensional dynamic vector - c++

Question

I have so far been able to use 1-d vectors, adding or removing elements with push/pop back.
However, when trying to fill out a 2-d vector which is supposed to represent a matrix, I run into problems. I haven't been able to use these functionalities with my 2-d vector. When the code does compile and run I get the bits asking for dimension and then it asks for the first element [1,1], for any value entered I get "Segmentation fault: core dumped". I have no idea what is going on and have been trying to modify bits of my code but to no great success, the internet has also been rather useless in giving an easy guide on how to fill out these damn things...
Thank you so much!
Here is my code
#include <iostream>
#include <vector>
using namespace std;
vector<vector<double> > readMatrix();
int main()
{
vector<vector<double> > matrix1 = readMatrix();
vector<vector<double> > matrix2 = readMatrix();
}
vector<vector<double> > readMatrix()
{
cout << "Entering a matrix" << endl;
cout << "Number of Lines : ";
int numberOfLines;
cin >> numberOfLines;
cout << "Number of Columns :";
int numberOfColumns;
cin >> numberOfColumns;
vector<vector<double> > matrix;
int i(0);
int j(0);
while(i<=numberOfLines && j<=numberOfColumns)
{
cout << "[" << i+1 << "," << j+1 << "] =" ;
int value;
cin >> value;
matrix[i].push_back(value);
cout << endl ;
j++;
if(j==numberOfColumns)
{
j=1;
i++;
}
}
return matrix;
}

Answer 1

Declare the matrix like this:
vector<vector<double> > matrix (y, vector<double> (x, 0));
Now you have an y*x matrix. If you want to use push_back you have to declare the x-vector when starting on a new row.

Answer 2

When you do vector<vector<double> > matrix; you initialize a vector of vectors, both of size 0 (that's the default), so when you do matrix[i].push_back(value) even when i==0 you access memory which you hasn't been allocated. it's better if you do pre-allocate memory like this:
vector<vector<double> > matrix(numberOfLines,vector<double>(numberOfColumns));
And then you don't need to push back or anything, you just do matrix[i][j]=whatever.

Answer 3

In C++ you can use the vector constructor to automatically define the size:
vector<vector<double> > matrix(numberOfLines, vector<double>(numberOfColumns, 0));
With vectors, for loops are better than while loops because the size is always known. I suggest you iterate over the vector rows and columns with these (and remove your push_back statements.)

Answer 4

you need to do something like:
matrix.resize( ... );
.
.
.
matrix[i].push_back( ... );
The problem here is that you don't have a two dimensional vector. You have a vector, of vectors, of doubles. You need to put some vectors in the first vector, calling resize() as above is one way. You could also pass an initial size to the constructor, you could push the vectors on one by one - though that could carry performance issues.