Accessing multiple logical array indexing results at the same time

Question

Shane Smith 2018-7-23

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https://ww2.mathworks.cn/matlabcentral/answers/411776-accessing-multiple-logical-array-indexing-results-at-the-same-time

评论： Walter Roberson 2018-7-24

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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Answer 1

Paolo 2018-7-23

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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{:}])

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Shane Smith 2018-7-24

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 2

Walter Roberson 2018-7-23

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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)

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Shane Smith 2018-7-24

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?

Walter Roberson 2018-7-24

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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