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Full Discussion: scripting....guru's help needed
Top Forums Shell Programming and Scripting scripting....guru's help needed Post 89438 by cbkihong on Sunday 13th of November 2005 01:23:45 AM
Old 11-13-2005
The FAQ has two Q&A on performance tuning:

http://perldoc.perl.org/perlfaq3.htm...-run-faster%3f
http://perldoc.perl.org/perlfaq3.htm...less-memory%3f

Whether you can make a lot of difference with such advice I doubt it, as in my opinion Perl is already very efficient provided you do not use a particularly dumb way to do something. And those advice do not generally make much sense unless you are really familiar with the internals of Perl.

But there are a few obvious observations, though. For example, sequential search for an item in an array is dumb because you need to search for all items before you can tell an item is not in the array. If you use a hash to store the items as keys in the first place, you will get significant performance improvements.

This benchmark test tells you so:

Code:
#!/usr/bin/perl -w

sub randstr {
	my $num = shift;
	my $str = '';
	for (my $i = 0; $i < $num; $i++) {
		$str .= chr(int(rand(26)) + 65);
	}
	return $str;
}

sub str_exists_in_array(\@$) {
	my ($array, $str) = @_;
	for (@$array) {
		if ($str eq $_) {
			return 1;
		}
	}
	return 0;
}
sub str_exists_in_hash(\%$) {
	my ($hsh, $str) = @_;
	return exists $$hsh{$str};
}

srand();
my @array = ();
my %hsh = ();
for (my $i = 0; $i < 50000; $i++) {
	my $str = randstr(20);
	push(@array, $str);
	$hsh{$str} = 1;
}

# Query 1000 times
for (my $i = 0; $i < 1000; $i++) {
	my $strToTest = randstr(20);
	str_exists_in_hash(%hsh, $strToTest);          # (1)
#	str_exists_in_array(@array, $strToTest);        # (2)
}

The program generates 50000 items and then try to randomly generate another set of 1000 items to search for in these 50000 items. I expect most if not all of them will not match. Here we comment out line (2) as shown leaving (1) open, and use time() to time the result on my Windows XP system (with cygwin):

$ time perl test1.pl

real 0m0.895s
user 0m0.015s
sys 0m0.015s

Good. Less than 1 sec. Now with the line (1) commented out but not (2):

$ time perl test1.pl

real 4m14.705s
user 0m0.015s
sys 0m0.015s

The search now takes more than 4 minutes to complete. The two only differ in the way of searching. The data generation step is common for both tests. So we can conclude using a bad way to search can hurt performance when the number of searches is large.
Last edited by cbkihong; 11-14-2005 at 07:58 AM..
 

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LEARN ABOUT XFREE86

shift

shift(1)                                                           User Commands                                                          shift(1)

NAME

       shift - shell built-in function to traverse either a shell's argument list or a list of field-separated words

SYNOPSIS

   sh
       shift [n]

   csh
       shift [variable]

   ksh
       * shift [n]

DESCRIPTION

   sh
       The positional parameters from $n+1 ... are renamed $1 ... . If n is not given, it is assumed to be 1.

   csh
       The  components of argv, or variable, if supplied, are shifted to the left, discarding the first component. It is an error for the variable
       not to be set or to have a null value.

   ksh
       The positional parameters from $n+1 $n+1 ... are renamed $1 ..., default n is 1. The parameter n can  be  any  arithmetic  expression  that
       evaluates to a non-negative number less than or equal to $#.

       On this man page, ksh(1) commands that are preceded by one or two * (asterisks) are treated specially in the following ways:

       1.  Variable assignment lists preceding the command remain in effect when the command completes.

       2.  I/O redirections are processed after variable assignments.

       3.  Errors cause a script that contains them to abort.

       4.  Words,  following a command preceded by ** that are in the format of a variable assignment, are expanded with the same rules as a vari-
           able assignment. This means that tilde substitution is performed after the = sign and word splitting and file name generation  are  not
           performed.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE         |      ATTRIBUTE VALUE        |
       +-----------------------------+-----------------------------+
       |Availability                 |SUNWcsu                      |
       +-----------------------------+-----------------------------+

SEE ALSO

       csh(1), ksh(1), sh(1), attributes(5)

SunOS 5.10                                                          15 Apr 1994                                                           shift(1)
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