Python "why" questions

Martin Gregorie martin at address-in-sig.invalid
Tue Aug 17 19:36:13 CEST 2010


On Tue, 17 Aug 2010 10:22:27 -0500, Robert Kern wrote:

> On 8/16/10 11:10 PM, Steven D'Aprano wrote:
>> On Mon, 16 Aug 2010 22:56:20 -0500, Robert Kern wrote:
>>
>>> On 8/16/10 9:29 PM, Roy Smith wrote:
>>>> In article<i4cqg0$olf$3 at lust.ihug.co.nz>,
>>>>    Lawrence D'Oliveiro<ldo at geek-central.gen.new_zealand>   wrote:
>>>>
>>>>> In message<roy-EE1B7F.21001716082010 at news.panix.com>, Roy Smith
>>>>> wrote:
>>>>>
>>>>>> 5) real intensity[160.0 : 30.0 : 0.01]
>>>>>
>>>>> How many elements in that array?
>>>>>
>>>>> a) 2999
>>>>> b) 3000
>>>>> c) neither of the above
>>>>
>>>> c) neither of the above.  More specifically, 13,001 (if I counted
>>>> correctly).
>>>
>>> 13000, actually. Floating point is a bitch.
>>>
>>> [~/Movies]
>>> |1>  import numpy
>>>
>>> [~/Movies]
>>> |2>  len(numpy.r_[160.0:30.0:-0.01])
>>> 13000
>>
>>
>> Actually, the answer is 0, not 13000, because the step size is given as
>> 0.01, not -0.01.
>>
>>>>> import numpy
>>>>> len(numpy.r_[160.0:30.0:-0.01])
>> 13000
>>>>> len(numpy.r_[160.0:30.0:0.01])
>> 0
> 
> Roy wasn't using numpy/Python semantics but made-up semantics (following
> Martin Gregorie's made-up semantics to which he was replying) which
> treat the step size as a true size, not a size and direction. The
> direction is determined from the start and stop parameters. It's an
> almost-reasonable design.

That wasn't a made-up example: AFAICR and ignoring a missing semi-colon 
it was an Algol 68 snippet. The semantics of the for statement and the 
use of lwb and upb operators to extract the bounds from a 1-dimensional 
array are correct A68, but OTOH its a very long time since I last 
programmed in that language. I used that rather than Python because Algol 
68 supports the non-zero lower bound and treats the array limits as 
attributes of the array.


-- 
martin@   | Martin Gregorie
gregorie. | Essex, UK
org       |



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