Perl 学习手札之十：subroutines

understanding subroutines

subroutines(sometimes called "functions") are a means of encapsulating code.

subroutines are reusable.

subroutines can hide complexity.

subroutines can call other subroutines.

subroutines can call themselves.

 this is called "recursion"http://www.cnblogs.com/hanleilei/admin/EditPosts.aspx?IsDraft=1

 defining.pl

 1 #!/usr/bin/perl
 2 
 3 use strict;
 4 use warnings;
 5 use subs qw( message error);#加上这个语句，就可以在调用函数的时候去掉括号，否则只能把函数定义放在调用前面
 6 
 7 main(@ARGV);
 8 
 9 sub main
10 {
11     message "This is the template.pl exercise file from Perl 5 Essential Training.";
12     message "message two";
13     error "message three";
14 }
15 
16 sub message
17 {
18     my $m = shift or return;
19     print("$m\n");
20 }
21 
22 sub error
23 {
24     my $e = shift || 'unkown error';
25     print("$0: $e\n");
26     exit 0;
27 }

 注意：上面的例子的message语句是没有括号的，因为从一开始先申明了调用message的子函数，否则需要把申明子函数放在main函数之前。

argument.pl

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

main(@ARGV);

sub main
{
    my $s = "This is the template.pl exercise file from Perl 5 Essential Training.";
    my $y = "42";
    my $z = "zee";
    message($s,$y,$z);
}

sub message
{
    my ($s,$y,$z)= @_;
    print("$s,($y),($z)\n");
}

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

 注意以上的message子函数的参数传递：先传递给一个默认数组，然后传递给标量。

scope.pl

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

my $g= "yet other string";

main(@ARGV);

sub main
{
    my $m = "some other string";
    message("This is the template.pl exercise file from Perl 5 Essential Training.");
    message($m);
}

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

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

上面的例子描述了标量的作用域。

return.pl

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

main(@ARGV);

sub main
{
    my $num = addnum(2,4);
    message($num);
    message(addnum(5,8));
}

sub addnum{
    my ($v1,$v2)=@_;
    return $v1+$v2;
}

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

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

上面的例子描述了子函数的返回值，和C一样