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From: bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: Checking the loop variable after the loop has ended (Was: Loops
 (was Re: do { quit; } else { }))
Date: Fri, 18 Apr 2025 18:40:52 +0100
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On 18/04/2025 17:46, Janis Papanagnou wrote:
> On 18.04.2025 17:57, bart wrote:
>> On 18/04/2025 16:24, Kaz Kylheku wrote:
>>> On 2025-04-17, Lew Pitcher <lew.pitcher@digitalfreehold.ca> wrote:
>>>> [...]
>>> [...]
>>>     for (n = NUM_ENTRIES - 1; ...
>>
>> That looks like another inconsistency in the syntax (and another error
>> opportunity).
>>
>> Since the loop for upward and downward iterations between 0 and N-1
>> inclusve would be:
>>
>>     for (i = 0;   i <  N; ++i)
>>     for (i = N-1; i >= 0; --i)
>>
>> 'N-1' is what I would have to write; it's odd that C people have write
>> it too. Unless perhaps you do this:
>>
>>     for (i = N; i-- > 0;)
>>
>> Here you now have a genuine 2-part loop!
>>
>> I guess this is C being 'flexible', in being able to reinvent for-loops
>> each time.
> 
> It is perfectly fine to write your loops in "C" as
> 
>      for (i = 1; i <= N; ++i)
>      for (i = N; i >= 1; --i)
> 
> (and access your array elements as [i-1] if you wish).

That's never going to happen. Only, possibly, if you have to port a 
1-based algorithm to C, without risking the bugs that will creep in if 
you try and convert it to 0-based.

Sure, with 1-based, upward and downward counting loops can be more 
symmetrical.



> 
> You should be aware that the problem you have here is *not* a "C"
> "loop-problem", it's a problem of the low-level _"C" arrays_ you
> have to use; they force you to define and use bounds of 0 and N-1
> respectively if you want to use an array of N elements.

I can use 0-base arrays in my language, and yes, for loops to iterate 
over bounds could involve 0 and N-1:

     for i 0 .. N-1 do

but they can also look like this:

    for i in A.lwb .. A.upb do

or just:

    for i in A.bounds do
    for i, x in A do                # iterate over bounds and values

'0' and 'N-1' or 'A.len-1', don't appear at all.

So while 0-based can be more troublesome, it can still be a language 
problem rather a 0-based one.