Perl函数

        Perl函数也叫子程序，用sub关键字定义，没有显示的参数列表，用‘@_’可以获取实参列表，用return返回value。具体用法在后面的demo中体现。

关于变量作用域

         Perl中的变量默认是全局的，如果需要限制变量的作用域，比如定义子程序的私有变量，就用关键字my做声明。

         local关键字可以为全局变量做临时声明，对其修改访问都只针对临时对象，退出作用域后，全局变量的value不变。这里其实我们可能会认为用my定义局部对象和用local声明全局变量的临时对象差不多，确实两者呈现的效果很类似，但是实际上在作用域上还是有些不同：例如，存在全局对象A，在子程序sub1用my定义了局部对象A，sub1随后调用了子程序sub2，那么sub2中访问A将是访问的全局对象，如果sub1中的A是用local做声明，那么在sub2中访问A访问的还是sub1中的临时对象，在CASE4中做了演示。

         state关键字可以为局部对象做持久化，也就是退出作用域后，该对象不被销毁，在CASE5有演示。

#!/usr/bin/perl -w

#CASE1:compute the average oh the all params
print "\n=====CASE1=====\n";
sub getAverage{
    my $paramNum = scalar(@_);
    print "the number of params: $paramNum\n";
    $paramSum = 0;

    foreach $param (@_){
        $paramSum += $param;
    }

    $average = $paramSum / $paramNum;
    return $average;
}

@array = (10, 20 ,30, 40);
$ave = getAverage(@array);
print "average value of array(@array): $ave\n";

#CASE2:delivery list params
print "\n=====CASE2=====\n";
sub printParams{
    my @list = @_;
    print "print all params: @list\n";
}

$aInt = 10;
@bArray = (20,30,40);
printParams($aInt, @bArray);

#CASE3: delivery hash param
print "\n=====CASE3=====\n";
sub printHash{
    my (%hash) = @_;
    foreach my $key (keys %hash){
        my $value = $hash{$key};
        print "$key : $value\n";
    }
}

%hash = ('name' => 'hunk', 'age' => 25);
printHash(%hash);

#CASE4: test local and global var
print "\n=====CASE4=====\n";
$helloString = "hello wolrld";
sub printHello{
    local $helloString = "hello hunk";
    print "local: $helloString\n";
    printHelloSub();
}

sub printHelloSub{
    print "sub: $helloString\n";
}
print "global: $helloString\n";
printHello();
print "global: $helloString\n";

#CASE5: state
print "\n=====CASE5=====\n";
use feature 'state';

sub printCount{
    state $count = 0;
    print "count: $count\n";
    $count++;
}

for(1..3){
    printCount();
}

Output：

root@n209-h246:/home/hunk/code/Project_Code_Exercise/perl/basic# ./function.pl 

=====CASE1=====
the number of params: 4
average value of array(10 20 30 40): 25

=====CASE2=====
print all params: 10 20 30 40

=====CASE3=====
name : hunk
age : 25

=====CASE4=====
global: hello wolrld
local: hello hunk
sub: hello hunk
global: hello wolrld

=====CASE5=====
count: 0
count: 1
count: 2

CASE4修改local为my：

#CASE4: test local and global var
print "\n=====CASE4=====\n";
$helloString = "hello wolrld";
sub printHello{
    my $helloString = "hello hunk";
    print "local: $helloString\n";
    printHelloSub();
}

sub printHelloSub{
    print "sub: $helloString\n";
}
print "global: $helloString\n";
printHello();
print "global: $helloString\n";

=====CASE4=====
global: hello wolrld
local: hello hunk
sub: hello wolrld
global: hello wolrld