Multiplication of a vector and a matrix in c++ - c++

Question

I tried to multiply a vector by a matrix but I can't get it to work because the loop always stops at one line. But with no error code, I've tried different ways to write the code into the resulting vector but it doesn't work. The outputs are to control where the loop stops, it stops after res[i] += (A[i][k] * B[k]);.
This is the specific function to perform the vector & matrix multiplication, if you need all the code let me know.
void vector_matrix_multiplication(vector<vector<int>> A,vector<int> B, int col1, int row1, int row2, vector<int>& res) {
int row = row1;
if(row1 < row2)
row = row2;
for(int i = 0; i < row; i++) {
cout << "Loop 1 ";
cout << i << endl;
for (int k = 0; k < col1; k++) {
cout << "Loop 2 " << i << " " << k << endl;
res[i] += (A[i][k] * B[k]);
cout << "Loop 2?" << endl;
}
}
The output of the function (with input A = {{2,3},{4,5}} & B = {1,2} is:
Loop 1 0
Loop 2 0 0
#include <iostream>
#include <vector>
using namespace std;
void vectorinput(vector<int>& a, int col){
cout << "Vector: " << endl;
for(int i = 0; i < col; i++) {
int x;
cin >> x;
a.push_back(x);
}
}
void matrixinput(vector<vector<int>>& a, int row, int col){
cout << "Matrix: " << endl;
for(int i = 0; i < row; i++) {
vector<int> vector;
for(int j = 0; j < col; j++) {
int x;
cin >> x;
vector.push_back(x);
}
a.push_back(vector);
}
}
int main(){
vector<int> vector;
vector<vector<int>> matrix; //Matrix is read in separate function
int row1 = 0; //Number of rows of first matrix
int col1 = 0; //Number of columns of first matrix
int row2 = 0; //Number of rows second matrix (redundant in this case)
int col2 = 0; //Number of columns second matrix
matrixinput(matrix1, row1, col1);
vectorinput(vector2, col2);
int row = row1; //Matrix with number of columns "col1", rows "row" - in this case both are 2
if(row2 > row1)
row = row2; //Vector with number of rows "row2" - in this case 2
vector<int> resvector(row, col1);
vector_matrix_multiplication(matrix2, vector1, col2, row, col1, resvector);
for(int i = 0; i < row; i++) {
cout << resvector[i] << endl;
}
return 0;
}
I hope this clarifies the purpose of the program and the function. I tried to cut it down a little because I have a lot of useless code in it. (Plus I struggled to input the code at first, was not quite sure how the code block works - sorry ^^)

Answer 1

There are following three bugs in your code:
Since you use the following ctor of std::vector
explicit vector(size_type count,
const T& value = T(),
const Allocator& alloc = Allocator());
, the second argument of resvector must be zero.
row1, row2, col1 and col2 are all zero through the main function and thus the loop in vector_matrix_multiplication does not work.
We can also reduce these variables to row and col of the matrix as #n.m. suggests in the comments.
matrix1, matrix2, vector1 and vector2 are not defined.
In summary, the following minimally fixed version will work fine for you:
int main()
{
std::vector<int> vec;
std::vector<std::vector<int>> mat;
int row = 2; //Number of rows of matrix
int col = 2; //Number of columns of matrix
matrixinput(mat, row, col);
vectorinput(vec, col);
std::vector<int> resvector(row, 0);
// 5th argument is redundant as #n.m. suggested
vector_matrix_multiplication(mat, vec, col, row, col, resvector);
for(int i = 0; i < row; i++) {
std::cout << resvector[i] << std::endl;
}
return 0;
}

Answer 2

and welcome to StackOverflow! I suggest you using mathematical libraries for this kind of stuff. The reason is:
they are already tested by someone else
they are efficient and optimized to use advanced CPU features
they are easier to use
I've used two different libraries: glm and Eigen
If you want to build your own library for learning purpose I suggest you looking how these libraries are written (they're open source)