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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: "undefined behavior"?
Date: Sat, 15 Jun 2024 22:22:09 +0200
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On 15/06/2024 06:45, DFS wrote:
> On 6/14/2024 11:56 PM, Keith Thompson wrote:
>> DFS <nospam@dfs.com> writes:
>>> On 6/14/2024 9:39 PM, Keith Thompson wrote:
>>>> DFS <nospam@dfs.com> writes:
>>>> [...]
>>>>> During conversion, I got a Python error I don't remember seeing in 
>>>>> the past:
>>>>>
>>>>> "TypeError: list indices must be integers or slices, not float"
>>>>>
>>>>> because division returns a float, and some of the array addressing was
>>>>> like this: nums[i/2].
>>>> [...]
>>>> C's "/" operator yields a result with the type of the operands
>>>> (after
>>>> promotion to a common type).
>>>> Python's "/" operator yields a floating-point result.  For C-style
>>>> integer division, Python uses "//".  (Python 2 is more C-like.)
>>>
>>> I was surprised python did that, since every division used in the
>>> array addressing results in an integer.
>>>
>>> After casting i to an int before any array addressing, // works.
>>
>> I'm surprised you needed to convert i to an int.  I would think that
>  > just replacing nums[i/2] by nums[i//2] would do the trick,
>  > as long> as i always has an int value (note Python's dynamic typing). 
>   If i
>> is acquiring a float value, that's probably a bug, given the name.
> 
> I spotted the issue.  Just prior to using i for array addressing I said:
> i = N/2.
> 
> The fix is set i = int(N/2)
> 
> 
> 
>> But if you want help with your Python code, comp.lang.python is the
>> place to ask.
> 
> Thanks for your help, but David Brown is a Python developer and I'll ask 
> him python questions here whenever I care to.
> 

I consider myself more of a C developer than a Python developer, but I 
use Python regularly.  I would say that my knowledge of the C language 
and standard, while not as deep as some others here, covers a far higher 
proportion of the language than my knowledge of Python covers of Python. 
  But I think you can make good use of Python while knowing a smaller 
fraction of the language and library than for C.

> In the recent past you were involved in discussions on perl, Fortran and 
> awk, among other off-topics.
> 
> Rules for thee but not for me?
> 

If occasional questions or discussions about other languages pop up 
here, people will often answer them.  But for more in-depth discussions 
or questions, this is not the newsgroup - comp.lang.python is the place 
for Python questions.  (You'll also probably get better answers there 
than I can give.)

The rules are for everyone, but they are a bit fuzzy.  (And different 
posters have different levels of fuzziness.)