# [Numpy-discussion] numarray.where confusion

Alok Singhal as8ca at virginia.edu
Wed May 26 07:49:40 CDT 2004

```Hi,

I am having trouble understanding how exactly "where" works in
numarray.

What I am trying to do:

I am preparing a two-level mask in an array and then assign values to
the array where both masks are true:

>>> from numarray import *
>>> a = arange(10)
>>> m1 = where(a > 5)
>>> a[m1]
array([6, 7, 8, 9])
>>> m2 = where(a[m1] < 8)
>>> a[m1][m2]
array([6, 7])
>>> # So far so good
>>> # Now change some values
>>> a[m1][m2] = array([10, 20])
>>> a[m1][m2]
array([6, 7])
>>> a
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
>>> # Didn't  work
>>> # Let's try a temporary variable
>>> t = a[m1]
>>> t[m2]
array([6, 7])
>>> t[m2] = array([10, 20])
>>> t[m2], t
(array([10, 20]), array([10, 20,  8,  9]))
>>> a
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

So, my assignment to a[m1][m2] seems to work (no messages), but it
doesn't produce the effect I want it to.

I have read the documentation but I couldn't find something that would
explain this behavior.

So my questions:

- did I miss something important in the documentation,
- I am expecting something I shouldn't, or
- there is a bug in numarray?

Thanks,
Alok

--
Alok Singhal (as8ca at virginia.edu)       __
Graduate Student, dept. of Astronomy   /  _
University of Virginia                 \_O \
http://www.astro.virginia.edu/~as8ca/    __/

```