Count the number of same elements in an array

Hi given a vector

V = [ 1 2 4 3 4 2 3 5 6 4 5 6 8 4 2 3 5 7 8 5 3 1 3 5 7 8 9 5 3 2 4 6 7 8]

I would like to count how many times the value 1,2,3,4,5,6,7,8,9 are repeated inside V, and obtain a vector that report this values:

C = [2 4 6 5 6 3 3 4 1]

where 1 is repeated 2 times, 2 is repetead 4 times, 3 is repeated 6 times and so on..

Answer 1

[madhan ravi]

[~,~,ix] = unique(V);
C = accumarray(ix,1).'

[luca]

may you explain your answer?

[luca]

there's a precision that I have to put.

If there is no 1 element like

V = [2 2 3 4 5 6 7 7 8 8 9 9]

I would like to obtain

C= [0 2 1 1 1 1 2 2 2]

Where the entities 1 is not present so the first element is 0.

Do you know how to do it?

[madhan ravi]

C=accumarray(V(:),1).'

%or
C=sum(V(:)==(1:max(V)))

As per your latter comment Stephen’s approach below will do what you want.

[Stephen23]

@madhan ravi : this is really quite neat:

accumarray(V(:),1)

Simple idea which works well:

>> V = [2 2 3 4 5 6 7 7 8 8 9 9]
>> accumarray(V(:),1)
ans =
  0
  2
  1
  1
  1
  1
  2
  2
  2

[madhan ravi]

Thank you Stephen :) !

Answer 2

[Stephen23]

Your 1st example:

>> V = [ 1 2 4 3 4 2 3 5 6 4 5 6 8 4 2 3 5 7 8 5 3 1 3 5 7 8 9 5 3 2 4 6 7 8];
>> C = hist(V,1:max(V))
C =
  2  4  6  5  6  3  3  4  1

Your 2nd example:

>> V = [2 2 3 4 5 6 7 7 8 8 9 9]
>> C = hist(V,1:max(V))
C =
  0  2  1  1  1  1  2  2  2

[luca]

thanks Stephen

Answer 3

[Vitek Stepien]

I found this function extremely useful, and doing exactly what you need:

V = [ 1 2 4 3 4 2 3 5 6 4 5 6 8 4 2 3 5 7 8 5 3 1 3 5 7 8 9 5 3 2 4 6 7 8];
[gc,grps] = groupcounts(V'); % <- need column vector here
grps'

ans = 1×9

1 2 3 4 5 6 7 8 9

gc'

ans = 1×9

2 4 6 5 6 3 3 4 1

Where grps lists the unique values in order, and gc provides the count of each unique values found in v.

This is very similar to madhan ravi's accumarray, but even simpler.

P.S. I turned gc and grps into row vectors only for compactness of the post, it's purely aesthetical. However groupcounts requires a column vector, not a row.

Answer 4

[Hugo Diaz]

I use sparse(V(:),V(:), 1) for large arrays with missing indices.