Question

I want to do a function that given 2 matrix returns the sum of both.I think the problem is in how I initialize the Matrix ‘t’.

#include <iostream>
#include <vector>

using namespace std;
typedef vector< vector<int> > Matrix;
    Matrix sum(const Matrix&a,const Matrix&b){
           Matrix t;
           for(int i=0;i<a.size();i++)
               for(int j=0;j<a.size();j++)
                   t[i][j] = a[i][j] + b[i][j];


           return t;
    }

Answer 1

You’ll need to initialize the rows and columns of t with something like:

Matrix t = vector< vector<int> >(row_count, vector<int>(col_count, 0));

That will make a row_count by col_count matrix filled with zeroes.

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On a side note about performance: comparing to .size() in a for loop means that before each iteration, .size() has to be calculated again. You can save a bit of processing (which adds up for massive data sets) by pre-calculating it like so:

for (int row = 0, row_ct = mat.size(); row < row_ct; ++row)