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From: bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: Loops (was Re: do { quit; } else { })
Date: Sun, 20 Apr 2025 00:34:57 +0100
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On 19/04/2025 22:07, Keith Thompson wrote:
> bart <bc@freeuk.com> writes:
>> On 19/04/2025 20:54, Keith Thompson wrote:
>>> bart <bc@freeuk.com> writes:
>>> [...]
>>>> I don't understand why you're happy using a crippled looping
>>>> mechanism, that is a PITA to use for the majority of iterating loops,
>>>> just so you have that flexibility for the whacky loops that are in the
>>>> minority, most of which can be trivially expressed with 'while'.
>>> [...]
>>> I agree.  You don't understand.  What confuses me is your complete
>>> lack
>>> of interest in trying to understand.
>>> Many of us are happy using C's for loop because we don't consider
>>> it to be crippled.  It is not a PITA to use it for the majority of
>>> iterating loops; we just use a very common coding pattern like
>>>       for (int i = 0; i < N; i ++) {
>>>           // ...
>>>       }
>>> We can write "i --" if we want the loop to run in reverse,
>>
>> That's not enough for it to run in reverse.
> 
> No, it's not.  I didn't mean to imply that it was.
> 
>>                                              I'd be interested in a
>> complete example.
> 
> Really?  It's such a simple example that I'm surprised you think you
> need my help.  But ok :
> 
>      for (int i = N-1; i >=0; i --) {
>          // ...
>      }
> 
> I wrote that off the top of my head.  I then wrote and compiled a
> program using it, and it worked.

I only recently noticed that the pattern was rather different from an 
upwards loop. For example, for a range of 0..N-1 inclusive the upper 
limit is usually written as N, with N-1 being something that a 1-based 
language is more likely to use.

I just wondered if you used a different approach. So it looks like 3 
things need to be changed, apart from reversing range limits:

                Start val  Comp  End Val  Increment
     Upwards       0        <      N       ++
     Downwards     N-1      >=     0       --

It's also just struck me that it would be more consistent if Upwards 
loops used this:

     Upward        0        <=     N-1     ++

I doubt this would be popular though with two extra characters to type 
(and possibly to omit or get wrong).

> 
> A similar loop with an iteration variable of an unsigned type would
> fail because `i >= 0` is always true.  I have some thoughts about
> how to deal with that, but I'll leave that aside.

Just insist on signed loop variables. Or if ints are 32 bits and start 
values are between 2**31 and 2**32, require an i64 loop variable.