Segmentation fault in matrix multiplication C++ - c++

Question

I'm trying to make a function that receives as parameter two matrixes and returns the multiplication of both. But when I try to test it, it returns a segmentation fault, can anyone tell me why? And help me to fix it?
This is my function code:
#include <iostream>
#include <math.h>
#include <fstream>
#include <vector>
using namespace std;
vector<vector<double> > MMultiplication(vector<vector<double> > a, vector<vector<double> > b) {
int a_columns = a[0].size();
int a_rows = a.size();
int b_columns = b[0].size();
int b_rows = b.size();
< vector<vector<double> > result;
result.resize(a_columns);
for (int i = 0; i < m; ++i)
{
//Grow Columns by b_rows
result[i].resize(b_rows);
}
for (int p = 0; p < a_rows; p++) {
for (int q = 0; q < b_columns; q++) {
result[p][q] = 0;
}
}
if (a_columns != b_rows) {
cout << "Error: The number of columns of the first matrix needs to be equal to the number of rows of the second matrix" << endl;
return result;
}
for (int i = 0; i < a_rows; i++) { //i iterate a rows
for (int j = 0; j = b_columns; j++) //j iterates b columns
{
for (int k = 0; k < a_columns; k++) { //k goes back to a and iterates its columns
result[i][j] += a[i][k] * b[k][j]; //sums all multiplications into result[i][j]
}
}
}
return result;
}
And this is the main i'm using to test my function:
int main ()
{
vector < vector < double >>a;
int m = 4, n = 2;
//Grow rows by m
a.resize (m);
for (int i = 0; i < m; ++i)
{
//Grow Columns by n
a[i].resize (n);
}
a[0] =
{
1, 0};
a[1] =
{
1, 1};
a[2] =
{
1, 2};
a[3] =
{
1, 3};
cout << a.size () << endl;
cout << a[0].size () << endl;
vector < vector < double >>b;
int o = 2, p = 4;
b.resize (o);
for (int i = 0; i < o; i++)
{
b[i].resize (p);
}
b[0] =
{
1, 2, 3, 4};
b[1] =
{
1, 3, 5, 7};
vector < vector < double >>result = MMultiplication (a, b);
cout << result.size () << endl;
cout << result[0].size () << endl;
/* for(int k = 0; k < result.size(); k++) {
for(int q = 0; q < result[0].size(); q++)
cout << result[k][q] << endl;
} */
}
Also, it's important to point out that the function, so far, is returning a matrix of zeros if it's impossible to multiply a and b.
Thank you.