Perl 简明教程

Perl - Arrays

An array is a variable that stores an ordered list of scalar values. Array variables are preceded by an "at" (@) sign. To refer to a single element of an array, you will use the dollar sign ($) with the variable name followed by the index of the element in square brackets.

Here is a simple example of using the array variables −

#!/usr/bin/perl

@ages = (25, 30, 40);
@names = ("John Paul", "Lisa", "Kumar");

print "\$ages[0] = $ages[0]\n";
print "\$ages[1] = $ages[1]\n";
print "\$ages[2] = $ages[2]\n";
print "\$names[0] = $names[0]\n";
print "\$names[1] = $names[1]\n";
print "\$names[2] = $names[2]\n";

Here we have used the escape sign (\) before the $ sign just to print it. Other Perl will understand it as a variable and will print its value. When executed, this will produce the following result −

$ages[0] = 25
$ages[1] = 30
$ages[2] = 40
$names[0] = John Paul
$names[1] = Lisa
$names[2] = Kumar

In Perl, List and Array terms are often used as if they’re interchangeable. But the list is the data, and the array is the variable.

Array Creation

Array variables are prefixed with the @ sign and are populated using either parentheses or the qw operator. For example −

@array = (1, 2, 'Hello');
@array = qw/This is an array/;

The second line uses the qw// operator, which returns a list of strings, separating the delimited string by white space. In this example, this leads to a four-element array; the first element is 'this' and last (fourth) is 'array'. This means that you can use different lines as follows −

@days = qw/Monday
Tuesday
...
Sunday/;

You can also populate an array by assigning each value individually as follows −

$array[0] = 'Monday';
...
$array[6] = 'Sunday';

Accessing Array Elements

When accessing individual elements from an array, you must prefix the variable with a dollar sign ($) and then append the element index within the square brackets after the name of the variable. For example −

#!/usr/bin/perl

@days = qw/Mon Tue Wed Thu Fri Sat Sun/;

print "$days[0]\n";
print "$days[1]\n";
print "$days[2]\n";
print "$days[6]\n";
print "$days[-1]\n";
print "$days[-7]\n";

这会产生以下结果 −

Mon
Tue
Wed
Sun
Sun
Mon

Array indices start from zero, so to access the first element you need to give 0 as indices. You can also give a negative index, in which case you select the element from the end, rather than the beginning, of the array. This means the following −

print $days[-1]; # outputs Sun
print $days[-7]; # outputs Mon

Sequential Number Arrays

Perl offers a shortcut for sequential numbers and letters. Rather than typing out each element when counting to 100 for example, we can do something like as follows −

#!/usr/bin/perl

@var_10 = (1..10);
@var_20 = (10..20);
@var_abc = (a..z);

print "@var_10\n";   # Prints number from 1 to 10
print "@var_20\n";   # Prints number from 10 to 20
print "@var_abc\n";  # Prints number from a to z

Here double dot (..) is called range operator. This will produce the following result −

1 2 3 4 5 6 7 8 9 10
10 11 12 13 14 15 16 17 18 19 20
a b c d e f g h i j k l m n o p q r s t u v w x y z

Array Size

The size of an array can be determined using the scalar context on the array - the returned value will be the number of elements in the array −

@array = (1,2,3);
print "Size: ",scalar @array,"\n";

The value returned will always be the physical size of the array, not the number of valid elements. You can demonstrate this, and the difference between scalar @array and $#array, using this fragment is as follows −

#!/usr/bin/perl

@array = (1,2,3);
$array[50] = 4;

$size = @array;
$max_index = $#array;

print "Size:  $size\n";
print "Max Index: $max_index\n";

这会产生以下结果 −

Size: 51
Max Index: 50

There are only four elements in the array that contains information, but the array is 51 elements long, with a highest index of 50.

Adding and Removing Elements in Array

Perl provides a number of useful functions to add and remove elements in an array. You may have a question what is a function? So far you have used print function to print various values. Similarly there are various other functions or sometime called sub-routines, which can be used for various other functionalities.

Sr.No.	Types & Description
1	push @ARRAY, LIST Pushes the values of the list onto the end of the array.
2	pop @ARRAY Pops off and returns the last value of the array.
3	shift @ARRAY Shifts the first value of the array off and returns it, shortening the array by 1 and moving everything down.
4	unshift @ARRAY, LIST Prepends list to the front of the array, and returns the number of elements in the new array.

#!/usr/bin/perl

# create a simple array
@coins = ("Quarter","Dime","Nickel");
print "1. \@coins  = @coins\n";

# add one element at the end of the array
push(@coins, "Penny");
print "2. \@coins  = @coins\n";

# add one element at the beginning of the array
unshift(@coins, "Dollar");
print "3. \@coins  = @coins\n";

# remove one element from the last of the array.
pop(@coins);
print "4. \@coins  = @coins\n";

# remove one element from the beginning of the array.
shift(@coins);
print "5. \@coins  = @coins\n";

这会产生以下结果 −

1. @coins = Quarter Dime Nickel
2. @coins = Quarter Dime Nickel Penny
3. @coins = Dollar Quarter Dime Nickel Penny
4. @coins = Dollar Quarter Dime Nickel
5. @coins = Quarter Dime Nickel

Slicing Array Elements

You can also extract a "slice" from an array - that is, you can select more than one item from an array in order to produce another array.

