Logical Indexing Failing when Matrix is loaded by matfile

Question

I have a matrix that was stored in a .mat file, and was then reloaded in matlab via the function matfile. I also have a logical index, like logical([1 0 1 0]), that I want to apply to the loaded matrix:

results = matfile('results.mat');
% id is my logical vector of the appropriate size
% IV is a matrix stored in results.mat
newIV = results.IV(:,id);  

However, I am running into a problem and getting this error:

'IV' cannot be indexed with class 'logical'. Indices must be numeric.

I do not understand what is causing this issue. I have been using this same code before and it was working, the only thing was that I did not have to load the struct results before, I already had it in memory. It gets weirder; this works:

IV = results.IV;
newIV = IV(:,id); % this works somehow

This also works:

results_raw = matfile('results.mat');
results = struct('IV',results_raw.IV);
newIV = IV(:,id); % this also works!!! why matlab, why???

I also tried resaving the results.mat file using the -v7.3 flag, but it did not solve the problem. The issue seems to be with loading the .mat file, because I created a struct with a matrix and used logical indexing and it worked fine.

Question: why does indexing work when I pass results.IV to IV? how can I make it work with results.IV?

Thanks for helping!!! :D

Answer 1

As @Adiel said in questions comments. You can't use logical indices. So, convert logical indices to numeric indices using find.

results = matfile('results.mat');
% id is my logical vector of the appropriate size
% IV is a matrix stored in results.mat
newIV = results.IV(:,find(id));