Perl introductory learning (VIII) subroutine

One 、 Definition

A subfunction is a piece of separate code defined by the user in the program , It can appear anywhere in the program , Its function is to avoid using the same piece of code multiple times , The writing is complicated , Setting it as a sub function can effectively reduce repeated code , Make the program easy to read .

The definition method is ：

sub subroutine
{
    statements;
}

Two 、 call

Subroutines can appear anywhere , In writing order, you can call first and then define , You can also define it first and then call , For reading experience , When calling first and then defining, you can define the subroutine name first , Examples are as follows ：

##  Call first and then define 
subroutine(  parameter list  );
sub subroutine {
body
}
OR 
sub subroutine;
subroutine(  parameter list  );
sub subroutine {
body
}

##  First define and then invoke 

sub subroutine {
body
}
subroutine(  parameter list  );

Subroutines can call each other , It is called recursive subroutine , Local variables should be used except for variables that will not be overwritten by subroutines （my,local) , in addition , You need code that can stop recursive calls .

3、 ... and 、 Return value

Subroutines can use return Statement to get the return value , without return sentence , The default return value is the value of the last line of statement . Examples are as follows ：

#!/usr/bin/perl
sub add {
return 1 + 2;
1+1;
}
print add();

The return value is 3, Because there is return.

Four 、 local variable

By default , All variables are global variables , It can be called anywhere in the program , If you want to create a local variable , have access to my perhaps local The operator ,my The function of is limited to the current subroutine ,local In addition to applying to the current subroutine , It is still useful in the subroutine called by this subroutine , Simple memory is ：my、local、 The variable names of global variables are the same ：

1、 The priority in the subroutine is local> Global variables >my. here local It can be understood as the temporary assignment of global variables ,

2、 Outside the subroutine, only global variables work .

Examples are as follows ：

#!/usr/bin/perl
$x = 1;
sub sub1 {
print "current $x\n";
}

sub sub2 {
my $x = 2;
print "my $x\n"
sub1;
}

sub sub3 {
local $x = 3;
print "local $x\n"
sub1;
}

sub2;
sub3;
print "$x\n"

The running result is ：

my 2
current 1 # my Only the current subroutine 
local 3
current 3 # local Can act on sub3 Subroutine called sub1 in .
1         #  Outside the subroutine , Only global variables work 

When sub2 Run time output my $x Is a local variable , When sub2 Call in sub1 when , local variable my $x Cannot be called , perform sub1 At the time of the $x The global variable definition value is 1.sub3 Run time output local $ Is a local variable , Act on the current subroutine , When sub3 Call in sub1 when ,local The defined variables apply to sub1, So execute sub1 when $x Time local variable local $x Value 3.

5、 ... and 、 Pass parameters to subroutines

PERL The parameter list passed to the subroutine is automatically stored in the array @_ in , Therefore, the first parameter of the subroutine is $_[0], The second parameter is $_[1], And so on . What needs to be noted here is , When the objects passed are scalars and arrays , You need to put the array behind .

Pass scalars and arrays ：

#!/usr/bin/perl
#  Defined function 
sub PrintList{
   my @list = @_;
   print " The list is  : @list\n";
}
$a = 10;
@b = (1, 2, 3, 4);
#  List parameters 
PrintList($a, @b);

The running result is ：

10 1 2 3 4

Pass hash ：

#!/usr/bin/perl
#  Method definition 
sub PrintHash{
   my (%hash) = @_;
   foreach my $key ( keys %hash ){
      my $value = $hash{$key};
      print "$key : $value\n";
   }
}
%hash = ('name' => 'it', 'age' => 3);
#  Pass hash 
PrintHash(%hash);

  The running result is ：

age ： 3
name ： it