How can I count the occurrences of each element in a vector in MATLAB?

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].

Answer 1

[MathWorks Support Team]

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.

[Walter Roberson]

Why is abbab 5 ?

If we number the positions 1:5 then you have (1,2), (1,3), (1,5), (4,5) which is 4 combinations.

[Walter Roberson]

S = 'aaabb'

nnz(triu((S'=='a') & S=='b'))

[Walter Roberson]

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')

Answer 2

[Andrei Bobrov]

[a,b] = histc(x,unique(x));
y = a(b);

[Andrei Bobrov]

y = accumarray(x(:),1)

Answer 3

[Razvan Carbunescu]

There is a simpler way of answering this now using groupcounts(R2019a) or grouptransform(R2018b):

>> 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

[Razvan Carbunescu]

I'm not sure I understand the question Masoud. Multiple columns can be fed into groupcounts and it will count the occurances of each unique line but I don't know if that's what you're asking.

[madhan ravi]

+1

[Razvan Carbunescu]

For the example you gave above how does the solution look and what does 'similar number' for the first column mean?

[Razvan Carbunescu]

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

Answer 4

[Julian Hapke]

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)

[Johannes Korsawe]

the second solution exhibits pathological tendencies...