Python "why" questions

Martin Gregorie martin at address-in-sig.invalid
Mon Aug 16 13:25:20 CEST 2010


On Mon, 16 Aug 2010 12:33:51 +1200, Gregory Ewing wrote:

> Ian Kelly wrote:
>> On Fri, Aug 13, 2010 at 11:53 AM, Martin Gregorie
>> <martin at address-in-sig.invalid> wrote:
> 
>>>       real sample[-500:750];
> 
>> Ugh, no.  The ability to change the minimum index is evil.
> 
> Not always; it can have its uses, particularly when you're using the
> array as a mapping rather than a collection.
>
Say you have intensity data captured from an X-ray goniometer from 160 
degrees to 30 degrees at 0.01 degree resolution. Which is most evil of 
the following?

1) real intensity[16000:3000]
   for i from lwb intensity to upb intensity
      plot(i/100, intensity[i]);

2) double angle[13000];
   double intensity[13000];
   for (int i = 0; i < 13000; i++)
	plot(angle[i], intensity[i]);

3) struct 
   { 
      double angle;
      double intensity 
   } measurement;
   measurement m[13000];
   for (int i = 0; i < 13000; i++)
	plot(m[i].angle, m[i].intensity);

4) double intensity[13000];
   for (int i = 0; i < 13000; i++)
      plot((16000 - i)/100, intensity[i])
 
To my mind (1) is much clearer to read and far less error-prone to write, 
while zero-based indexing forces you to use code like (2), (3) or (4), 
all of which obscure rather than clarify what the program is doing.


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



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