I'm trying to perform a simple matrix times vector multiplication and for some reason I am getting the wrong sign in my results in a couple of my multiplications. I have no idea why this is happening, any pointers would be greatly appreciated.
Here is my whole code, i.e. the matrix * vector function and a caller function (or whatever it's called) from mex - I'm running the code from Matlab via mex.
#include "mex.h"
void mxv(int m, int n, double *A, double *b, double *c) {
double sum;
int i, j;
for (i = 0; i < m; i++) {
sum = 0.0;
for (j = 0; j < n; j++) {
sum += A[i * n + j] * b[j];
}
c[i] = sum;
}
}
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) {
double *A, *b, *c;
int i, j, Am, An;
A = mxGetPr(prhs[0]);
Am = (int)mxGetM(prhs[0]);
An = (int)mxGetN(prhs[0]);
c = malloc(Am * sizeof(double));
b = mxGetPr(prhs[1]);
mxv(Am, An, A, b, c);
for (i = 0; i < Am; i++)
printf("c[%d] = %1.4f\n", i, c[i]);
}
I call the mex function with the following inputs:
A = [-865.6634 0 0 0;
0 -17002.6822 0 0;
0 0 -1726.2421 2539.6267;
0 0 -2539.6267 -1726.2421;]
b = [-0.00153521; -0.00011165; -0.00037659; 0.00044981]
The correct result should be:
1.3290
1.8983
1.7924
0.1799
But I get
c[0] = 1.3290
c[1] = 1.8983
c[2] = -0.4923
c[3] = -1.7329
So the first two are correct (c[0] and c[1]), but not the latter two.
I added a bunch of print statements into my code to try and figure out where the error occurs:
#include "mex.h"
void mxv(int m, int n, double *A, double *b, double *c) {
double sum;
int i, j;
for (i = 0; i < m; i++) {
sum = 0.0;
printf("********\n");
for (j = 0; j < n; j++) {
printf("A[%d][%d] = %1.10f\nb[%d] = %1.10f\n", i , j, A[i + n * j], j, b[j]);
printf("A[%d][%d]*b[%d] = %1.10f\n", i, j, j, A[i * n + j] * b[j]);
sum += A[i * n + j] * b[j];
printf("sum = %1.10f\n", sum);
}
c[i] = sum;
}
}
void mexFunction(int nlhs, mxArray *plhs[],
int nrhs, const mxArray *prhs[]) {
double *A, *b, *c;
int i, j, Am, An;
A = mxGetPr(prhs[0]);
Am = (int)mxGetM(prhs[0]);
An = (int)mxGetN(prhs[0]);
printf("size(A) = (%d,%d)\n", Am, An);
for (i = 0; i < Am; i++) {
for (j = 0; j < An; j++) {
printf("A[%d][%d] = %1.4f\n", i, j, A[i + Am * j]);
}
}
c = malloc(Am *sizeof(double));
b = mxGetPr(prhs[1]);
for (i = 0; i < Am; i++) {
printf("b[%d] = %1.4f\n", i, b[i]);
}
mxv(Am, An, A, b, c);
for (i = 0; i < Am; i++)
printf("c[%d] = %1.4f\n", i, c[i]);
}
I'm pretty confident that I'm getting the right inputs from Matlab:
size(A) = (4,4)
A[0][0] = -865.6634
A[0][1] = 0.0000
A[0][2] = 0.0000
A[0][3] = 0.0000
A[1][0] = 0.0000
A[1][1] = -17002.6822
A[1][2] = 0.0000
A[1][3] = 0.0000
A[2][0] = 0.0000
A[2][1] = 0.0000
A[2][2] = -1726.2421
A[2][3] = 2539.6267
A[3][0] = 0.0000
A[3][1] = 0.0000
A[3][2] = -2539.6267
A[3][3] = -1726.2421
b[0] = -0.0015
b[1] = -0.0001
b[2] = -0.0004
b[3] = 0.0004
But when I look into the matrix-vector multiplication I find that the multiplication resuts are for some reason having the wrong sign in some cases. This is what it prints out for i = 2:
********
A[2][0] = 0.0000000000
b[0] = -0.0015352100
A[2][0]*b[0] = -0.0000000000
sum = 0.0000000000
A[2][1] = 0.0000000000
b[1] = -0.0001116500
A[2][1]*b[1] = -0.0000000000
sum = 0.0000000000
A[2][2] = -1726.2421000000
b[2] = -0.0003765900
A[2][2]*b[2] = 0.6500855124
sum = 0.6500855124
A[2][3] = 2539.6267000000
b[3] = 0.0004498100
A[2][3]*b[3] = -1.1423494859 <- THIS SHOULD BE 1.142349... (no minus sign)
sum = -0.4922639735
********
Something similar happens for the i=3 case.
Thanks in advance!
In:
printf("A[%d][%d] = %1.10f\nb[%d] = %1.10f\n", i , j, A[i + n * j], j, b[j]);
printf("A[%d][%d]*b[%d] = %1.10f\n", i, j, j, A[i * n + j] * b[j]);
The first printf uses A[i + n*j], while the second uses A[i*n + j]. These are transposed positions in the array.