#!/usr/bin/perl

@days = qw/Mon Tue Wed Thu Fri Sat Sun/;

@weekdays = @days[3,4,5];

print "@weekdays\n";

这会产生以下结果 −

Thu Fri Sat

The specification for a slice must have a list of valid indices, either positive or negative, each separated by a comma. For speed, you can also use the .. range operator −

#!/usr/bin/perl

@days = qw/Mon Tue Wed Thu Fri Sat Sun/;

@weekdays = @days[3..5];

print "@weekdays\n";

这会产生以下结果 −

Thu Fri Sat

Replacing Array Elements

Now we are going to introduce one more function called splice(), which has the following syntax −

splice @ARRAY, OFFSET [ , LENGTH [ , LIST ] ]

This function will remove the elements of @ARRAY designated by OFFSET and LENGTH, and replaces them with LIST, if specified. Finally, it returns the elements removed from the array. Following is the example −

#!/usr/bin/perl

@nums = (1..20);
print "Before - @nums\n";

splice(@nums, 5, 5, 21..25);
print "After - @nums\n";

这会产生以下结果 −

Before - 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
After - 1 2 3 4 5 21 22 23 24 25 11 12 13 14 15 16 17 18 19 20

Here, the actual replacement begins with the 6th number after that five elements are then replaced from 6 to 10 with the numbers 21, 22, 23, 24 and 25.

Transform Strings to Arrays

Let’s look into one more function called split(), which has the following syntax −

split [ PATTERN [ , EXPR [ , LIMIT ] ] ]

This function splits a string into an array of strings, and returns it. If LIMIT is specified, splits into at most that number of fields. If PATTERN is omitted, splits on whitespace. Following is the example −

#!/usr/bin/perl

# define Strings
$var_string = "Rain-Drops-On-Roses-And-Whiskers-On-Kittens";
$var_names = "Larry,David,Roger,Ken,Michael,Tom";

# transform above strings into arrays.
@string = split('-', $var_string);
@names  = split(',', $var_names);

print "$string[3]\n";  # This will print Roses
print "$names[4]\n";   # This will print Michael

这会产生以下结果 −

Roses
Michael

Transform Arrays to Strings

We can use the join() function to rejoin the array elements and form one long scalar string. This function has the following syntax −

join EXPR, LIST

This function joins the separate strings of LIST into a single string with fields separated by the value of EXPR, and returns the string. Following is the example −

#!/usr/bin/perl

# define Strings
$var_string = "Rain-Drops-On-Roses-And-Whiskers-On-Kittens";
$var_names = "Larry,David,Roger,Ken,Michael,Tom";

# transform above strings into arrays.
@string = split('-', $var_string);
@names  = split(',', $var_names);

$string1 = join( '-', @string );
$string2 = join( ',', @names );

print "$string1\n";
print "$string2\n";

这会产生以下结果 −

Rain-Drops-On-Roses-And-Whiskers-On-Kittens
Larry,David,Roger,Ken,Michael,Tom

Sorting Arrays

The sort() function sorts each element of an array according to the ASCII Numeric standards. This function has the following syntax −

sort [ SUBROUTINE ] LIST

This function sorts the LIST and returns the sorted array value. If SUBROUTINE is specified then specified logic inside the SUBTROUTINE is applied while sorting the elements.

#!/usr/bin/perl

# define an array
@foods = qw(pizza steak chicken burgers);
print "Before: @foods\n";

# sort this array
@foods = sort(@foods);
print "After: @foods\n";

这会产生以下结果 −

Before: pizza steak chicken burgers
After: burgers chicken pizza steak

Please note that sorting is performed based on ASCII Numeric value of the words. So the best option is to first transform every element of the array into lowercase letters and then perform the sort function.

The $[ Special Variable

So far you have seen simple variable we defined in our programs and used them to store and print scalar and array values. Perl provides numerous special variables, which have their predefined meaning.

We have a special variable, which is written as $[. This special variable is a scalar containing the first index of all arrays. Because Perl arrays have zero-based indexing, $[ will almost always be 0. But if you set $[ to 1 then all your arrays will use on-based indexing. It is recommended not to use any other indexing other than zero. However, let’s take one example to show the usage of $[ variable −

#!/usr/bin/perl

# define an array
@foods = qw(pizza steak chicken burgers);
print "Foods: @foods\n";

# Let's reset first index of all the arrays.
$[ = 1;

print "Food at \@foods[1]: $foods[1]\n";
print "Food at \@foods[2]: $foods[2]\n";

这会产生以下结果 −

Foods: pizza steak chicken burgers
Food at @foods[1]: pizza
Food at @foods[2]: steak

Merging Arrays

Because an array is just a comma-separated sequence of values, you can combine them together as shown below −

#!/usr/bin/perl

@numbers = (1,3,(4,5,6));

print "numbers = @numbers\n";

这会产生以下结果 −

numbers = 1 3 4 5 6

The embedded arrays just become a part of the main array as shown below −

#!/usr/bin/perl

@odd = (1,3,5);
@even = (2, 4, 6);

@numbers = (@odd, @even);

print "numbers = @numbers\n";

这会产生以下结果 −

numbers = 1 3 5 2 4 6

Selecting Elements from Lists

The list notation is identical to that for arrays. You can extract an element from an array by appending square brackets to the list and giving one or more indices −

#!/usr/bin/perl

$var = (5,4,3,2,1)[4];

print "value of var = $var\n"

这会产生以下结果 −

value of var = 1

Similarly, we can extract slices, although without the requirement for a leading @ character −

#!/usr/bin/perl

@list = (5,4,3,2,1)[1..3];

print "Value of list = @list\n";

这会产生以下结果 −

Value of list = 4 3 2