Perl 学习手札之六： Loops

loops are used for repeating code

eg: while(condition){

　　statement1;

　　statement2;

}

perl provide two basic types of loops

while/until loops

for/foreach loops

while.pl

#!/usr/bin/perl
use strict;
use warnings;

main(@ARGV);

sub main
{
    open(FH, "linesfile.txt");
    while(my $line = <FH>){
#        message("$line");    
        print $line;
    }
    
    close FH;
}

sub message
{
    my $m = shift or return;
    print("$m\n");
}

sub error
{
    my $e = shift || 'unkown error';
    print("$0: $e\n");
    exit 0;
}

first setup a file named linesfile.txt, which content is:

This is line 1
This is line 2
This is line 3
This is line 4
This is line 5

until.pl

#!/usr/bin/perl
use strict;
use warnings;

main(@ARGV);

sub main
{
    my $line;
    open(FH, "linesfile.txt");
    #until(($line=<FH>)=~/3/){
    #    message("$line");
    #}
    message("$line") until  ($line=<FH>)=~/3/;
    close FH;
}

sub message
{
    my $m = shift or return;
    print("$m\n");
}

sub error
{
    my $e = shift || 'unkown error';
    print("$0: $e\n");
    exit 0;
}

this script will print the content of file linesfile.txt until the script match 3

foreach.pl

#!/usr/bin/perl
use strict;
use warnings;

main(@ARGV);

sub main
{
    my @list= ("one","two","three","four","five");
#    foreach my $s (@list){
#        message($s);
#    }

#    foreach (@list){
#        message($_);
#    }
    message($_) foreach @list;
}

sub message
{
    my $m = shift or return;
    print("$m\n");
}

sub error
{
    my $e = shift || 'unkown error';
    print("$0: $e\n");
    exit 0;
}

注意注释掉的部分，写法和之前的condition语句一样。

loop.pl

#!/usr/bin/perl
use strict;
use warnings;

main(@ARGV);

sub main
{
    my @list= ("one","two","three","four","five");
    my $i;
    for($i=0;$list[$i];$i++){
        message($list[$i]);
    }
   
}

sub message
{
    my $m = shift or return;
    print("$m\n");
}

sub error
{
    my $e = shift || 'unkown error';
    print("$0: $e\n");
    exit 0;
}

C分格的for 循环写法

control.pl

#!/usr/bin/perl
# loops.pl by Bill Weinman <http://bw.org/contact/>
# Copyright (c) 2010 The BearHeart Group, LLC
#
use strict;
use warnings;

main(@ARGV);

sub main
{
    open(FH, "linesfile.txt");
    while(my $line=<FH>){
        #last if $line =~/3/;
        next if $line =~/3/;
        print $line;
    }
    close FH;
}

sub message
{
    my $m = shift or return;
    print("$m\n");
}

sub error
{
    my $e = shift || 'unkown error';
    print("$0: $e\n");
    exit 0;
}

注意注释掉的部分，在使用last和next的时候，得到的结果的不同。

profix.pl

#!/usr/bin/perl
use strict;
use warnings;

main(@ARGV);

sub main
{
    my $line='';
    open(FH, "linesfile.txt");
    #print while(<FH>);
    #print $_ while($_=<FH>);
    print $line=<FH> until($line=~/3/);
    close FH;
}

sub message
{
    my $m = shift or return;
    print("$m\n");
}

sub error
{
    my $e = shift || 'unkown error';
    print("$0: $e\n");
    exit 0;
}

注意：注释的两行得到的结果是等价的，这里描述了$_的用法。

profix2.pl

#!/usr/bin/perl
use strict;
use warnings;

main(@ARGV);

sub main
{
    my @list = ('one','two','three','four','five');
    print "$_\n" foreach @list;
}

sub message
{
    my $m = shift or return;
    print("$m\n");
}

sub error
{
    my $e = shift || 'unkown error';
    print("$0: $e\n");
    exit 0;
}

在用到foreach的时候的用法；