select part of array using two conditions

Bill Baxter wbaxter at gmail.com
Wed Oct 11 13:04:27 CDT 2006


On 10/11/06, Nils Wagner <nwagner at iam.uni-stuttgart.de> wrote:
> Mark Bakker wrote:
> > Hello -
> >
> > I want to select part of an array using two conditions.
> > I know how to do it with one condition (and it works great), but when
> > I use two conditions I get an error message?
> > This is probably easy, but I cannot figure it out.
> > Thanks for any help, Mark
> >
> > >>> a = arange(10)
> > >>> a
> > array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
> > >>> a[ a>2 ]
> > array([3, 4, 5, 6, 7, 8, 9])
> > >>> a[ a>2 and a<8 ]
> > Traceback (most recent call last):
> >   File "<pyshell#52>", line 1, in ?
> >     a[ a>2 and a<8 ]
> > ValueError: The truth value of an array with more than one element is
> > ambiguous. Use a.any() or a.all()
> >
> > ------------------------------------------------------------------------
> >
> a[ (a>2) & (a<8) ]
>

& is bitwiase and which works fine for the booleans you get back from
comparisons like (a>2).  So in this case & is ok.

For arrays with non-boolean values (any non-zero is True) use logical_and:

    a[ logical_and(c, d) ]

Logical_and works always to give you the boolean result.
'&' gives you the bitwise result, which is sometimes equivalent to the
boolean result.

--bb

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