CODEWITHSUNDEEP
matlab
Zane Kaminski
Zane Kaminski

Reputation: 539

Find indices where array changes

I have an array something like this:

[0 0 0 0 1 1 1 1 1 0 0 0 0 1 1 1 0 0 0 1 1 ... ]

I want to find the indices where 0 changes to 1 and 1 changes to 0. So for the following array:

changes = [5 10 14 17 20]

I know how to use find and a vector predicate expression like find(A > 3) to search for simpler conditions, but this has me stuck. I could write a big loop and do it, but I feel there must be something built-in or otherwise easier to achieve the same thing.

Upvotes: 3

Views: 2392

Answers (3)

Carl Witthoft
Carl Witthoft

Reputation: 21532

Dennis K's solution works for your binary data. If you wish to find transitions in general, I recommend Run Length Encoding. I don't think MATLAB has a builtin, but (shameless plug), you can use seqle .

Upvotes: 0

Dennis Klopfer
Dennis Klopfer

Reputation: 769

A very simple approach which works with all values as start would be:

changes = find(diff(value))+1;

This will also return the expected result changes = [5 10 14 17 20].

Upvotes: 5

Zane Kaminski
Zane Kaminski

Reputation: 539

Figured out a solution.

find(circshift(value, [0, 1]) ~= value)

For my application, the array is guaranteed to begin and end with 0s, otherwise it will not work properly.

Upvotes: 3

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