Problem 51740. Find all the indices when a value repetition occurs

Created by Armando Longobardi

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Find all the sequences of consecutive repeated numbers in a numeric array, returning the indices where each sequence starts and ends. The output is a matrix nx2: the starting indices of the n sequences are in the first column, the ending indices are in the second one.
For example, if x=[0 7 7 3 8 7 1 3 5 5 5], we have that the number '7' appears consecutevely from index 2 to 3, while the number '5'  from index 9 to 11, so y = [2 3; 9 11]

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31 Solutions
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Last Solution submitted on Jun 02, 2025

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Armando Longobardi on 10 May 2021

It would be nice making this working with nan and adding the repeated values as second output

Christian Schröder on 17 Jun 2023

Could I convince you to use isempty() for the last test, rather than isequal(..., [])? Since -- for n>0 runs in the input array -- the output is n-by-2, it is quite natural for the output to be 0-by-2 when there are no runs, but a 0-by-2 empty array is not equal to [].

Dyuman Joshi on 17 Jun 2023

The last test case has been changed to use isempty() instead of isequal().

Christian Schröder on 17 Jun 2023

Thanks, Dyuman!

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