我觉得这一系列的标题应该是：PERL，从入门到放弃

USE IT OR U WILL LOSE IT

参考资料：

https://qntm.org/files/perl/perl.html

在线perl编译器：

https://www.tutorialspoint.com/execute_perl_online.php

---

use strict;
use warnings;

no block comments

variables: use 'my' for declaration

1. scalars: undef(like NULL), number, string, reference to any other variables

no distinguish between float and int

my $num = 12.34;

use '.' for string concatenation

my $str = "hello, "."m'lady";
print $str."\n";

no boolean type. but "false" in condition means: 0,"","0",undef

2. arrays: use @variableName

trailing comma is OK

my @arr = (1, "hi", 3,);

use '$' because it's scala when retrived

print $arr[0]."\n";

negative index means retriving from backend

print "$arr[-2]\n";

'' will not interpret, just output raw string

"" will interpret variables

they're same: print 'Hello \$str \@arr'

print "Hello \$str \@arr"

OPERATORS

Numerical operators: <, >, <=, >=, ==, !=, <=>, +, *

String operators: lt, gt, le, ge, eq, ne, cmp, ., x

啦啦啦今天继续啊

数组长度：

my @arr = (2, "Hi", "my", "lady", "!");

the length of array

print scalar @arr;
print "\n";

the end index of array. aka, length-1

print $#arr;

• hash Tables:

use strict;
use warnings;

hash variable

my %hashTable = ("1st" => "hi",
"2nd" => "my",
"3rd" => "lady");

since some element is a scalar

print $hashTable{"1st"};

use strict;
use warnings;

my %hashTable = (1=>"hi", 2=>"my", "3rd"=>"lady");

convert to array

my @arr = %hashTable;

1hi2my3rdlady

print @arr;

output 1

print $arr[0];

索引时hash用{},array[],""有无无所谓；构建时两者都用()　

#print $hashTable{1};
#print $hashTable{"1"};

• list　　

列表指的是用()构建的东西，array和hash都是。列表可以作为一个元素放在列表中间，但是会失去层次结构。hash的话，

use strict;
use warnings;

my @array = (
"apples",
"bananas",
(
"inner",
"list",
"several",
"entries",
),
"cherries",
);
my $i = 0;
while ($i <= $#array) {
print "array[$i] is ".$array[$i]."\n";
$i = $i + 1;
}

但其实list与array还是不同的，如下例：

my @arr = ("alpha", "beta", "gamma", "pie");

这里得到arr的长度

my $num = @arr;
print "$num\n";

output: pie　$num1存的是最后一项

my $num1 = ("alpha", "beta", "gamma", "pie");
print "$num1\n";

Every expression in Perl is evaluated either in scalar context or list context

而对很多函数来说，对scalar和list的处理方式是不同的，如reverse：

# link. no reverse
print reverse "hello, my lady", 3, " ";
print "\n";

reverse each letter

my $scalar = reverse "hello, my lady";
print "$scalar\n";

可以在前面强制加scalar，让函数可以按照scalar的方式处理：

print scalar reverse "hello world"; # "dlrow olleh"　　

而在下面的例子中，给$outer[3]赋值时，由于它是scalar，所以取得的值只能是数组长度，而非数组。

my @outer = ("Sun", "Mercury", "Venus", undef, "Mars");
my @inner = ("Earth", "Moon");

$outer[3] = @inner;

print $outer[3]; # "2"　　

好吧，为了解决这个问题，使用引用：

my $colour = "Indigo";

add reference

my $scalarRef = \$colour;
print $colour; # "Indigo"
print $scalarRef; # e.g. "SCALAR(0x182c180)"
print ${ $scalarRef }; # "Indigo"

如果不会混淆，可以省略{}

print $$scalarRef;

my @colours = ("Red", "Orange", "Yellow", "Green", "Blue");
my $arrayRef = \@colours;

print $colours[0]; # direct array access
print ${ $arrayRef }[0]; # use the reference to get to the array
print $arrayRef->[0]; # exactly the same thing

my %atomicWeights = ("Hydrogen" => 1.008, "Helium" => 4.003, "Manganese" => 54.94);
my $hashRef = \%atomicWeights;

print $atomicWeights{"Helium"}; # direct hash access
print ${ $hashRef }{"Helium"}; # use a reference to get to the hash
print $hashRef->{"Helium"}; # exactly the same thing - this is very common

我们希望定义自己的数据类型，类似C++里类的概念，所以我们这样做：

# Braces denote an anonymous hash
my $owner1Ref = {
"name" => "Santa Claus",
"DOB" => "1882-12-25",
};

my $owner2Ref = {
"name" => "Mickey Mouse",
"DOB" => "1928-11-18",
};

Square brackets denote an anonymous array

my $ownersRef = [ $owner1Ref, $owner2Ref ];

my %account = (
"number" => "12345678",
"opened" => "2000-01-01",
"owners" => $ownersRef,
);

注意，这里$owner1Ref和$owner2Ref存的其实是一个匿名hash的引用，$ownersRef存的则是匿名array的引用。既然时候引用，

它们的实际值就是地址。所以%account中才能够保存完整的信息。等价于：

my %account = (
"number" => "31415926",
"opened" => "3000-01-01",
"owners" => [
{
"name" => "Philip Fry",
"DOB" => "1974-08-06",
},
{
"name" => "Hubert Farnsworth",
"DOB" => "2841-04-09",
},
],
);　

如何打印？使用->

print "Account #", $account{"number"}, "\n";
print "Opened on ", $account{"opened"}, "\n";
print "Joint owners:\n";
print "\t", $account{"owners"}->[0]->{"name"}, " (born ", $account{"owners"}->[0]->{"DOB"}, ")\n";
print "\t", $account{"owners"}->[1]->{"name"}, " (born ", $account{"owners"}->[1]->{"DOB"}, ")\n";