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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: else ladders practice
Date: Tue, 5 Nov 2024 14:29:21 +0100
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On 05/11/2024 13:42, Waldek Hebisch wrote:
> Bart <bc@freeuk.com> wrote:
>>
>> Then we disagree on what 'multi-way' select might mean. I think it means
>> branching, even if notionally, on one-of-N possible code paths.
> 
> OK.

I appreciate this is what Bart means by that phrase, but I don't agree 
with it.  I'm not sure if that is covered by "OK" or not!

>   
>> The whole construct may or may not return a value. If it does, then one
>> of the N paths must be a default path.
> 
> 
> You need to cover all input values.  This is possible when there
> is reasonably small number of possibilities.  For example, switch on
> char variable which covers all possible values does not need default
> path.  Default is needed only when number of possibilities is too
> large to explicitely give all of them.  And some languages allow
> ranges, so that you may be able to cover all values with small
> number of ranges.
> 

I think this is all very dependent on what you mean by "all input values".

Supposing I declare this function:

// Return the integer square root of numbers between 0 and 10
int small_int_sqrt(int x);


To me, the range of "all input values" is integers from 0 to 10.  I 
could implement it as :

int small_int_sqrt(int x) {
	if (x == 0) return 0;
	if (x < 4) return 1;
	if (x < 9) return 2;
	if (x < 16) return 3;
	unreachable();
}

If the user asks for small_int_sqrt(-10) or small_int_sqrt(20), that's 
/their/ fault and /their/ problem.  I said nothing about what would 
happen in those cases.

But some people seem to feel that "all input values" means every 
possible value of the input types, and thus that a function like this 
should return a value even when there is no correct value in and no 
correct value out.

This is, IMHO, just nonsense and misunderstands the contract between 
function writers and function users.

Further, I am confident that these people are quite happen to write code 
like :

// Take a pointer to an array of two ints, add them, and return the sum
int sum_two_ints(const int * p) {
	return p[0] + p[1];
}

Perhaps, in a mistaken belief that it makes the code "safe", they will add :

	if (!p) return 0;

at the start of the function.  But they will not check that "p" actually 
points to an array of two ints (how could they?), nor will they check 
for integer overflow (and what would they do if it happened?).



A function should accept all input values - once you have made clear 
what the acceptable input values can be.  A "default" case is just a 
short-cut for conveniently handling a wide range of valid input values - 
it is never a tool for handling /invalid/ input values.