Count number of times value occurs

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Hi,
I have an array a = [ 1 2 3 2 2 3 1], and another array, b = [1:5].
How can I create an array c, that counts the number of times that each value in b occurs in a?
I want c = [2 3 2 0 0] (ie. 1/2/3/4/5 from b, appears 2/3/2/0/0 times in a)
Any help would be appreciated. Thanks,
DB

Accepted Answer

Bruno Luong
Bruno Luong on 12 Oct 2018
Edited: Bruno Luong on 12 Oct 2018
Method for large a and b
[u,~,j] = unique(b(:));
[b, i] = ismember(a(:),u);
c = accumarray(i(b),1,size(u));
c = c(j)'
  3 Comments
Benedict Greenwood
Benedict Greenwood on 6 Jul 2021
Cheers Bruno great piece of code! Can I double-check why the bottom line is necessary? For my data the output is the same without the bottom line (although not transposed because of the apostrophe).
Bruno Luong
Bruno Luong on 6 Jul 2021
The last line is needed in case your data is not consecutive integers 1, 2, 3 ...

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More Answers (2)

Image Analyst
Image Analyst on 12 Oct 2018
Yet another way to get the histogram is to use histcounts():
a = [ 1 2 3 2 2 3 1]
b = [1:5]
counts = histcounts(a, 'BinEdges', [b, inf])
  1 Comment
Bruno Luong
Bruno Luong on 12 Oct 2018
Edited: Bruno Luong on 12 Oct 2018
@Dan: double check if you really want IA's method, for example with
a = [2];
b = [1 3];
counts = histcounts(a, 'BinEdges', [b, inf])
> counts =
1 0
to me 1 and 3 from b appear 0 time in a.

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Bruno Luong
Bruno Luong on 12 Oct 2018
Edited: Bruno Luong on 12 Oct 2018
Assume a and b are row vectors, and one of them is not too big
c = sum(a(:)==b,1)

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