Extract group of identical values from an array

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I have the array
a = [0 0 0 1 1 1 0 0 0 0 0 1 1 1 1 0 0 1 1];
I want to extract the first and last index where a==1 as a group
o/p: [4 6; 12 15; 18 19];
Is there any efficient way to do this?
  1 Comment
Adam
Adam on 7 Jul 2017
Edited: Adam on 7 Jul 2017
firstIndices = find( diff(a) == 1) + 1;
lastIndices = find( diff(a) == -1);
will give you most of what you need, but you would need to make a special case for the last index which it will not find, and obviously it is more efficient to do the diff just once, etc. Shaping the results into the format you want shouldn't be difficult.

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Accepted Answer

José-Luis
José-Luis on 7 Jul 2017
If you have the image-processing toolbox.

More Answers (1)

C.J. Harris
C.J. Harris on 7 Jul 2017
a = [0 0 0 1 1 1 0 0 0 0 0 1 1 1 1 0 0 1 1];
nStart = find(diff([0 a 0]) == 1);
nEnd = find(diff([0 a 0]) == -1) - 1;
nResult = [nStart' nEnd'];

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