How to index cell Matrix with a logical matrix?

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Diego Makasevicius Barbosa 2015-11-26

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https://ww2.mathworks.cn/matlabcentral/answers/257656-how-to-index-cell-matrix-with-a-logical-matrix

评论： Florian B 2022-2-24

采纳的回答： Guillaume

Hello all, I tried to index a cell matrix with logical matrix, but it does not return me the correct output.

I want to:

Index: A ={'a','b','c','d'; 'a','b','c','d'; 'a','b','c','d'}

Matrix: B =[1,0,1,0; 0,1,1,0; 0,0,1,1]

Output: C =['a','','c',''; '','b','c',''; '','','c','d'];

I tried 'C=A(logical(B));' but without success.

Could someone help me, please?

Thank you!

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Guillaume 2015-11-26

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Please note that

A =['a','b','c','d'; 'a','b','c','d'; 'a','b','c','d']

is not a cell array. It is a plain matrix of character and is the same as:

A = ['abcd';'abcd';'abcd'];

A cell array is:

A = {'a','b','c','d'; 'a','b','c','d'; 'a','b','c','d'}

Note the use of curly brackets. The distinction is important for your desired output since

C =['a','','c',''; '','b','c',''; '','','c','d'];

is the same as

C = ['ac'; 'bc'; 'cd'];

Diego Makasevicius Barbosa 2015-11-26

Sorry Guillaume, my fault. Actually is exactly as you said (A ={'a','b','c','d'; 'a','b','c','d'; 'a','b','c','d'}). I only typed the question incorrectly.

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Answer 1

Guillaume 2015-11-26

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Assuming you indeed have a cell array (unlike your example):

A = {'a','b','c','d'; 'a','b','c','d'; 'a','b','c','d'}
B = logical([1,0,1,0; 0,1,1,0; 0,0,1,1]);
%option 1: create an empty cell array and use logical indexing to copy A
C = cell(size(A));
C(B) = A(B)
%option 2: copy A, and use logical indexing to replace unwanted elements 
D = A;
D(~B) = {''}

Note that for all intent and purposes '' and [] are the same, even if matlab does not display them the same

isequal(C, D) %returns true

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Ahmad Suliman 2017-7-30

Guillaume, when I do: D(~B) = {''}, the MATLAB throws an error "Function 'subsindex' is not defined for values of class 'cell'." Could it be due to version? maybe 2016 does not support this way of indexing? Thanks,

Florian B 2022-2-24

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@Thorsten: Using option 2 does indeed give you an array with "empty" values (actually ' '). You can however just assign [ ] directly, which MATLAB interprets as deletion command.

%option 2: copy A, and use logical indexing to replace unwanted elements 
D = A;
D(~B) = []; % this will delete all arguments where B(i) ~= true. -> before: {''}

This question - solving the problem for a 2D-cell array - may also help. A full alternative to the snippet above would be:

% copy A, and use logical indexing to replace unwanted {elements} with {''}
D = A;
D(~B) = {''};
% Identify empty cells
idx = cellfun(@isempty, D);
% Delete rows with empty cell
D(~any(idx)) = [];