Numpy

Cet article porte sur la librairie python numpy.

API

where

Exemple d'utilisation de numpy.where

>>> from numpy import *
>>> a = array([3,5,7,9])
>>> b = array([10,20,30,40])
>>> c = array([2,4,6,8])
>>> where(a <= 6, b, c)
array([10, 20,  6,  8])
>>> where(a <= 6, b, -1)
array([10, 20, -1, -1])
>>> indices = where(a <= 6)
 # returns a tuple; the array contains indices.
>>> indices
(array([0, 1]),)
>>> b[indices]
array([10, 20])
>>> b[a <= 6]
# an alternative syntax
array([10, 20])
>>> d = array([[3,5,7,9],[2,4,6,8]])
>>> where(d <= 6)
# tuple with first all the row indices, then all the column indices
(array([0, 0, 1, 1, 1]), array([0, 1, 0, 1, 2]))

Be aware of the difference between x[list of bools] and x[list of integers]!
 #!python numbers=disable
>>> from numpy import *
>>> x = arange(5,0,-1)
>>> print x
[5 4 3 2 1]
>>> criterion = (x <= 2) | (x >= 5)
>>> criterion
array([True, False, False, True, True], dtype=bool)
>>> indices = where(criterion, 1, 0)
>>> print indices
[1 0 0 1 1]
>>> x[indices]
 # integers!
array([4, 5, 5, 4, 4])
>>> x[criterion]
 # bools!
array([5, 2, 1])
>>> indices = where(criterion)
>>> print indices
(array([0, 3, 4]),)
>>> x[indices]
array([5, 2, 1])

Liens externes

• Numpy Home Page
• Namespace Index
• Tutorial
• Exemples
• Numpy Example Fetcher