[Numpy-discussion] Most efficient trim of arrays

2010-12-14 Thread Mark Janikas 
Hello All,

I was wondering what the best way to trim an array based on some values I do 
not want  I could use NUM.where or NUM.take... but let me give you an 
example:

import numpy as NUM
n = 100 (Length of my dataset)
data = NUM.empty((n,), float)
badRecords = []
for ind, record in enumerate(records):
if record == someValueIDOntWant:
badRecords.append(ind)
else:
data[ind] = record


Now, I want to trim my array using badRecords.  I guess I want to avoid 
copying.  Any thoughts on the best way to do it?  I do not want to use lists 
and then subsequently array the result as it is nice to pre-allocate the space.

Thanks much,

MJ


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Re: [Numpy-discussion] Most efficient trim of arrays

2010-12-14 Thread Robert Kern 
On Tue, Dec 14, 2010 at 12:20, Mark Janikas mjani...@esri.com wrote:
 Hello All,

 I was wondering what the best way to trim an array based on some values I do
 not want….  I could use NUM.where or NUM.take… but let me give you an
 example:

 import numpy as NUM

 n = 100 (Length of my dataset)
 data = NUM.empty((n,), float)
 badRecords = []
 for ind, record in enumerate(records):
     if record == someValueIDOntWant:
     badRecords.append(ind)
     else:
     data[ind] = record

 Now, I want to “trim” my array using badRecords.  I guess I want to avoid
 copying.  Any thoughts on the best way to do it?  I do not want to use lists
 and then subsequently array the result as it is nice to pre-allocate the
 space.

Don't fear the copy. Use boolean indexing.

http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html#boolean

-- 
Robert Kern

I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth.
  -- Umberto Eco
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