Logical indexing Matrix vs Matrix inside Struct loaded by matfile

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
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 even weirder, this works:

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

It can get even more weird:

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

[Walter Roberson]

matfile is not equivalent to loading the file. matfile() accesses the array "in place", in pieces, loading from the file or writing to the file as needed.

It appears that matfile() does not permit logical indexing. I do not see that documented, but there is no point in fighting it.

newIV = results.IV(:,find(id));

[Guilherme Salome]

Found the limitations in this page. If "id" is a vector, in the case of linear indexing, then converting to numeric values, as in "find(id)", will not work, since it is not supported by "matfile". It seems that the most straightforward solution is to just copy "results.IV" into a new matrix in the workspace, and avoid all these restrictions.

[Walter Roberson]

You are not doing linear indexing as you have : for the first dimension. The find() should work for this.