multiply two negative numbers in c++ - c++

Question

when I tried to multiple two negative numbers the value it is zero in c++,
for example -5 * -3
the result is zero,
why?
this is my code
#include <bits/stdc++.h>
#include <iostream>
using namespace std;
void Multiply(const int v_arr[], const int m_arr[][3], int signed
o_arr[], int size)
{
for (int i = 0; i < size; i++) {
for (int j = 0; j < size; j++) {
o_arr[i] = 0;
for (int k = 0; k < 3; k++)
o_arr[i] += v_arr[k] * m_arr[k][i];
}
}
//End your code here
}
int main()
{
int n;
cin >> n;
int v_array[n];
int m_array[n][3];
int signed o_array[3];
for (int i = 0; i < n; i++) {
cin >> v_array[i];
}
for (int i = 0; i < n; i++) {
for (int j = 0; j < 3; j++) {
cin >> m_array[i][j];
}
}
//fuction
Multiply(v_array, m_array, o_array, n);
for (int j = 0; j < 3; j++) {
cout << o_array[j] << " ";
}
return 0;
}
how to fix it to get the correct result?
the input is
2
2 -3
2 -3
2 -4

Answer 1

Your issue is here:
for (int k = 0; k < 3; k++)
o_arr[i] += v_arr[k] * m_arr[k][i];
}
You access elements at indices 0, 1 and 2 in v_arr, but it only has 2 elements. That's Undefined Behaviour.
Assuming this is matrix*vector multiplication code, it should look like this (untested):
for (int k = 0; k < 3; k++)
o_arr[k] += v_arr[i] * m_arr[i][k];
}
Also, your loop based on j is useless. You can remove it:
void Multiply(const int v_arr[], const int m_arr[][3], int signed o_arr[], int size)
{
for(int k = 0; k < 3; k++) { //initialize output array
o_arr[k] = 0;
}
for (int i = 0; i < size; i++) {
for (int k = 0; k < 3; k++)
o_arr[k] += v_arr[i] * m_arr[i][k];
}
}