Python Programming – Copying Array

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Python Programming – Copying Array

Copying array

When operating and manipulating arrays, their elements sometimes needs to be copied into a new array, and sometimes not. This is often a source of confusion for beginners. Simple assignment does not make copy of array data.

>>> a=np . arange ( 12 )
>>> b=a 
>>> id ( a )
85660040
>>> id ( b )
85660040
>>> b is a                    # a and b are two names for the same ndarray object
True

Python passes mutable objects as references, so function calls make no copy.

>>> def F ( x ) :
. . .             print id ( x )
. . .
>>> id ( a )
85660040 
>>> F ( a )
85660040

The copy method makes a complete copy of the array data.

>>> c=a . copy ( )
>>> c 
array ( [ 0 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 ] )
>>> c is a 
False
>>> c [ 0 ]=999 
>>> c
array ( [ 999 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 ] )
>>> a
array ( [ 0 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 ] )