New Feature in perl5: Add array elements to the list

The following is perl5's Reading Notes.

1. Why should I use the array elements of the list?

Assume that the following data is available:

Beijing, China

Tianjin, China

Tokyo, Japan

Seoul, South Korea

If it is stored in the list, you want:

China => Beijing, Tianjin

Japan => Tokyo

Korea => Seoul

In perl4, you can only treat the elements after => as strings. During each processing, you need to extract, sort, and then access the strings. for Perl, these operations should have been implemented internally.

2. Create a list with array elements

1) perl reference symbol "/"

You can use the symbol "/" to create a reference to an array, list, or common variable, as shown in

$ Aref = // @ array; # array reference

$ Href =/% list; # reference to the list

$ Scalar =/$ var; // The value

References can be passed:

$ Xy = $ Aref; # $ reference created by xy to @ Array

$ P [3] = $ href; # $ P [3] creates a reference to % list

$ Z = $ P [3]; # $ Z also creates a reference to % list

References are similar to pointers in C language. The difference is that references own variable types. For example:

Print $ Aref; the result is array (0x275d18)

Print $ href; the result is hash (0x1830ef4)

2) use [...] to create a reference to the unknown array, and use {key => value,...} to create a reference to the unknown list.

For example: $ Aref = [1, 2, "foo", UNDEF, 13];

$ Href = {key1 => val1, key2 => val2 };

The effect is as follows:

@ Array = (1, 2, "foo", UNDEF, 13 );

$ Aref = // @ array;

3) access objects with reference

For a sufficient number, you can use:

@ A equals @ {$ Aref}

Reverse @ a reverse @ {$ Aref}

$ A [3] $ {$ Aref} [3]

$ A [3] = 127; $ {$ Aref} [3] = 17

For hash tables, the situation is very similar

% H % {$ href}

Keys % H keys % {$ href}

$ H {'red' }$ {$ href} {'red '}

$ H {'red'} = 7 $ {$ href} {'red'} = 7

It can also be used wherever you want, such

For my $ elment (@ {$ Aref }){

}

Like a C pointer, Perl can also use "->" to reference its sub-objects, such

$ {$ Aref} [3] ==$ Aref-> [3]

$ {$ Href {red }==$ href-> {red}

You can also create multiple pseudo arrays.

@ A = ([, 3], [, 6], [, 9]);

$ A [n] is a reference to the Child array. You can use $ {$ A [n]} [N] or $ A [n]-> [N] to access the object.

It also supports mixed array and hash phenomena, such

@ A = ([1, 2, 3, 5],
{A => 'A', B => 'B '}
);

You can use $ A [0]-> [1] ($ {$ A [0]} [0]), $ A [1]-> {A} ($ {A [0]} {A}) Access

For a reference with more than two levels, "->" can be omitted. $ A [0]-> [1] can be written as $ A [0] [1], $ {$ A [0]} [1]} [2] can be written as $ A [0] [1] [2];

3. Specific solutions

Let's look at how the program we started should be written:

My % table;

Print "input lick/" city, country/". And end input press enter directly/N ";

While (<> ){

Chomp;
If ($ _ eq ") {last ;}

My ($ City, $ country) = Split/, [] */;

$ Table {$ country} = [] Unless exists $ table {$ country}; # create an empty anonymous List reference if it does not exist.

Push @ {$ table {$ country}, $ city;

}

# Output

Foreach $ country (sort keys % table ){

Print "$ country :";

My @ cities =@{$ table {$ country }};

Print join ',', sort @ cities;

Print "./N ";

}