Count number of times value occurs

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

Answer 1

[Bruno Luong]

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)'

[dan berkowitz]

Thanks, Bruno. I'm dealing with some very large matrices. This does the trick!

[Benedict Greenwood]

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]

The last line is needed in case your data is not consecutive integers 1, 2, 3 ...

Answer 2

[Image Analyst]

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])

[Bruno Luong]

@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.

Answer 3

[Bruno Luong]

Assume a and b are row vectors, and one of them is not too big

c = sum(a(:)==b,1)