[Numpy-discussion] numarray iterator question
Robert Kern
robert.kern@gmail....
Wed Mar 10 18:22:40 CST 2010
On Wed, Mar 10, 2010 at 18:19, Neal Becker <ndbecker2@gmail.com> wrote:
> This is a bit confusing to me:
>
> import numpy as np
>
> u = np.ones ((3,3))
>
> for u_row in u:
> u_row = u_row * 2 << doesn't work
>
> print u
> [[ 1. 1. 1.]
> [ 1. 1. 1.]
> [ 1. 1. 1.]]
>
> for u_row in u:
> u_row *= 2 << does work
> [[ 2. 2. 2.]
> [ 2. 2. 2.]
> [ 2. 2. 2.]]
>
> Naively, I'm thinking a *= b === a = a * b.
http://docs.python.org/reference/simple_stmts.html#augmented-assignment-statements
"""An augmented assignment expression like x += 1 can be rewritten as
x = x + 1 to achieve a similar, but not exactly equal effect. In the
augmented version, x is only evaluated once. Also, when possible, the
actual operation is performed in-place, meaning that rather than
creating a new object and assigning that to the target, the old object
is modified instead."""
> Is this behavior expected? I'm asking because I really want:
>
> u_row = my_func (u_row)
Iterate over indices instead:
for i in range(len(u)):
u[i] = my_func(u[i])
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
-- Umberto Eco
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