[Numpy-discussion] Piecewise functions.

[Travis Oliphant]

Andrea Riciputi wrote:

> Hi all,
> this is probably an already discussed problem, but I've not been able  
> to find a solution even after googling a lot.
>
> I've a piecewise defined function:
>
>        /
>        | f1(x)    if x <= a
> f(x) = |
>        | f2(x)    if x > a
>        \
>
> where f1 and f2 are not defined outside the above range. How can I  
> define such a function in Python in order to apply (map) it to an  
> array ranging from values smaller to values bigger than a?

This is not a trivial problem in current versions of Numeric.

What are you using, Numeric, numarray?

In new scipy core (which replaces Numeric) and, I think, in numarray, 
you could say

gta = x>a
lea = x<=a
y = x.copy()
y[gta] = f2(x[gta])
y[lea] = f1(x[lea])

I've also just written a piecewise function for the new scipy core so 
you could write

y = piecewise(x, x<=a, [f1,f2])

-Travis Oliphant