Re: Loops (was Re: do { quit; } else { })

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.