Question about array alternating between positive and negative
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Hello,
I have solved a differential equation symbolically and then evaulated it based on user input. The result is different every time. Each time I obtain an array from the graph. For example, lets say I get an array, a=[-1,1,2,-2,1,5,-4,1]. Is there a way to find out how many times the sign of an array changes from positive to negative? The reason I am trying to do this is I need to know how many times the curve crosses the origin.
Thanks!
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James Tursa
2019-5-15
编辑：James Tursa
2019-5-15
Seems like there should be a shorter way, but assuming you are looking for all 0 crossings and not just the positive to negative ones:
sum(logical(diff(sign(a))))
This will double book the exact 0's, however. So if you expect you might get those, you will need something more sophisticated.
2 个评论
William Fantin
2019-5-15
James Tursa
2019-5-15
E.g.,
>> a = [-2 -3 0 1 2 -3 5]
a =
-2 -3 0 1 2 -3 5
>> sum(logical(diff(sign(a))))
ans =
4
There's really only three 0 crossings, but the calculation above gave a result of 4. The 0 crossing from -3 --> 0 --> 1 got double booked because the sign changed twice even though it was only one crossing. So if you expect you might get exact 0's you will need a more sophisticated method to detect them and check for 0 crossings vs 0 peaks etc and adjust the answer accordingly.
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