Introduction to Perl (IX) quotation

A reference can be understood as a pointer , Play a directional role , Can point to scalar 、 Array 、 Hash tables and even sub functions .

One 、 Create reference

Reference is a scalar type , Prefix the referenced variable name with “\”, It means that the variable is referenced , Examples are as follows ：

$scalarref = \$foo;     #  Scalar variable reference 
$arrayref  = \@ARGV;    #  References to arrays 
$hashref   = \%ENV;     #  Hash reference 
$coderef   = \&handler; #  Sub function reference 

Multi dimensional arrays can be constructed by reference , Use here "[ ]" Define an anonymous array , If you refer to the array assignment method to use “（）”, Then what you get is not a quote , It's the length of the returned array .

my $aref = [
        [1, 2, 3],
        [4, 5, 6],
        [7, 8, 9],
]

Except for anonymous arrays , And anonymous hash table , Use “{ }” To define .

$href= { APR =>4, AUG =>8 };

Sub function reference format : \&.

Call reference function format : & + Created reference name .

#!/usr/bin/perl
#  Function definition 
sub PrintHash{
   my (%hash) = @_;
   foreach $item (%hash){
      print " Elements  : $item\n";
   }
}
%hash = ('name' => 'it', 'age' => 3);
#  Create a reference to a function 
$cref = \&PrintHash;
#  Use references to call functions 
&$cref(%hash);

  The operation results are as follows ：

 Elements  : age
 Elements  : 3
 Elements  : name
 Elements  : it

Two 、 Dereference

Dereference can be used according to different types $, @ or % + Variable name to cancel , Scalar $, Array with @, Hash with %.

#!/usr/bin/perl
$var = 10; # $r  quote  $var  Scalar 
$r = \$var; #  Output locally stored  $r  Variable value 
print "$var  by  : ", $$r, "\n";

@var = (1, 2, 3); # $r  quote   @var  Array 
$r = \@var; #  Output locally stored  $r  Variable value 
print "@var  by : ",  @$r, "\n";

%var = ('key1' => 10, 'key2' => 20); # $r  quote   %var Hash 
$r = \%var; #  Output locally stored  $r  Variable value 
print %var ," by  : ", %$r, "\n";

sub add {
$count = @_[0] + @_[1];
}
$value = \&add;
print add(1,2), "=" &$value "\n"; 

It should be noted here that neither hash nor subfunction can be placed in double quotation marks ” “ in ,print You can use commas between strings of ”,“ Separate . The running result is ：

10  by  : 10
1 2 3  by : 123
key110key220  by  : key110key220
3=3

When you are not sure what type the reference is, you can use ref To judge .

#!/usr/bin/perl
$var = 10;
$r = \$var;
print "r  The type of reference  : ", ref($r), "\n";
@var = (1, 2, 3);
$r = \@var;
print "r  The type of reference  : ", ref($r), "\n";
%var = ('key1' => 10, 'key2' => 20);
$r = \%var;
print "r  The type of reference  : ", ref($r), "\n";

The operation results are as follows ：

r  The type of reference  : SCALAR
r  The type of reference  : ARRAY
r  The type of reference  : HASH