[Numpy-discussion] Bug: extremely misleading array behavior

[Pearu Peterson]

On Tue, 28 May 2002, Larry Denneau wrote:

> Hello,
> 
> I recently discovered the following behavior when fetching values
> from a Numeric array.  Can somebody offer some insight?
> 
> #1)
> 
>   import Numeric
> 
>   a = Numeric.zeros((2, 2), 'i')
>   n = a[1, 1]     # fetch interesting value from array
>   print n 
>   a[1, 1] = 10    # change array
>   print n         # blam
>   print type(n)   # huh
> 
>   [bash]$ python 1.py
>   0
>   10
>   <type 'array'>
> 
> but
> 
> #2)
> 
>   import Numeric
> 
>   a = Numeric.zeros((2,), 'i')
>   n = a[1]
>   print n
>   a[1] = 10
>   print n
>   print type(n)
> 
>   [bash]$ python 2.py
>   0
>   0
>   <type 'int'>
> 
> #2 works the way one would expect, and #1 does not (n changes).
> They should at least both behave the same. :-)  At a minimum, naive
> use of arrays can lead to confusing or disastrous results, since
> a single value fetched from an array can change behind your back.

Use
     
    a[1][1] = 10
     
and the output will be
  
0
0 
<type 'int'>

I find it is an useful feature in Numeric to have both behaviours of
either using a[1,1] or a[1][1]. You may want to dig into Numeric's
userguide to get a more detailed explanation of the differences.

Regards,
	Pearu