Create a loop using part of variable name in MATLAB

Question

I am a beginner in Matlab and have not been able to find an answer to my question so far. Your help will definitely be very much appreciated.

I have 70 matrices (100x100), named SUBJ_1, SUBJ_2 etc. I would like to create a loop so that I would calculate some metrics (i.e. max and min values) for each matrix, and save the output in a 70x2 result matrix (where each row would correspond to the consecutively named SUBJ_ matrix).

I am struggling with both stages - how to use the names of individual variables in a 'for' loop and how to properly save individual outputs in a combined array.

Many thanks and all the best!

See related answer. The eval(sprintf(...)) is the code/workaround/hack you want, but please note the warning in the beginning (similar to what Daniel and Jonas mentioned). — Dev-iL
– Dev-iL, Commented Sep 1, 2014 at 9:06

Daniel · Accepted Answer · 2014-09-01 08:45:41Z

4

Don't use such variable names, create a big cell array named SUBJ and put each Matrix in it.

r=zeros(numel(SUBJ),2)
for idx=1:numel(SUBJ)
   r(idx,1)=min(min(SUBJ{idx}))
   r(idx,2)=max(max(SUBJ{idx}))
end

min and max are called twice because first call creates maximum among rows, second call among columns.

answered Sep 1, 2014 at 8:45

Daniel

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1 Comment

Jonas Heidelberg Over a year ago

A cell array containing the matrices also works, but I would expect it to be slower than the 3d matrix I give in my solution. It has the advantage though that all 70 matrices do not have to be the same size.

Jonas Heidelberg · Accepted Answer · 2014-09-01 08:45:14Z

3

Even though this is in principle possible in Matlab, I would not recommend it: too slow and cumbersome to implement.

You could instead use a 3-D matrix (100x100x70) SUBJ which would contain all the SUBJ_1 etc. in one matrix. This would allow you to calculate min/max etc. with just one line of code. Matlab will take care of the loops internally:

OUTPUT(:,1) = min(min(SUBJ,[],1)[],2);
OUTPUT(:,2) = max(max(SUBJ,[],1)[],2);

Like this, OUTPUT(1,1) contains min(min(SUBJ(:,:,1))) and so on...

answered Sep 1, 2014 at 8:45

Jonas Heidelberg

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4 Comments

Daniel Over a year ago

A big matrix is even better than a cell array, but your code is not correct. ALLMINS = min(min(SUBJ,[],1)[],2); would produce the correct result.

Jonas Heidelberg Over a year ago

@Daniel, thanks for that - didn't have MATLAB open and too long since I used it much :-/

rochned Over a year ago

Hi both! Many thanks for your suggestions. While this is practical for this particular (first) task, for a later stage of my data processing, I would still need to somehow create a loop through arrays corresponding to individual subjects. I will need to run sone previously prepared scripts for which the arrays would need to be kept at their original sizes. Would it therefore still be somehow possible to create a loop through individual arrays based on subject names? Thanks!

beaker Over a year ago

To loop through individual subjects you simply loop over the index ii in SUBJ(:,:,ii). SUBJ(:,:,1) is identical to SUBJ_1(:,:); the only difference is notation. The advantage is that it is trivial to increment the index for SUBJ(:,:,1) in Matlab, while incrementing the variable name for SUBJ1(:,:) is difficult and ugly.

Highman · Accepted Answer · 2014-09-01 14:12:05Z

1

As to how to use the names of individual variables in a 'for' loop, here gives an example:

SUBJ = [];
for idx = 1:70
    term = eval(['SUBJ_',num2str(idx)]);
    SUBJ = [SUBJ; max(max(term)),min(min(term))];
end

answered Sep 1, 2014 at 14:12

Highman

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3 Comments

Jonas Heidelberg Over a year ago

This is what @rochned actually asked for, but as mentioned in the other comments it is not really recommended, e.g. for speed reasons. Whenever you are tempted to use eval, there is probably a more efficient way to do things...

Highman Over a year ago

Yeah, I admit that, especially when you have massive data to deal with.

rochned Over a year ago

Thanks, @Highman - this is exactly what I needed! For the current dataset (and other scripts), this seems to work really well. Many thanks also to everyone else for your advice and explanations!

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