Product between matrices and vector

GAMS
1

Hi, I’m new with GAMS and I’m struggling with a very simple problem.
I wish to define the product between a matrix(k,m) and a vector(k), e.g.:
A = [0.082 0.036 ; 0.036 0.127] and v = [1 ; 0]
Multiplying Av I expect to obtain the first column of A, so: x = Av = [0.082; 0.036]

If I try:
x(k) = sum(m,A(k,m)*v(k))
My result is a single element that is the sum of the two elements I expect to be in the column, hence 0.118.
If I try with the product (x(k) = prod(…)), my result is a single element that is the product of the two elements I expect to be in the column, hence 0.003.

What am I doing wrong? Thanks a lot to anyone will help.

2

Hi,

Try to use loops. I wrote a simple code for you.

set      col     /1*4/
set      row     /1*5/

Table    matrix(row, col)
         1       2       3       4
1        2       3       4       5
2        4       6       2       1
3        9       7       1       0
4        6       7       1       8
5        7       2       3       1
;

Parameter        vector(col)/
         1       1
         2       1
         3       0
         4       0
/;

Parameter        result(row);

loop(row,
         result(row) = sum(col, matrix(row, col) * vector(col));
);

display result;

If you need to assign the matrix and vector by index, just add one more index to vector and matrix and one more outer loop.

Cheers!
Gabriel

3

Hi Gabriel, many thanks! :slight_smile: