Need help with a matrix task for my school project

Question

Hello guys,

I am currently working on a project for my school. Our task is to make a matrix 4x4 multiplied with a vector 4x1. I am sure my code is right but the result keeps beeing false.
https://code.sololearn.com/cuhR7NA0426e/#cpp
help is appreciated

Answer 1

I think if you have to multiply a matrix with a vector your result will be another matrix. Each line of the new matrix is = each line of old matrix * vector.
But in your code you use 2 vector and a matrix and in the function mul you attempt to save the result in the second vector

Answer 2

Yes...give me some time please

Answer 3

something like this, without scanf 4 semplicity

#include <conio.h>
#include <iostream>
#include <stdlib.h>
#include <math.h>
using namespace std;

typedef double vec[4];

typedef double mat[4] [4];

void mul(mat m_res, mat m, vec v)
{
int i, j;

for (i = 0; i < 4; i++)
for (j = 0; j < 4; j++)
m_res[i][j]= m[i][j] * v[i];
}

int main()
{
vec v;
mat m, m_res;
int i, j;

for (i = 0; i<4; i++)
v[i] = i;

for (i = 0; i < 4; i++)
for( j = 0; j < 4; j++)
m[i][j] = 10*i + j;

mul(m_res, m, v);

for (i = 0; i < 4; i++)
{
cout << endl;
for( j = 0; j < 4; j++)
cout << m_res[i][j] << " ";
}

return 0;
}

Answer 4

maybe you have to adjust the formula but now it work

Answer 5

yes, the m_res matrix

Answer 6

v is 0 1 2 3
m is 0 1 2 3
10 11 12 13
20 21 22 23
30 31 32 33

so if u multiply each line with v

first line 0*0 0*1 0*2 0*3
second line 1*10 1*11 1*12 1*13
third line 2*20 2* 21 2*22 2*23
last line 3*30 3*31 3*32 3*33

Answer 7

try also

m_res[i][j] = m[i][j] * v[j]

for a different moltiplication

Answer 8

maybe the second is what you need

Answer 9

do you have any idea how to change my code to do that?

Answer 10

the output is just
0 0 0 0
10 11 12 13
40 42 44 46
90 93 96 99