Numeric: 'where' function conditions

Jorl Shefner v451v at yahoo.com
Mon Nov 29 18:05:37 EST 2004


Could anyone tell me the efficient way to do this?   Extracting values 
from an array for a single condition (say all values greater than 'x') 
using 'where' and 'compress' is simple enough.  

>>> from Numeric import arange,where,compress
>>> data= arange(10)
>>> data= [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> mask= where((data > 3),1,0)
>>> result= compress(mask,data)
>>> result
array([4, 5, 6, 7, 8, 9])

But, 'where' doesn't appear to allow for multiple conditions in one 
statement.  For instance, I'd like to do something like:

mask= where((3 < data <= 7),1,0)

but, this won't work.

So, the best I could come up with was this more complicated process that 
requires 2 separate masks and an extra temporary array.    

>>> mask1= where((data > 3),data,0)
>>> mask1
array([0, 0, 0, 0, 4, 5, 6, 7, 8, 9])
>>> mask2= where((mask1<= 7),mask1,0)
>>> mask2
array([0, 0, 0, 0, 4, 5, 6, 7, 0, 0])
>>> r= compress(mask2,data)
>>> r
array([4, 5, 6, 7])

Is there a more concise way?

Thanks,

J.S.



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