Keep a tally of the number of occurences of a value in an array

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FredMat 2022-10-17

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https://ww2.mathworks.cn/matlabcentral/answers/1829048-keep-a-tally-of-the-number-of-occurences-of-a-value-in-an-array

评论： FredMat 2022-10-17

Hello,

I have an array with integers from 1 to n. I would like to create a second array with the occurence number of the value in the corresponding location (index) of the initial array. For instance, if I have a first array [2 3 1 2 1 4 3 2 4], I want to generate a second array [1 1 1 2 2 1 2 3 2].

I'd like to avoid a series of n conditional statements.

Thx!

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Jiri Hajek 2022-10-17

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Hi, you don't need conditional statements, but a loop is probably necessary... You can use logical indexing:

LogicalIndexes = FirstArray == i;
SecondArray(LogicalIndexes) = nnz(LogicalIndexes);

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Answer 1

Stephen23 2022-10-17

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https://ww2.mathworks.cn/matlabcentral/answers/1829048-keep-a-tally-of-the-number-of-occurences-of-a-value-in-an-array#answer_1077183

编辑：Stephen23 2022-10-17

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A = [2,3,1,2,1,4,3,2,4];
% method 1
F = @(n)nnz(A(1:n)==A(n));
B = arrayfun(F,1:numel(A))
B = 1×9
     1     1     1     2     2     1     2     3     2
% method 2
B = sum(triu(A==A.'),1)
B = 1×9
     1     1     1     2     2     1     2     3     2