How can I count the occurrences of each element in a vector in MATLAB?
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MathWorks Support Team
on 11 Jan 2012
Commented: Walter Roberson
on 26 Apr 2022
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].
Accepted Answer
MathWorks Support Team
on 27 Feb 2020
Edited: MathWorks Support Team
on 27 Feb 2020
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:
---
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:
---
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:
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.
3 Comments
Walter Roberson
on 26 Apr 2022
https://www.mathworks.com/help/matlab/ref/double.groupcounts.html#mw_92fbcf5a-2ab5-45d0-ac09-68c1986c269f
when you use groupcounts then the groups are in the order returned by unique('sorted')
More Answers (3)
Razvan Carbunescu
on 9 May 2019
>> x=[10 25 4 10 9 4 4]';
>> grouptransform(x,x,@numel)
ans =
2
1
3
2
1
3
3
>>[GC,GR]=groupcounts(x)
GC =
3
1
2
1
GR =
4
9
10
25
4 Comments
Razvan Carbunescu
on 6 Jun 2019
For the example you gave above how does the solution look and what does 'similar number' for the first column mean?
Razvan Carbunescu
on 7 Jun 2019
This seems like a very different type of problem so unlikely the functions in this topic will help you directly. I'd post this question as a separate thread with the example input/output
Julian Hapke
on 1 Jun 2017
Edited: Julian Hapke
on 1 Jun 2017
here is another one:
sum(bsxfun(@eq,x,x'),1)
or if you want the output to be the same orientation as input
sum(bsxfun(@eq,x,x'),(size(x,2)==1)+1)
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