MATLAB: Question about array alternating between positive and negative

graphguiMATLABMATLAB and Simulink Student Suite

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!

Best Answer

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.

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MATLAB: How do you enter the command for a cube root

Two simple options:

x^(1/3)

Or,

nthroot(x,3)

Be very careful though. If x is negative, it will return a complex number, because there are indeed THREE cube roots of a negative number. Two of them are complex. nthroot will give you the root you would expect however.

(-2)^(1/3)
ans =
      0.62996 +     1.0911i
nthroot(-2,3)
ans =
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In your case, it is not relevant, since the number will be non-negative.

MATLAB: Negative x axis meaning

This is confusing as a question. You have a plot of some numbers. Some of them are negative. Of course MATLAB will extend the axis out to include the values you plotted. That some of them are negative and you don't want them to be is irrelevant, since MATLAB just plots what you tell it to plot.

If you don't want anything negative, then force it to be so in advance. Or correct what ever bug was there to prevent that from happening.

I suppose, you can always set the axis limits after the fact, effectively ignoring any negative values. But that just closes your eyes to the problem. I would suggest fixing the problem, rather than just pretending it does not exist.

Best Answer

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