Prod giving a different answer than repeated multiplication

X(:,:,1) = [0.0000 + 0.0000i   0.0000 + 0.0000i   1.0000 + 0.0000i   0.0000 + 0.0000i;0.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i   1.0000 + 0.0000i;1.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i;0.0000 + 0.0000i   1.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i];
    X(:,:,2) = [0.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 - 1.0000i   0.0000 + 0.0000i;0.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 - 1.0000i;0.0000 + 1.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i;0.0000 + 0.0000i   0.0000 + 1.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i];
    X(:,:,3) = [0.0000 + 0.0000i   1.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i;1.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i;0.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i  -1.0000 + 0.0000i;0.0000 + 0.0000i   0.0000 + 0.0000i  -1.0000 + 0.0000i   0.0000 + 0.0000i];
    X(:,:,4) = [0.0000 + 0.0000i   0.0000 - 1.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i;0.0000 + 1.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i;0.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 1.0000i;0.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 - 1.0000i   0.0000 + 0.0000i];

I have the given 3D array. For some reason prod(X(:,:,1:4),3) gives 0 as the output. That does not match with the output for X(:,:,1)*...X(:,:,4). I do not want to use a loop, I want to use a vectorised method. However, prod is giving me issues. Am I doing something wrong? Is there another method?

[Divij Gupta]

By 0 as the output I mean the 4x4 zero matrix

Answer 1

[KSSV]

prod gives you element by element multiplication.

Check:

A = rand(2,2,3) ; 
X = prod(A,3) ; 
Y = A(:,:,1).*A(:,:,2).*A(:,:,3) ;   % element by element mulltiplication 
Z = A(:,:,1)*A(:,:,2)*A(:,:,3) ;
isequal(X,Y)

ans = logical
   1

isequal(X,Z)

ans = logical
   0

[Divij Gupta]

Thank you so much, that makes sense. What function can I use to vectorise matrix multiplication then?

[Walter Roberson]

You cannot vectorize matrix multiplication.

You can make the process a little more compact by using fold() https://www.mathworks.com/help/symbolic/fold.html . However that requires the Symbolic Toolbox

The elements you would implicitly loop over would be created by using num2cell(A,3)