[PYTHON MATRIX-SIG] empty array

[Groma Geza]

Konrad Hinsen writes:
 
        > array() should accept empty lists in its arguments. Then array([])
        > would be a rank-1 array of length zero, array([[],[]]) a rank-2 array
        > of shape (2,0) and so on. 

That is what I expected, too, but it does not work, at least on my 
system (built from v0.35). What is wrong in this code?
------------------------------------------------------------------- 
>>> from Numeric import *
>>> array([])
Traceback (innermost last):
  File "<stdin>", line 1, in ?
TypeError: illegal argument type for built-in operation
>>> 
-------------------------------------------------------------------

        > A single "empty array" does not make sense,
        > since there can be many empty arrays that differ in shape.  Matlab can
        > get away with that because its matrix concept is much more limited.

Even if array([]) were correct, that would be useful only to create 
an empty array, and not to test it. In TestUtils.py I found a simple 
and natural way to check the equality of array a and b:

	     a.notEqual(b).nonzero() == None

In v0.35 this returns false. The equality can be tested by

	    len(a.notEqual(b).nonzero()) == 0

but I personally prefer the previous style. None is a 'general empty 
object', something similar could be introduced for arrays, too.
Another problem is that an empty array is not a printable object:

--------------------------------------------------------------
>>> array([0]).nonzero()
Traceback (innermost last):
  File "<stdin>", line 1, in ?
  File "./PrettyPrinter.py", line 28, in arrayToString
    max_str_len = max(len(str(maximum.reduce(data))),
ArrayError: can't take anything from an empty array
>>> 
-----------------------------------------------------------------

 


Geza I. Groma
Institute of Biophysics
Biological Research Centre of Hungarian Academy of Sciences
Szeged, Hungary

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