Count the number of same elements in an array

Question

luca 2019-9-10

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/479719-count-the-number-of-same-elements-in-an-array

编辑： Vitek Stepien 2021-8-14

采纳的回答： madhan ravi

在 MATLAB Online 中打开

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..

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Answer 1

madhan ravi 2019-9-10

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https://ww2.mathworks.cn/matlabcentral/answers/479719-count-the-number-of-same-elements-in-an-array#answer_391230

编辑：madhan ravi 2019-9-10

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

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Stephen23 2019-9-10

编辑：Stephen23 2019-9-10

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@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 2019-9-10

Thank you Stephen :) !

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Answer 2

Stephen23 2019-9-10

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https://ww2.mathworks.cn/matlabcentral/answers/479719-count-the-number-of-same-elements-in-an-array#answer_391237

编辑：Stephen23 2019-9-10

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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

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luca 2019-9-10

thanks Stephen

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Answer 3

Vitek Stepien 2021-8-14

4
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https://ww2.mathworks.cn/matlabcentral/answers/479719-count-the-number-of-same-elements-in-an-array#answer_767062

编辑：Vitek Stepien 2021-8-14

在 MATLAB Online 中打开

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.