Alok Singhal wrote:
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) # First mask m1 = where(a > 5) a[m1] array([6, 7, 8, 9]) # Second mask 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?
(due to confusions with "a" in text I'll use x in place of "a") I believe the problem you are seeing (I'm not 100% certain yet) is that although it is possible to assign to an array-indexed array, that doing that twice over doesn't work since Python is, in effect, treating x[m1] as an expression even though it is on the left side. That expression results in a new array that the second indexing updates, but then is thrown away since it is not assigned to anything else. Your second try creates a temporary t which is also not a view into a so when you update t, a is not updated. try x[m1[0][m2]] = array([10,20]) instead. The intent here is to provide x with the net index array by indexing m1 first rather than indexing x first. (note the odd use of m1[0]; this is necessary since where() will return a tuple of index arrays (to allow use in multidimensional cases as indices, so the m1[0] extracts the array from the tuple; Since m1 is a tuple, indexing it with another index array (well, tuple containing an index array) doesn't work). Perry Greenfield