[Numpy-discussion] Why is the truth value of ndarray not simply size>0 ?
Neil Martinsen-Burrell
nmb@wartburg....
Mon Sep 7 08:46:35 CDT 2009
On 2009-09-07 07:11 , Robert wrote:
> Is there a reason why ndarray truth tests (except scalars)
> deviates from the convention of other Python iterables
> list,array.array,str,dict,... ?
>
> Furthermore there is a surprising strange exception for arrays
> with size 1 (!= scalars).
Historically, numpy's predecessors used "not equal to zero" as the
meaning for truth (consistent with numerical types in Python). However,
this introduces an ambiguity as both any(a != 0) and all(a != 0) are
reasonable interpretations of the truth value of a sequence of numbers.
Numpy refuses to guess and raises the exception shown below. For
sequences with a single item, there is no ambiguity and numpy does the
(numerically) ordinary thing.
The ndarray type available in Numpy is not conceptually an extension of
Python's iterables. If you'd like to help other Numpy users with this
issue, you can edit the documentation in the online documentation editor
at http://docs.scipy.org/numpy/docs/numpy-docs/user/index.rst
-Neil
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