# Find a series of consecutive numbers and change index of them

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Jaehwi Bong on 12 Aug 2019
Answered: Andrei Bobrov on 12 Aug 2019
Hi,
I have this kind of data. The first column and second column of the data refers to Index and time series(not consecutive) relatively.
Data = [1 1; 1 2; 1 3; 1 5; 1 6; 1 10; 1 11; 1 13; 1 20; 1 21; 1 22;];
I'd like to change this index number using Matlab code(to apply to thousands of data) when the value of the second colmn is not consecutive numbers.
So in my example, the first column should be changed [1; 1; 1; 2; 2; 3; 3; 4; 5; 5; 5;] .
If anyone can help, it would be greatly appreciated.
Thank you!

the cyclist on 12 Aug 2019
Edited: the cyclist on 12 Aug 2019
Data(:,1) = cumsum(diff([Data(1,2); Data(:,2)]) ~= 1);
will change the first column to what you want.
That's might be a bit "obfuscated" for you, but it is pretty straightforward to parse what is going on.
[Data(1,2); Data(:,2)]
is appending another copy of the first element at the front of the vector. I do that so that the first element of your vector will look non-consecutive to the "prior" one, and therefore be assigned index 1.
diff() ~= 1
is identifying the locations with non-consecutive jumps, putting a 1 there.
cumsum()
takes the cumulative sum along the vector, which means that each time a 1 is encountered, your index is incremented.

Andrei Bobrov on 12 Aug 2019
[~,~,Data(:,1)] = unique(cumsum(Data(:,1)) - Data(:,2),'stable');