CODEWITHSUNDEEP
matlabvector
Fixed Point
Fixed Point

Reputation: 145

Check a sequence of integers to see if it is non-decreasing

I have a vector, a very long vector composed of integers in MATLAB. My question is what is a fast efficient way to check to see if that vector is non-decreasing in MATLAB? The vector has a few thousand elements, all positive integers. The vector starts with some positive integer, repeats a few dozens times and then is supposed to increment by one and repeat. For example, the vector should look something like

a = [5 5 5 5 6 6 6 6 6 6 6 7 7 8 8 9 9 9 10 10 10 10 11];

The problem is that we suspect that the counter may have screwed up occasionally so once in a while the counter decreases and then increases, something like

b = [5 5 5 5 6 6 6 7 7 6 6 7 7 8 8 9 9 9 10 11 10 10 11];

so first I need to see if the sequence is non-decreasing. Then if it ever does decrease, it would be nice to know all the indices where the sequence decreases like that. I could only think of using first differences and checking to see if all entries are either zero or one. Any other clever ideas?

Thanks.

Upvotes: 1

Views: 763

Answers (2)

sebii
sebii

Reputation: 516

To check whether the sequence is non-decreasing, you can use one of the following:

all(diff(b) >= 0)
~any(diff(b) < 0)

To find the specific positions where it decreases:

find(diff(b) < 0) + 1

Upvotes: 6

bla
bla

Reputation: 26069

use diff and any :

 any(diff(b)<0)

this will return true (1) if the sequence is decreasing and false (0) for non-decreasing.

Upvotes: 5

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