MATLAB: How to count the occurrences of each element in a vector in MATLAB

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I would like to be able to return the count of occurences of each element in a vector.
For example if I have a vector:
 x=[10 25 4 10 9 4 4]
I expect the result to be
 y=[2 1 3 2 1 3 3].

Best Answer

As of MATLAB R2019a, you can use the “groupcounts” function to compute the number of times an element appears in a vector as a summary. In other words, the elements of the below output “GC” are the counts of the corresponding element values in “GR” (from the original input vector “x”):
x = [10 25 4 10 9 4 4]'; 
[GC,GR] = groupcounts(x) 
GC = 
       3 
       1 
       2 
       1 
GR = 
       4 
       9 
      10 
      25
For more information on "groupcounts", please see the documentation link below:
https://www.mathworks.com/help/matlab/ref/double.groupcounts.html
---
As of MATLAB R2018b, you can use the “grouptransform” function if you want to compute the number of times an element appears in a vector and output that count for each corresponding element of the input vector. For example:
x = [10 25 4 10 9 4 4]'; 
y = grouptransform(x,x,@numel) 
y = 
       2 
       1 
       3 
       2 
       1 
       3 
       3
For more information on "grouptransform", please see the documentation link below:
https://www.mathworks.com/help/matlab/ref/double.grouptransform.html
---
Prior to MATLAB R2018b, while there is no single function to count occurrences of each element, there are a few ways to count elements in a vector:
1. Logical Indexing:
The following code snippet will give the desired output:
y = zeros(size(x));
for i = 1:length(x)
y(i) = sum(x==x(i));
end
For MATLAB R2016b and later, you can use implicit expansion to further simplify the code:
y = sum(x==x')
2. Binning:
You can use the "hist" and "unique" functions as shown here to do the same:
x = [10 25 4 10 9 4 4]
[a,b]=hist(x,unique(x))
3. Third-Party Tools:
For another workaround, see the following file, 'CountMember.m', that was contributed by a MATLAB user to do the same from a single function:
https://www.mathworks.com/matlabcentral/fileexchange/7738-countmember
Note that MathWorks does not guarantee or warrant the use or content of submissions to the MATLAB Central File Exchange. Any questions, issues, or complaints should be directed to the contributing author.
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