multiplying matrix by a vector on an element by element basis using for loops

Hi All,

i kindly need to multiply matrix A and the vector B on an element-by-element basis using for loop:

code:

A = [1 12 22 10 18; 20 8 13 2 25; 6 19 3 23 14; 4 24 17 15 7; 11 21 16 5 9];

B = [9 7 11 4 23];

[rowsA, colsA] = size(A);

[rowsB, colsB] = size(B);

theMatrixProduct = zeros(rowsA, colsB);

for row = 1 : rowsA

row % Print progress to command window.

for col = 1 : colsB

theSum = 0;

for k = 1 : colsA

theSum = theSum + A(row, k) * B(k, col);

end

theMatrixProduct(row, col) = theSum;

end

end

i get the following error : (Index exceeds matrix dimensions.)

kindly support me what is wrong in the code.

thanks in advance

Answer 1

[Arif Hoq]

try this:

A = [1 12 22 10 18; 20 8 13 2 25; 6 19 3 23 14; 4 24 17 15 7; 11 21 16 5 9];
B = [9 7 11 4 23];
for i = 1:size(A,1) 
    for j= 1:size(A,2)
        C(i,j) = A(i,j)*B(j);
    end
end
output=C;

% But if you want make it simple vectorized solution is the most efficient
output2=A.*B;   % element wise multiplication

% checking loop output and vectorized output equal or not
isequal(output,output2)

ans = logical
   1

Answer 2

[Bruno Luong]

To be able to compute A*B; You need B is column (vector): the number of rows of B much match the number of rows of A, 5 in this case, B must be initialized as

B = [9; 7; 11; 4; 23];