binary constants (??)

[Dr A. N. Walker]

In article <1989Nov29.164913.1794 at utzoo.uucp> henry at utzoo.uucp (Henry
Spencer) writes:
>In article <20989 at mimsy.umd.edu> chris at mimsy.umd.edu (Chris Torek) writes:
>> [proposes]	       <base>r<text>, 2r11001 = 8r31 = 25 = 16r19.
> [introduces]  a new class of subtle errors.  (How many programs would
>notice if 16r19 was mistyped as 15r19?  Or 19r16?)

	Many moons ago, when my students were writing their programs
in a sane language such as Chris suggests, it took me *ages* to spot
the bug in [re-written in the D or P idiom!]:

		int a[20];
		for (r = 1; r <= 10; r++)
			a[2r-1] = r;
		/* Bzzt!  Core dropped! Subscript out of bounds! */

Well, r runs from 1 to 10, so 2r-1 runs from 1 to 19, so that should
be OK.  You used the same array name, didn't you?  No mis-typings?
Semi-colons all in the right place.  Hmm.  Umm.  Shouldn't there be
a * between 2 and r?  I'm surprised the compiler accepted that, quite
neat getting implicit multiplication.

	CLANG!  2r-1 means -1 in binary.  (Actually, it should still be
a syntax error, but one can see why a compiler might allow it.)

-- 
Andy Walker, Maths Dept., Nott'm Univ., UK.
anw at maths.nott.ac.uk