[Matrix-SIG] array assignments

[Janko Hauser]

Pedro Miguel Frazao Fernandes Ferreira writes:
 > Hi All,
 > 
 > 	Can anyone tell me if this is the correct behaviour:
 > 
 > Python 1.5.1 (#1, Dec 17 1998, 20:58:15)  [GCC 2.7.2.3] on linux2
 > Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
 > Executing startup script
 > End
 > >>> a=arange(10)
 > >>> b=arange(10)
 > >>> a=b
 > >>> b[5]=10
 > >>> print a
 > [ 0  1  2  3  4 10  6  7  8  9]
 > >>>
 > 
 > 	Should a and b be two different objects ?
 > 	= operator should copy ?
 > 

No and no.
>>> a=arange(10)
>>> b=a
>>> b[5]=10
>>> a
array([ 0,  1,  2,  3,  4, 10,  6,  7,  8,  9])
>>> b
array([ 0,  1,  2,  3,  4, 10,  6,  7,  8,  9])
>>> b=a*1. # or copy.copy(a)
>>> b[5]=5
>>> a
array([ 0,  1,  2,  3,  4, 10,  6,  7,  8,  9])
>>> b
array([ 0.,  1.,  2.,  3.,  4.,  5.,  6.,  7.,  8.,  9.])
>>> b=array(a,copy=1)
>>> b[5]=5
>>> a
array([ 0,  1,  2,  3,  4, 10,  6,  7,  8,  9])
>>> b
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

The following is special for NumPy arrays not like the behaviour of
python lists.

>>> b=a[:]
>>> b[5]=10
>>> a
array([ 0,  1,  2,  3,  4, 10,  6,  7,  8,  9])
>>> b
array([ 0,  1,  2,  3,  4, 10,  6,  7,  8,  9])

HTH

__Janko