How to use ismember in a cell array?

Question

Dominik Mattioli 2017-2-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/326213-how-to-use-ismember-in-a-cell-array

编辑： Emmanuel Lasso 2019-10-6

I would like to change the value of some value within a cell array that has only numeric data, preferably not using loops because I will be performing this operation with large cell arrays many times.

    % Given:
    A = {[1 2 3] [3 4 5 6];[7 8] [9 1 2 3 4];[5] [6 7 8 9]};
    B = 1;
    % Set all values in A equal to B as NaN.
    % If A were a double then it would look like this:
    A(ismember(A,B)) = NaN;

I've been trying to use this

A(cellfun(@(x) x == B(1),A,'un',0))

But I get this error:

"Function 'subsindex' is not defined for values of class 'cell'."

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Answer 1

Jan 2017-2-21

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https://ww2.mathworks.cn/matlabcentral/answers/326213-how-to-use-ismember-in-a-cell-array#answer_255749

在 MATLAB Online 中打开

Try it with loops, because there is no general rule that loops are slow:

A = {[1 2 3] [3 4 5 6];[7 8] [9 1 2 3 4];[5] [6 7 8 9]};
B = 1;
for iA = 1:numel(A)
  tmpA  = A{iA};
  tmpA(tmpA == B) = NaN;
  A{iA} = tmpA;
end

Measure the timings using timeit or tic/toc. cellfun with anonymous function is not really fast also, because handling the anonymos function for each element needs some time, perhaps more than the loop overhead.

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Jan 2017-2-21

Note that strrep is very efficient with replacing characters in cell strings. Unfortunately this command does not accept numerical values as e.g. strfind does. But a C-Mex might be much faster, so consider to mex this, if it is the bottleneck of your code.

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Answer 2

Stephen23 2017-2-21

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/326213-how-to-use-ismember-in-a-cell-array#answer_255746

编辑：Stephen23 2017-2-21

在 MATLAB Online 中打开

>> A = {[1,2,3],[3,4,5,6];[7,8],[9,1,2,3,4];[5],[6,7,8,9]};
>> B = 1;
>> C = cellfun(@(a)a~=B,A,'Uni',0);
>> D = cellfun(@(a,c)a.*(c./c),A,C,'Uni',0);
>> D{:}
ans =
   NaN     2     3
ans =
   7   8
ans =  5
ans =
   3   4   5   6
ans =
     9   NaN     2     3     4
ans =
   6   7   8   9

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Dominik Mattioli 2017-2-21

This is what I expected, however Jan's right about the loops. For my code, a loop was faster. Must be the overhead of cellfun. Thanks though!

Emmanuel Lasso 2019-10-6

编辑：Emmanuel Lasso 2019-10-6

And if I want to replace a value of an array, i mean, in this case B = [3 2 1 0], and C should find the values that match from A, A variates between 3 and 0 like randi function, so how D can use cellfun for search in A the values that match and replace with another values?. For example if a~=3 then replace it with -3

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