help regarding extracting a smaller list from larger one

Prasad, Ramit ramit.prasad at jpmorgan.com
Fri Sep 16 01:16:05 CEST 2011


>Nevertheless, I think list comprehension is what you want:
>     [ x  for x in A  if 3.0 <= x <= 8.0 ]
>will extract all the values between 3 and 8 from the array A and create 
>a new list containing them.

>If you want the new list to be a numpy array then
>     numpy.array([ x  for x in A  if 3.0 <= x <= 8.0 ])


I have been told that the following is much faster than list generation for filtering if you are using numpy arrays. I have not looked to see if this claim is accurate, but since it was one of the SciPy creators....I trust him :) YMMV

foo = numpy.array([
	[1.2,2.3,1.3],
	[4.3,2.1,6.2],
	[3.2,1.2,6.5],
	[1.4,2.1,4.3]])
#if we want 2nd element > 2 and 3rd element < 6
mask = (foo[:,2]>2)&(foo[:,2]<6)
v = foo[mask] #this is a view and if you want a new list, you will have to wrap it
# I assume np.array(v) will create new array (untested).

Ramit


Ramit Prasad | JPMorgan Chase Investment Bank | Currencies Technology
712 Main Street | Houston, TX 77002
work phone: 713 - 216 - 5423



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