Loop semantics: FOR C := A TO B DO ...

Thu Feb 9 09:39:45 AEST 1989

>     Find a suitable translation in C for
>     a, b, c: CHAR
>     ...
>     FOR c := a TO b DO <Body>
> 
> that correctly accounts for c's remaining inside its subrange 0..127 or
> 0..255 or whatever.

I haven't seen the following answer yet.  (I'm sure that net propagation
delay means that I'll see three such answers tomorrow, but anyway....)

As a non-speaker of Modula2, I am assuming that the original statement
calls for the greater of 0 and b-a+1 iterations, i.e., including both the
a and b cases if a >= b but nothing if a < b.  Otherwise it'd be simple.

The problem is of course that c might loop through all values that its
type can hold, so there's no single test such as c < b that you can use
in a for- or while-header, no matter how you fiddle the increment side.
However, this doesn't mean that you have to repeat the whole body.
Just provide a separate test before the first iteration:

	if (a <= b) {
		c = a;
		do {
			<Body>
		} while (c++ < b);
	}

This does what was asked for -- and yes, I tested it.  So you might
keep it in mind the next time you need a loop whose index that might
exactly reach the minimum legal maximum value for its data type.

But I don't think it really meets the requirement of a *suitable* C
translation... it's more of a transliteration.  A suitable C translation
would use the suitable datatype for a loop index whose absolute value
doesn't exceed 32767: int.  Since the loop index isn't going to reach 32767,
the loop can now be coded in (wait "for" it) a straight"for"ward fashion:

	for (c = a; c <= b; ++c) {
		<Body>
	}

Of course, the loop body might have to be altered if it does something
that depends on the type of the loop index being char.  Such things
are relatively rare, usually involving & operations.

Mark Brader, Toronto		sed -e "s;??\\([-=(/)'<!>]\\);?\\\\?\\1;g"
utzoo!sq!msb, msb at sq.com	will fix them...	-- Karl Heuer