[Numpy-discussion] *= operator not intuitive
Paul Anton Letnes
paul.anton.letnes at gmail.com
Wed Mar 16 10:59:45 EDT 2011
On 16. mars 2011, at 15.57, Dag Sverre Seljebotn wrote:
> On 03/16/2011 02:35 PM, Paul Anton Letnes wrote:
> Hei der,
>> On 16. mars 2011, at 14.30, Dag Sverre Seljebotn wrote:
>>> On 03/16/2011 02:24 PM, Paul Anton Letnes wrote:
>>>> This little snippet of code tricked me (in a more convoluted form). The *= operator does not change the datatype of the left hand side array. Is this intentional? It did fool me and throw my results quite a bit off. I always assumed that 'a *= b' means exactly the same as 'a = a * b' but this is clearly not the case!
>>> In : a = np.ones(5)
>> Here, a is numpy.float64:
>>> In : b = a
>>> In : c = a * 2
>>> In : b
>>> Out: array([ 1., 1., 1., 1., 1.])
>>> In : a *= 2
>> So since a is already float, and b is the same object as a, the resulting a and b are of course floats.
>>> In : b
>>> Out: array([ 2., 2., 2., 2., 2.])
>> This is not the case I am describing, as in my case, a was of dtype integer.
>> Or did I miss something?
> I was just trying to demonstrate that it is NOT the case that "a = a * 2
> is exactly the same as a *= 2". If you assume that the two statements
> are the same, then it does not make sense that b = [1, 1, ...] the first
> time around, but b = [2, 2, 2...] the second time around. And in trying
> to figure out why that happened, perhaps you'd see how it all fits
> OK, it perhaps wasn't a very good explanation...
> Dag Sverre
I see, I misunderstood your point. That's another interesting aspect of this, though.
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