[Numpy-discussion] u in [u+1]

[josef.pktd at gmail.com]

On Fri, Feb 5, 2010 at 8:48 AM, Zachary Pincus <zachary.pincus at yale.edu> wrote:
>> I'm having some trouble here.  I have a list of numpy arrays.  I
>> want to
>> know if an array 'u' is in the list.
>
> Try:
>
> any(numpy.all(u == l) for l in array_list)
>
> standard caveats about float comparisons apply; perhaps
> any(numpy.allclose(u, l) for l in array_list)
> is more appropriate in certain circumstances.
>
> Can of course replace the first 'any' with 'all' or 'sum' to get
> different kinds of information, but using 'any' is equivalent to the
> 'in' query that you wanted.
>
> Why the 'in' operator below fails is that behind the scenes, 'u not in
> [u+1]' causes Python to iterate through the list testing each element
> for equality with u. Except that as the error states, arrays don't
> support testing for equality because such tests are ambiguous. (cf.
> many threads about this.)
>
> Zach
>
>
> On Feb 5, 2010, at 6:47 AM, Neal Becker wrote:
>
>> I'm having some trouble here.  I have a list of numpy arrays.  I
>> want to
>> know if an array 'u' is in the list.
>>
>> As an example,
>> u = np.arange(10)
>>
>> : u not in [u+1]
>> ---------------------------------------------------------------------------
>> ValueError                                Traceback (most recent
>> call last)
>>
>> /home/nbecker/raysat/test/<ipython console> in <module>()
>>
>> ValueError: The truth value of an array with more than one element is
>> ambiguous. Use a.any() or a.all()
>>
>> What would be the way to do this?
>>

maybe np.in1d(u, u+1)  or np.in1d(u,u+1).all()  is what you want

>>> help(np.in1d)
Help on function in1d in module numpy.lib.arraysetops:

in1d(ar1, ar2, assume_unique=False)
    Test whether each element of a 1D array is also present in a second array.

    Returns a boolean array the same length as `ar1` that is True
    where an element of `ar1` is in `ar2` and False otherwise.


Josef