[Numpy-discussion] Combining Sigmoid Curves
Christopher Barker
Chris.Barker@noaa....
Fri May 2 15:24:01 CDT 2008
Rich,
this could use some serious vectorization/numpyification! Poke around
the scipy Wiki and whatever other tutorials you can find -- you'll be
glad you did. A hint:
When you are writing a loop like:
> for i in xL:
> x.append(xL[i])
You should be doing array operations!
Specifics:
xL = nx.arange(ll,mid,0.1)
# you've now created an array of your X values
for i in xL:
x.append(xL[i])
#this dumps them into a list - why not keep them an array?
# if you really need an array (you don't here), then you can use:
xL.tolist()
That's why you get this error:
TypeError: unsupported operand type(s) for -: 'list' and 'float'
you're trying to do array operations on a list.
Also, you probably want np.linspace, rather than arange, it's a better
option for floats.
Here is my version:
import numpy as np
ll, lr, mid = (-10, 10, 0)
flexL = (mid - ll)/2.0
flexR = (lr - mid)/2.0
tau = (mid - ll)/10.0
xL = np.linspace(ll, mid, 5)
yL = 1.0 / (1.0 + np.exp(-(xL-flexL)/tau))
print xL
print yL
xR = np.linspace(mid, lr, 5)
yR = 1 - (1.0 / (1.0 + np.exp(-(xR-flexR)/tau)))
print xR
print yR
# now put them together:
x = np.hstack((xL, xR[1:])) # don't want to duplicate the midpoint
y = np.hstack((yL, yR[1:]))
print x
print y
Though it doesn't look like the numbers are right.
Also, you don't need to create the separate left and right arraysand put
them together, slicing gives a view, so you could do:
numpoints = 11 # should be an odd number to get the midpoint
x = linspace(ll,lr, numpoints)
y = zeros_like(x)
xL = x[:numpoints/2]
yL = y[:numpoints/2]
yL[:] = 1.0 / (1.0 + np.exp(-(xL-flexL)/tau))
you could also use something like:
np.where(x<0, .....)
left as an exercise for the reader...
-Chris
--
Christopher Barker, Ph.D.
Oceanographer
Emergency Response Division
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Chris.Barker@noaa.gov
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