How to index within an array using another array?

I have an array 'A' of vectors of length 3 stored in a 5D matrix of size 'mXnXoXpX3'. Then, I define arrays B and C, such that:

B = sum(A.*A, 5); % Magnitude of the vectors
[~, C] = max(B, [], 1); % Location of the maximum vector magnitude along the first dimension

Here, C is of the size 1XnXoXp, and contains the location of maximum amplitudes of A, which is calculated in B. Now I wish to store these vectors (as identified in C) in a matrix D:

for i = 1 : n
  for j = 1 : o
    for k = 1 : p
      D(1, i, j, k, :) = A(C(1, i, j, k), i, j, k, :);
    end
  end
end

I am wondering if there is a better way to get 'D'? Perhaps without using the for loops?

Answer 1

[Walter Roberson]

As = size(A);
CAs = cumprod(As);
[I,J,K,L] = ndgrid(1:n, 1:m, 1:o, 1:p);
Aidx = squeeze(C(1,:,:,:)) + (I-1)*CAs(1) + (J-1)*CAs(2) + (K-1)*CAs(3) + (L-1)*CAs(4);

D1 = A(Aidx);
D(1,:,:,:,:) = D1;

If the first dimension of D is to be singular, then the last two statements could instead be

D = reshape(A(Aidx), 1, n, m, o, p);

Equivalently,

Aidx = sub2ind(size(A), squeeze(C(1,:,:,:)), I, J, K, L);
D = reshape(A(Aidx), 1, n, m, o, p);

[DK]

Sorry about an error in the subscripts. Edited it. Please see if that changes anything in your response.