Count number of times value occurs

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dan berkowitz
dan berkowitz 2018-10-12
评论： Bruno Luong 2021-7-6
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

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Bruno Luong
Bruno Luong 2018-10-12
编辑：Bruno Luong 2018-10-12
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)'
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Benedict Greenwood
Benedict Greenwood 2021-7-6
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 2021-7-6
The last line is needed in case your data is not consecutive integers 1, 2, 3 ...

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Image Analyst
Image Analyst 2018-10-12
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 个评论
Bruno Luong
Bruno Luong 2018-10-12
编辑：Bruno Luong 2018-10-12
@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 2018-10-12
编辑：Bruno Luong 2018-10-12
Assume a and b are row vectors, and one of them is not too big
c = sum(a(:)==b,1)

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