Re: "undefined behavior"?

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.