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From: Janis Papanagnou <janis_papanagnou+ng@hotmail.com>
Newsgroups: comp.lang.c
Subject: Re: else ladders practice
Date: Sat, 9 Nov 2024 07:54:44 +0100
Organization: A noiseless patient Spider
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On 04.11.2024 23:25, David Brown wrote:
> 
> If you have a function (or construct) that returns a correct value for
> inputs 1, 2 and 3, and you never pass it the value 4 (or anything else),
> then there is no undefined behaviour no matter what the code looks like
> for values other than 1, 2 and 3.  If someone calls that function with
> input 4, then /their/ code has the error - not the code that doesn't
> handle an input 4.

Well, it's a software system design decision whether you want to
make the caller test the preconditions for every function call,
or let the callee take care of unexpected input, or both.

We had always followed the convention to avoid all undefined
situations and always define every 'else' case by some sensible
behavior, at least writing a notice into a log-file, but also
to "fix" the runtime situation to be able to continue operating.
(Note, I was mainly writing server-side software where this was
especially important.)

That's one reason why (as elsethread mentioned) I dislike 'else'
to handle a defined value; I prefer an explicit 'if' and use the
else for reporting unexpected situations (that practically never
appear, or, with the diagnostics QA-evaluated, asymptotically
disappearing).

(For pure binary predicates there's no errors branch, of course.)

Janis

PS: One of my favorite IT-gotchas is the plane crash where the
code specified landing procedure functions for height < 50.0 ft
and for height > 50.0 ft conditions, which mostly worked since
the height got polled only every couple seconds, and the case
height = 50.0 ft happened only very rarely due to the typical
descent characteristics during landing.