Re: size_t vs long.

[Paul Eggert <eggert@cs.ucla.edu>]

On 11/17/22 18:11, Maciej W. Rozycki wrote:

>>> for (size_t i = 41; i < sizeof A / sizeof A[0]; --i) {
>>>     A[i] = something_nice;
>>> }

>  ... what exactly is bogus with the first loop?
> 
>   AFAICT if index 41 is within the bounds of A, it fills elements [0..41]
> with something_nice and otherwise it does nothing.

Yes, and that's precisely what is bogus about it. Most people who read 
that code won't easily see that you're summarizing it correctly 
(assuming INT_MAX < SIZE_MAX). And if I saw that code in a real program, 
my first guess - and it most likely would be the correct guess - is that 
the *author* of the code didn't know what it does, it's so confusingly 
written.

Certainly Alejandro was confused by that bogus loop, as his most recent 
email said the following:

> For code that means exactly the same as
> 
> for (size_t i = 41; i < nitems(A); --i) {
>     A[i] = something_nice;
> }
> 
> 
> we need:
> 
> 
> for (size_t i = 0; i < nitems(A) && i <= 41; ++i) {
>     A[i] = something_nice;
> }
> 
> or
> 
> for (idx_t i = 0; i < nitems(A) && i <= 41; ++i) {
>     A[i] = something_nice;
> }
> 
> or
> 
> for (idx_t i = 41; i < nitems(A) && i > 0; --i) {
>     A[i] = something_nice;
> }
> 
> (always assuming SIZE_MAX > INT_MAX.) 

and if we call those four loops A, B, C and D, then Alejandro was 
incorrect about B and C because they are not equivalent to A. And 
although D is equivalent to A, D is still confusing and is still Bad Code.

Code like this should be written more the way you said it. E.g.:

    idx_t nice_count = 42;
    if (nice_count <= nitems(A))
      for (idx_t i = 0; i < nice_count; i++)
        A[i] = something_nice;

This is much easier to understand than the other alternatives given so 
far, for reasons that I hope are obvious. And as a bonus, it doesn't 
assume INT_MAX < SIZE_MAX.