Re: OT:Perl %= operator

[will trillich <will@serensoft.com>]

On Thu, Aug 02, 2001 at 08:28:32PM -0400, Ken Januski wrote:
> I've been reading the most recent Randall Schwartz column in SysAdmin
> magazine and his code has this line:
> ($start +=1) %=7;
> If $start = 1 then I think that this line means 2%7. I believe that the
> intended result is 2 but I'm just not sure what 2 % 7 equals. I know
> that 7%2 = 1 but I'm not sure what happens with modulus when the 2nd
> number is greater than the first. I've gone through about 6 books
> looking for the answer but none of them consider the case where the 2nd
> number is larger than the first.

% is modulo, which is the remainder after division:

	print $value - (int($value / $divisor) * $divisor),"==",$value % $divisor;

for any $value and $divisor.

	@weekday = qw(sun mon tue wed thu fri sat);
	$offset = 1; # maybe our span starts on monday
	foreach $day ( 1..60 ) {
		my $wd = $day + $offset;
		print "day $day is a ",$weekday[$wd % 7]," (",$wd % 7,")\n";
	}

with randall's algorithm, he's wrapping his variable around
so that it always stays between 0<=$start<7 with %=:

	$start++;
	$start = $start % 7;
	# or ($start++) %= 7 for short

he could just calculate the modulo when it's needed and let
$start creep higher and higher -- after all, 52 % 7 is still
3 -- but <guess> it may take a few more clock cycles to
calculate modulos of larger values </guess> but i don't really
know for sure.

$val % $val is not to be confused with '%hash' as in

	%mon = (
		jan => 31,
		feb => ($year % 400 ? 29 : $year % 100 ? 28 : $year % 4 ? 29 : 28),
		mar => 31,
		apr => 30,
		#...
	);
	$hash{$key} %= 31;

or

	my $href = \%hash;
	&function( %$href );

perl can make your head swim, huh?

-- 
DEBIAN NEWBIE TIP #3 from Will Trillich <will@serensoft.com>
:
Wondering how to FIND WHICH PACKAGE CONTAINS x-y-or-z? Just enter
"dpkg -S part/of/path" and you'll get a list of all packages that
affect paths like that. For an example, try "dpkg -S http".

Also see http://newbieDoc.sourceForge.net/ ...