# [Numpy-discussion] *= operator not intuitive

Paul Anton Letnes paul.anton.letnes@gmail....
Wed Mar 16 09:59:45 CDT 2011

```On 16. mars 2011, at 15.57, Dag Sverre Seljebotn wrote:

> On 03/16/2011 02:35 PM, Paul Anton Letnes wrote:
>> Heisann!
>
> Hei der,
>
>> On 16. mars 2011, at 14.30, Dag Sverre Seljebotn wrote:
>>
>>> On 03/16/2011 02:24 PM, Paul Anton Letnes wrote:
>>>> Hi!
>>>>
>>>> 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 [1]: a = np.ones(5)
>> Here, a is numpy.float64:
>>>>> numpy.ones(5).dtype
>> dtype('float64')
>>
>>> In [2]: b = a
>>>
>>> In [3]: c = a * 2
>>>
>>> In [4]: b
>>> Out[4]: array([ 1.,  1.,  1.,  1.,  1.])
>>>
>>> In [5]: 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 [6]: b
>>> Out[6]: 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
> together...
>
> 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.

Paul.

```