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Accessing multiple logical array indexing results at the same time

Asked by Shane Smith on 23 Jul 2018
Latest activity Commented on by Walter Roberson
on 24 Jul 2018
For my application I'm working with a large array of signals stored inside of a cell array. I'm building a set of name-value pairs to pass to another function, so I'm using cells to store the signals that match certain names. I've used logical indexing successfully to grab one such signal, but I'd like to grab a full set of signals that all have different names. I've included a simplified example below illustrating what I'm trying to do and the error I get.
test = magic(4)
test =
16 2 3 13
5 11 10 8
9 7 6 12
4 14 15 1
K>> equalToTwo = test == 2
equalToTwo =
4×4 logical array
0 1 0 0
0 0 0 0
0 0 0 0
0 0 0 0
K>> equalToSixteen = test == 16
equalToSixteen =
4×4 logical array
1 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
K>> tests = {equalToTwo,equalToSixteen}
tests =
1×2 cell array
[4×4 logical] [4×4 logical]
K>> test2 = test(tests{:})
Index exceeds matrix dimensions.
So from the example above, I have two logical arrays that should return one value each. I can use those arrays to index into the main array just fine with only one, but I'd like to be able to return n-number of values based on how many sets of logical arrays I have. This is a much simpler example, but I think it is easier to understand then trying to explain the larger set of code I'm working with and the problem presents in the same way.

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2 Answers

Answer by Paolo
on 23 Jul 2018
 Accepted Answer

If I am understanding correctly the following code should solve your problem:
test = magic(4);
equalToTwo = find(test == 2);
equalToSixteen = find(test == 16);
tests = {equalToTwo,equalToSixteen};
test2 = test([tests{:}])

  1 Comment

Thanks, I don't know why I didn't think to use find, I had used it initially elsewhere, then got rid of it since you could just use logical indexing instead (to get one value).

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Answer by Walter Roberson
on 23 Jul 2018

If more than one logical value can be true, or if any of the entries might have no matches:
cellfun(@(mask) test(mask), tests, 'uniform', 0)
If each entry is certain to have exactly one match:
cellfun(@(mask) test(mask), tests)

  2 Comments

Can you elaborate more on what this is doing? I understand that cell function is applying the function to each cell, but it looks like you are using the variable itself as a function? What does the 'uniform' argument do?
This is just applying each cell as logical indexing. Remember in MATLAB, indexing and function calls use the same syntax.
'uniform', 0 means that each output from the call should be placed into an individual cell entry. You need that unless the result of the expression is always guaranteed to be a scalar.

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