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From: Bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: else ladders practice
Date: Fri, 1 Nov 2024 18:05:02 +0000
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On 01/11/2024 17:35, David Brown wrote:
> On 01/11/2024 16:59, Bart wrote:
>> On 01/11/2024 14:17, fir wrote:
>>> fir wrote:
>>>> Bart wrote:
>>>>> On 01/11/2024 12:55, fir wrote:
>>>>>> Bart wrote:
>>>>>>> On 01/11/2024 11:32, fir wrote:
>>>>>>>> Bart wrote:
>>>>>>>>> ral clear patterns here: you're testing the same variable 'n' 
>>>>>>>>> against
>>>>>>>>> several mutually exclusive alternatives, which also happen to be
>>>>>>>>> consecutive values.
>>>>>>>>>
>>>>>>>>> C is short of ways to express this, if you want to keep those
>>>>>>>>> 'somethings' as inline code (otherwise arrays of function 
>>>>>>>>> pointers or
>>>>>>>>> even label pointers could be use
>>>>>>>>
>>>>>>>> so in short this groupo seem to have no conclusion but is tolerant
>>>>>>>> foir various approaches as it seems
>>>>>>>>
>>>>>>>> imo the else latder is like most proper but i dont lkie it 
>>>>>>>> optically,
>>>>>>>> swich case i also dont like (use as far i i remember never in my 
>>>>>>>> code,
>>>>>>>> for years dont use even one)
>>>>>>>>
>>>>>>>> so i persnally would use bare ifs and maybe elses ocasionally
>>>>>>>> (and switch should be mended but its fully not clear how,
>>>>>>>>
>>>>>>>> as to those pointer tables im not sure but im like measurad it 
>>>>>>>> onece
>>>>>>>> and it was (not sure as to thsi as i dont remember exactly) slow 
>>>>>>>> maybe
>>>>>>>> dependant on architecture so its noth wort of use (if i remember
>>>>>>>> correctly)
>>>>>>>
>>>>>>> Well, personally I don't like that repetition, that's why I 
>>>>>>> mentioned
>>>>>>> the patterns. You're writing 'n' 5 times, '==' 5 times, and you're
>>>>>>> writing out the numbers 1, 2, 3, 4, 5.
>>>>>>>
>>>>>>> I also don't like the lack of exclusivity.
>>>>>>>
>>>>>>> However I don't need to use C. If those 'somethings' were simple, or
>>>>>>> were expressions, I could use syntax like this:
>>>>>>>
>>>>>>>     (n | s1, s2, s3, s4, s5)
>>>>>>>
>>>>>>
>>>>>> on a C ground more suitable is
>>>>>>
>>>>>> {s1,s2,s3,s4,s5)[n]
>>>>>>
>>>>>> //which is just array indexing
>>>>>
>>>>> No, it's specifically not array indexing, as only one of s1 - s5 is
>>>>> evaluated, or nothing is when n is not in range, eg. n is 100.
>>>>>
>>>>> You could try something like that in C:
>>>>>
>>>>>      int x;
>>>>>
>>>>>      x = ((int[]){(puts("a"),10), (puts("b"),20), (puts("c"), 30),
>>>>> (puts("d"),40)})[3];
>>>>>
>>>>>      printf("X=%d\n", x);
>>>>>
>>>>> The output is:
>>>>>
>>>>>     a
>>>>>     b
>>>>>     c
>>>>>     d
>>>>>     X=40
>>>>>
>>>>> Showing that all elements are evaluated first. If index is 100, the
>>>>> result is also undefined.
>>>>>
>>>>>
>>>> :-O
>>>> what is this, first time i see such thing
>>>>
>>> im surprised that it work, but in fact i meant that this syntax is 
>>> old c compatible but sych thing like
>>>
>>>
>>> {printf("ONE"), printf("TWO"), printf("THREE")} [2]
>>>
>>> shouldn evaluate al just the one is selected
>>> like in array tab[23] not eveluates something other than tab[23]
>>
>> It's a 'compound literal'. It allows you to have the same {...} 
>> initialisation data format, but anywhere, not just for initialing. 
>> However it always needs a cast:
>>
>>    (int[]){printf("ONE"), printf("TWO"), printf("THREE")}[2];
>>
>> This prints ONETWOTHREE, it also then indexes the 3rd value of the 
>> array, which is 5, as returned by printf, so this:
>>
>>    printf("%d\n", (int[]){printf("ONE"), printf("TWO"), 
>> printf("THREE")}[2]);
>>
>>    prints ONETWOTHREE5
>>
>>
> 
> What you have written here is all correct, but a more common method 
> would be to avoid having three printf's :
> 
> void shout_a_number(int n) {
>      printf( (const char* []) { "ONE", "TWO", "THREE" } [n] );
> }
> 
> That's more likely to match what people would want.

I was also trying to show that all elements are evaluated, so each has 
to have some side-effect to illustrate that.

A true N-way-select construct (C only really has ?:) would evaluate only 
one, and would deal with an out-of-range condition.

(In my implementations, a default/else branch value must be provided if 
the whole thing is expected to return a value.)