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Date: Sat, 15 Jun 2024 07:39:40 -0400
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From: DFS <nospam@dfs.com>
Subject: Re: "undefined behavior"?
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On 6/15/2024 1:03 AM, Janis Papanagnou wrote:
> On 15.06.2024 06:45, DFS wrote:
>> On 6/14/2024 11:56 PM, Keith Thompson wrote:
>>> DFS <nospam@dfs.com> writes:
>>>>
>>>> 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)
> 
> Given what Keith suggested, and assuming N is an integer, wouldn't it
> be more sensible to use the int division operator '//' and just write
> i = N // 2 ?  I mean, why do a float division on integer operands and
> then again coerce the result to int again?

Python bytecode
$ python3 -m dis file.py

i = N//2
  1068 LOAD_NAME               12 (N)
  1070 LOAD_CONST              10 (2)
  1072 BINARY_FLOOR_DIVIDE
  1074 STORE_NAME              10 (i)

i = int(N/2)
  1068 LOAD_NAME               11 (int)
  1070 LOAD_NAME               12 (N)
  1072 LOAD_CONST              10 (2)
  1074 BINARY_TRUE_DIVIDE
  1076 CALL_FUNCTION            1
  1078 STORE_NAME              10 (i)

Fewer ops is better, so I'll go with your suggestion.  Good catch.