Accessing a data matrix using indices stored at another matrix

Question

In matlab, I commonly have a data matrix of size NxMxLxK which I wish to index along specific dimension (e.g. the forth) using an indices matrix of size NxMxL with values 1..K (assume the are all in this range):

>>> size(Data)
ans =
     7    22    128    40
>>> size(Ind)
ans =
     7    22    128

I would like to have code without loops which achieve the following effect:

Result(i,j,k) = Data(i,j,k,Ind(i,j,k))

for all values of i,j,k in range.

Yes, that's great! But what are you expecting from SO? Please elaborate your question according to the guidelines. — RC0993
– RC0993, Commented Feb 4, 2019 at 8:44

Adiel · Accepted Answer · 2019-02-04 13:53:30Z

3

You can vectorize your matrices and use sub2ind :

% create indices that running on all of the options for the first three dimensions:
A = kron([1:7],ones(1,22*128));
B = repmat(kron([1:22],ones(1,128)),1,7);
C = repmat([1:128],1,7*22);

Result_vec = Data(sub2ind(size(Data),A,B,C,Ind(:)'));
Result = reshape(Result_vec,7,22,128);

edited Feb 4, 2019 at 13:53

answered Feb 4, 2019 at 9:11

Adiel

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3 Comments

Uri Cohen Over a year ago

This is specifically for 4-D matrix, am I correct that for a 3-D matrix the solution would have been even simpler, using [A,B]=meshgrid(...)?

Adiel Over a year ago

Sounds like it is.

Adiel Over a year ago

anyway, some vectors from the output matrices.

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