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Date: Tue, 16 Apr 2013 16:48:20 -0400
From: Dave Angel <davea@davea.name>
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To: python-list@python.org
Subject: Re: a couple of things I don't understand wrt lists
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On 04/16/2013 11:37 AM, aaB wrote:
> hello,
>
> I am a beginner programmer. I started learning programming about a year and a
> half ago, using C. I picked up python a few months ago, but only wrote very few
> scripts.
>
> I am currently trying to learn more about the python way of doing things by
> writing a script that generates png images using a 1D cellular automaton.
>
> While writing preliminary code for that project, I ran into a behaviour that I
> don't understand.
> I am using python 2.7 on a linux system.
>
> I represent the CA's rule with a list of integers, of value 1 or 0.
> Here is the function I use to generate the list:
>
> def get_rule(rulenum):
>    rule = []
>    while rulenum > 0:
>      rule.append(rulenume % 2)
>      rulenum /= 2
>    while len(rule) < 8:
>      rule.append(0)
>    rule.reverse()
>    return rule
>
> if i call it by writing:
>
> rule = getrule(int(8))
>
> and then call:
>
> print rule
>
> the output is good:
>
> [0, 0, 0, 0, 1, 0, 0, 0]
>
>
> I then tried to print each item of the list using a for loop:
>

There are copy/paste errors in your following pieces.  Did you retype 
them instead of using the clipboard?

> for i in range(rule):
>    print rule[i]
>
> the output is, as expected:
> 0
> 0
> 0
> 0
> 1
> 0
> 0
> 0
>

Here's what I get, and how I fix it:

 >>> rule = [0,0,0,0,1,0,0,0]
 >>> for i in range(rule):
...     print i
...
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: range() integer end argument expected, got list.
 >>> for i in range(len(rule)):
...     print rule[i]
...
0
0
0
0
1
0
0
0


> but when I do:
>
> for i in rule:
>    print rule[i]
You should be printing i here, not rule[i]

>
> I get the "complement":
> 1
> 1
> 1
> 1
> 0
> 1
> 1
> 1

I don't.  And don't expect to.  It's nothing like the complement.

 >>> for i in rule:
...     print rule[i]
...
0
0
0
0
0
0
0
0

Anyway, to fix it, just print the value, don't try to use it as a subscript.

 >>> for value in rule:
...     print value
...
0
0
0
0
1
0
0
0

>
> There must be something I didn't understand correctly in the for statement, but
> I really can't think of a reason why the output is what it is.
> I tried this using the interactive console, and the results are the same,
> whatever the length of the list, i always get the complement of my bit pattern.

You should never get the complement of the bit pattern with any code 
you've shown above.
>

Hope that helps.


-- 
DaveA