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convert an array into its counting sequence..

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VISHWATEJ N
VISHWATEJ N on 2 Aug 2019
Edited: madhan ravi on 2 Aug 2019
i want to convert an array into its counting sequence. for example
if the input is :
x = [1 1 4 4 6 7]
two 1's two 4's one 6 one 7...
output must be :
y=[2 1 2 4 1 6 1 7]
can anyone help?

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Answers (2)

madhan ravi
madhan ravi on 2 Aug 2019
Edited: madhan ravi on 2 Aug 2019
According to what I understand you want to count the number of occurances of each unique elements in the array x, then:
x = [1 1 4 4 6 7];
[u,~,idx]=unique(x);
v=accumarray(idx,1);
y=reshape([v,u(:)].',1,[])
Gives:
y =
2 1 2 4 1 6 1 7

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Jos (10584)
Jos (10584) on 2 Aug 2019
This fails for inputs like [1 1 2 1 1], where values are repeated.
I admit, the question is a little unclear, but "counting sequence" suggests, at least to me, more than just counting ;-)
madhan ravi
madhan ravi on 2 Aug 2019
I'm not sure either let the OP verify it ;)

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Jos (10584)
Jos (10584) on 2 Aug 2019
This is called run-length encoding, for which there are very efficient functions available on the File Exchange.
I point you to my own function RUNINDEX which can also return this:
[~, RLE] = runindex(x)
y = reshape(RLE(:,[3 1]).', 1, []) % your format

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