[Numpy-discussion] How do I do this?

Christopher Barker Chris.Barker@noaa....
Fri Aug 29 16:35:59 CDT 2008


HI all,

I need to do something I thought would be simple -- set all the values 
of an array to some minimum. so I did this:

 >>> min_value = 2
 >>> a = np.array((1, 2, 3, 4, 5,))
 >>> np.maximum(a, min_value)
array([2, 2, 3, 4, 5])

all was well... then I realized that a could have negative numbers in 
in, and I really wanted the absolute value to be greater than than minimum:

 >>> a = np.array((1, 2, 3, 4, -5,))
 >>> np.maximum(a, min_value)
array([2, 2, 3, 4, 2])

oops!

so I added a sign() and abs():

 >>> np.sign(a) * np.maximum(np.abs(a), min_value)
array([ 2,  2,  3,  4, -5])

all was well. However it turns out a could contain a zero:

 >>> a = np.array((0, 1, 2, 3, 4, -5,))
 >>> np.sign(a) * np.maximum(np.abs(a), min_value)
array([ 0,  2,  2,  3,  4, -5])

Darn! I want that zero to become a 2, but sign(0)  = 0, so that doesn't 
work.

How can I do this without another line of code special casing the 0, 
which isn't that big I deal, but it seems kind of ugly...

 >>> a[a==0] = min_value
 >>> np.sign(a) * np.maximum(np.abs(a), min_value)
array([ 2,  2,  2,  3,  4, -5])


thanks,
-Chris



-- 
Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
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Chris.Barker@noaa.gov


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