http://www.perlmonks.org?node_id=998515

truthseeker66 has asked for the wisdom of the Perl Monks concerning the following question:

Dear Monks,

1. '$first' and '$second' are 2 different variables. Why is
'$_[0] . $_1' used instead of '$_[0] . $_[0]' to combine
two strings? Doesn't the first '$_' for '$first' and the
second '$_' for '$second'?

2. In this code " . $_[0] . $_1 . "\n"; I understand why there
are two dots. But I don't understand why the third dot is
there.

Thank you, 
#!/usr/bin/perl

#prompt user for two strings. Remove newline and 
#store into variables
print "\nEnter the first string: ";
chomp($first = <STDIN>);
print "\nEnter the second string: ";
chomp($second = <STDIN>);

sub combine{
print "\nThe combined strings are: " . $_[0] . $_[1] . "\n";
}

&combine($first,$second);

[download]

C:\JPARK\JPERL>sub-ex.pl
Enter the first string: I go
Enter the second string: to school.
The combined strings are: I go to school.

Replies are listed 'Best First'.
Re: subroutine function
by roboticus (Chancellor) on Oct 11, 2012 at 19:05 UTC

    truthseeker66:

    The dot operator combines two strings. So given:

    my $t = "apple";
my $u = "banana";

my $x = "Combined: " . $t . $u . "\n";

    [download]

    Perl starts to combine the strings:<\p> 

    my $x = "Combined: " . $t . $u . "\n";            # before we start
      = "Combined: " . "apple" . "banana" . "\n"; # expanded variables
      = "Combined: apple"      . "banana" . "\n"; # combined first two
      = "Combined: applebanana"           . "\n"; # then the next
      = "Combined: applebanana\n";                # then the last

    [download]

    ...roboticus

    When your only tool is a hammer, all problems look like your thumb.

[reply]
[d/l]
[select]
Re: subroutine function
by blue_cowdawg (Monsignor) on Oct 11, 2012 at 19:07 UTC

    First off fellow monk understand just what you are doing here.

    Those dots you are referring to are the string concatenation operator. Each operator has a left hand side and a right hand side. I won't even start getting into precedence here since it reall y doesn't matter all that much. 

    my $thing = "Coca" . "Cola";

    [download]
    In the above example when $thing is printed it will result in "CocaCola" being printed.

    As far as the variables them selves consider this code block: 

    my ($left,$right)=@_;

    [download]
    that will give you the same result as 
    my $left=$_[0];
my $right=$_[1];

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    which is the same as 
    my $left = shift @_;
my $right = shift @_;

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    but not necessarily the same as 
    #
# NEVER ASSUME!!!
my $right = pop @_;
my $left = pop @_;
#
# You may not get the result you are after!!

    [download]

    Clear as mud?

    Peter L. Berghold -- Unix Professional
    Peter -at- Berghold -dot- Net; AOL IM redcowdawg Yahoo IM: blue_cowdawg
[reply]
[d/l]
[select]

      my $left=$_[0];
my $right=$_[1];

      [download]

      which is the same as

      my $left = shift @_;
my $right = shift @_;

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      It is not the same at all. The first version doesn't modify @_. The second one does modify it. The difference is often very important. (For example, if you're planning on using goto.)

      use Test::More;
use Data::Dumper;

sub func1 {
    my $left=$_[0];
    my $right=$_[1];
    goto \&Data::Dumper::Dumper;
}

sub func2 {
    my $left = shift @_;
    my $right = shift @_;
    goto \&Data::Dumper::Dumper;
}

my @args = qw( a b );

# Are they the same??
is(
    func1(@args),
    func2(@args),
    "they're the same!",
);

done_testing();

      [download]
      perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'
[reply]
[d/l]
[select]
        The difference is often very important.

        And the difference often doesn't matter at all. As in the OP's example. In which case they are essentially the same. Similar. Whatever. Yes, I know what the difference is; I often pass @_ to other methods after shifting off the object. And often I don't.

[reply]
      Thank you
[reply]

      Regarding question# 2 i believe now you do understand about dot(.); so first dot(.) is to concatenate left side variable. and right side dot(.) is to concatenate right of that variable. If string or variable is not available then it will simply ignore.

[reply]

        No. "If a string or variable is not available" then it is an empty string or undef and an empty string will be concatenated (not simply ignored).

[reply]

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