How to iterate through 4Th array dimension a summation over the first 3 dimensions

Dear all,

I am struggling to program this passage of my code:

being S a 4D-double 3x3x4x181 I need to iterate the sum of the matrix(3x3x4) over all the the 181 stored matrices. The following code gives wrong results and I do not understand why.

A = [0.0 0.0 0.0 ; 0.0 0.0 0.0 ; 0.0 0.0 0.0] ;
for i = 1:1:181 ;
  for k = 1:1:4
  A(:,:,i) = A(i) + S(:,:,k,i)*[ 0.1 0.1 0.1 0.1] ;
  end
end

even if I pick a single stored matrix to operate lead to a wrong result...

A = [0.0 0.0 0.0 ; 0.0 0.0 0.0 ; 0.0 0.0 0.0] ;
for i = 1:1:181 ;
  for k = 1:1:4
  A = A + S(:,:,k,61)*[ 0.1 0.1 0.1 0.1] ;
  end
end

I obtain the right results only using a single loop as follow:

A = [0.0 0.0 0.0 ; 0.0 0.0 0.0 ; 0.0 0.0 0.0] ;
for k = 1:1:4
A = A + S(:,:,k,61)*[ 0.1 0.1 0.1 0.1] ;
end

But I really need to iterate this operation along the 4Th dimension of the array, and I do not understand why the first approach does not work.

I hope to find some help here.

Kind regards, Edoardo

Answer 1

[Roger Stafford]

   A = sum(reshape(S,3*3*4,181),1);

This will give you a vector with 181 elements, each being the sum over all the first three dimensions for each of the 181 fourth dimension levels.

Added later:

In case I have misinterpreted your request and what you want is the sum along the 4th dimension for each of the 3*3*4 = 36 positions in the first three dimensions, that can be done this way:

   A = sum(S,4):

It will give you a 3 x 3 x 4 array of sums.