japhy
Enthusiast
Sep 8, 2000, 6:24 AM
Post #8 of 15
(3310 views)
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Hmm, I might end up writing an article on what I'm about to explain. It's about programming more intelligently, more or less.
Take a simple function:
<BLOCKQUOTE><font size="1" face="Arial,Helvetica,sans serif">code:</font><HR>
# DOW($N) -- returns day of week for numerical week day (0 - 6)
sub DOW {
my @days = qw( Sunday Monday Tuesday Wednesday Thursday Friday Saturday );
return $days[shift];
}
</pre><HR></BLOCKQUOTE>
That's all well and good. It does what I ask of it -- returns the string for the day of the week. But let's look at something a bit more involving:
<BLOCKQUOTE><font size="1" face="Arial,Helvetica,sans serif">code:</font><HR>
sub getDate {
my @months = qw(
January February March April May June
July August September October November December
);
my @days = qw(
Sunday Monday Tuesday Wednesday
Thursday Friday Saturday
);
# get abbreviated names...
my @smon = map substr($_,0,3), @months;
my @sday = map substr($_,0,3), @days;
my ($mode,$time) = @_;
$mode = 'l' if not $mode; # default to long format
# get date information
# default to current time
my ($sec,$min,$hour,$day,$mon,$year,$wday) = localtime(defined $time ? $time : time);
# short date
if (lc($mode) eq 's') {
return sprintf "%s, %s %d, %d", $sday[$wday], $smon[$mon], $day, $year+1900;
}
# long date (good kisser?) (that was a joke)
if (lc($mode) eq 'l') {
return sprintf "%s, %s %d, %d, at %02d  02d 02d",
$days[$wday], $months[$mon], $day, $year + 1900, $hr, $min, $sec;
}
die "bad mode ($mode), expected 's' or 'l'";
}
</pre><HR></BLOCKQUOTE>
By the way, those angry icons (WHICH I WISH I COULD SHUT OFF) are a colon followed by a percent sign.
Now, let's say I have to call this 10,000 times. Here's a benchmark:
timethis 10000: 15 wallclock secs (15.49 usr + 0.00 sys = 15.49 CPU)
Now, I'm going to make a magical adjustment to this function... done. Now let's check the running time:
timethis 10000: 3 wallclock secs ( 4.51 usr + 0.00 sys = 4.51 CPU)
WOW! What could I possibly have done to get nearly a 350% speed increase? I only declared the @months, @days, @smon, and @sday arrays ONCE. They won't ever CHANGE, will they? Of course not, so why declare them EVERY time the function gets called?
But how did I do that? I didn't create global variables (since that would interfere if I wanted another array called @days later). I didn't make my() variables in the main program, pretty much for the same reason. But I didn't make my() variables IN the subroutine either.
I declared them in a block, and defined the subroutine in that block:
<BLOCKQUOTE><font size="1" face="Arial,Helvetica,sans serif">code:</font><HR>
{
my $x = 1;
sub next_x { return $x++ }
}
</pre><HR></BLOCKQUOTE>
The next_x() function is the ONLY function that can see that $x variable, and the ONLY function that can modify it. So I intelligently rewrote my function as:
<BLOCKQUOTE><font size="1" face="Arial,Helvetica,sans serif">code:</font><HR>
{
my @months = qw(
January February March April May June
July August September October November December
);
my @days = qw(
Sunday Monday Tuesday Wednesday
Thursday Friday Saturday
);
# get abbreviated names...
my @smon = map substr($_,0,3), @months;
my @sday = map substr($_,0,3), @days;
sub getDate {
my ($mode,$time) = @_;
# rest of function....
}
}
</pre><HR></BLOCKQUOTE>
Defining those arrays over and over again was a silly thing to do, so I fixed that.
Oh, and there might be VERY little need to use this getDate() function anyway. Calling localtime() in scalar context returns a highly usable string-formatted date. Look into it.
<BLOCKQUOTE><font size="1" face="Arial,Helvetica,sans serif">code:</font><HR>
$date = localtime; # or localtime $seconds;
</pre><HR></BLOCKQUOTE>
------------------
Jeff "japhy" Pinyan -- accomplished author, consultant, hacker, and teacher
[This message has been edited by japhy (edited 09-08-2000).]
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