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

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FredMat
FredMat 2022-10-17
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!
  1 个评论
Jiri Hajek
Jiri Hajek 2022-10-17
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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Stephen23
Stephen23 2022-10-17
编辑：Stephen23 2022-10-17
Ran in:
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

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