Re: [Matplotlib-users] Splitting arrays into chunks that satisfy a condition?

[Benjamin R. <ben...@ou...>]

On Fri, Aug 2, 2013 at 1:36 PM, Drain, Theodore R (392P) <
the...@jp...> wrote:

> I have three arrays (x,y,z).  I want plot x vs y and draw the line
> segments differently depending on whether or not z is positive or negative.
>  So I'm trying to split the x,y arrays into chunks depending on the value
> of z.  Using numpy.where, I can find the indeces in z that satisfy a
> condition but I can't figure out an efficient way (other than brute force)
> to split the array up into continuous chunks.  Does anyone know of a numpy
> trick that would help with this?
>
> Here's a simple example:
>
> # index:    0  1  2  3   4  5  6  7 8 9
> z=numpy.array([-1,-1,-1, 1, -1,-1,-1, 1,1,1] )
> x=numpy.array([-2,-3,-4, 2, -5,-6,-7, 3,4,5] )
>
> # Want: xneg = [ x[0:3], x[4:7] ], xpos = [ x[3:4], x[7:10] ]
> xneg = [ [-2,-3,-4], [-5,-6,-7] ]
> xpos = [ [ 2 ], [ 3, 4, 5 ] ]
>
> idxneg = numpy.where( z < 0 )[0]
> # == [ 0,1,2, 4,5,6 ]
> idxpos = numpy.where( z >= 0 )[0]
> # == [ 3, 7,8,9 ]
>
> Thanks,
> Ted
>

One way I would go about it is to do this:

z1 = numpy.where(z < 0, z, numpy.nan)
z2 = numpy.where(z >= 0, z, numpy.nan)

And then plot those against x.  matplotlib ignores nans and would break up
the line where-ever a nan shows up (assuming that is the effect you want).

Cheers!
Ben Root