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I need to count the number of times a vector increases/decreases. E.g

vec = [0 0 1 2 4 4 2 0 1 2 3]

diff(vec)

ans =

[0 1 1 2 0 -2 -2 1 1 1]

would evidently yield 4 trend changes. I don't wish to count the zero as a trend.

Tips please!

Update:

If you were to plot(vec) it's easier to see what I mean. I wish to simply count the number of row changes in the plot. A flat line (derivative of 0) isn't considered a row change in my application.

Take this as an example:

vec = [3 3 3 5 5 4 3 2 4 5 5 5];

plot(vec)

As you can see from the plot, it's 4 row changes (even though it's 8 in pure diff).

Update:

Ok, I'll try to do better.

Imagine a matrix that describes position over time, where row is position and column is time. When going in a straight line (along a single row) I wish to do nothing. But when changing course over to another row-line I wish to count each new "course" as an occurrence.

E.g

- Travelling at row 5 and then moving up to row 6 and then going straight, that's one course change.
- [5 5 5 6 6 6] => 1 "course change"
- Travelling at row 5 and then moving up to row 7 is similarly one change.
- [5 5 5 7 7 7] => 1 "course change"
- Travelling at row 5, moving up to 7 and then directly back to row 6 is then two changes.
- [5 5 5 7 6 6 6] => 2 "course changes"
- Travelling at row 5, moving up to 7 and then to 12 and then going straight is two changes as the row increment is 2 and then 5 (not a straight course).
- [5 5 5 7 12 12 12] => 2 "course changes"

So basically I wish to count each time you change position (or row) in a straight course.

Andrei Bobrov
on 16 Jul 2012

Edited: Andrei Bobrov
on 16 Jul 2012

EDIT

a = diff(vec(:));

out = nnz(unique(cumsum([true;diff(a)~=0]).*(a~=0)));

Doug Hull
on 12 Jul 2012

You would need to remove the zeros from the first diff vector and diff it again, then count number of non-zeros. The only complication is the case where the first diff vector looks like:

[1 1 0 1 1]

Once the zero is removed, it looks constant. That can be dealt with, but would be messy.

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