Perl_notesby Rajeev 2013

7/28/2019 Perl_notesby Rajeev 2013

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Perl notesCurro Prez Bernal

$Id: perl_notes.sgml,v 1.3 2011/12/12 15:41:58 curro Exp curro $

Abstract

Short notes for an introduction to Perl based on R.L. Schwartz et al. Learning Perl (See refer-ences).


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Copyright Notice

Copyright 2011 by Curro Perez-Bernal

This document may used under the terms of the GNU General Public License version 3 orhigher. (http://www.gnu.org/copyleft/gpl.html)
http://www.gnu.org/copyleft/gpl.htmlhttp://www.gnu.org/copyleft/gpl.htmlhttp://www.gnu.org/copyleft/gpl.html


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i

Contents

1 Scalar Data 1

1.1 Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Numeric Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2.1 String Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 Scalar Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.4 Basic Output with print . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.5 Operator Associativity and Precedence . . . . . . . . . . . . . . . . . . . . . . . . 5

1.6 The if Control Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.6.1 Comparison Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.6.2 Using the if Control Structure . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.7 Getting User Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.8 The while Control Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.9 The undef Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 Lists and Arrays 9

2.1 Defining and Accessing an Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1.1 List Literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2 List Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3 Interpolating Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.4 Array Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.4.1 Operators pop and push . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.4.2 Operators shift and unshift . . . . . . . . . . . . . . . . . . . . . . . . 12

2.4.3 The splice Operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12


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CONTENTS ii

2.4.4 The reverse Operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.4.5 The sort Operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.4.6 The each Operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.4.7 Array clearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.5 The foreach control structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.5.1 Perls default scalar variable $_. . . . . . . . . . . . . . . . . . . . . . . . 14

2.6 Scalar and List context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.6.1 List-producing expressions in scalar context. . . . . . . . . . . . . . . . . . 15

2.6.2 Scalar-producing expressions in list context. . . . . . . . . . . . . . . . . . 16

2.6.3 STDIN in list context. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3 References 17


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1

Chapter 1

Scalar Data

The simplest kind of data in Perl are scalar data, that can be mostly numbers or strings ofcharacters.

1.1 Numbers

Both integers and floating-point numbers have an internal double-precision floating point rep-resentation.

Examples of floating-point literals1

1.24

255.005

5.235E45

-25.0E-11

251.0

-1.9221

Examples of integer literals

24

5

0

-45

1A literal is how a particular value is represented in Perl.


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Chapter 1. Scalar Data 2

-2501

19221

2519988585883

2_519_988_585_883

1.1.1 Numeric Operators

2 + 4

5.6 - 34.4447

4.5 * 4

- 4 / 3

55.5 / 3

7 5 % 2

7 0 % 3

1.2 Strings

Strings are character sequences that may contain any possible compination of characters. Wemay differentiate between single- and double-quoted string literals.

Single-Quoted

Markus

Lena

Shannon

let\s include an apostrophe!

and a backslash: \\

a backslash and n: \n

Double-Quoted

In this case the backslash is used to specify certain control characters.

"Barbara"

"Ana"

"Hello Karen!\n"

"Black\tWhile"


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The most important string backslash escapes are the following

\a Beep

\b Backspace

\c "Control" caracter. \cD = CTRL-D

\e Escape

\f Form feed

\l Make the next letter lowercase

\n New line, return.

\r Carriage return.

\t Tab.

\u Make the next letter uppercase

\x Enables hex numbers\v Vertical tab

\\ Print backslash

\" Print double quotes

\ Escape next character if known otherwise print. Also allows octal numbers.

\L Make all letters lowercase until the \E

\U Make all letters uppercase until the \E

\Q Add a backslash-quote to all the nonalphanumerics until the \E

\E Terminates the effects of \L, \U, or \Q

\007 Any octal ASCII value

\x7f Any hexadecimal value

\cx Control-x

1.2.1 String Operators

hello.world

GNU./.Linux

This is. .a sentence.\n

Tuxie x 5

7 0 x 3

Perl performs the conversion between numbers and strings when it is necessary.

1.3 Scalar Variables

Variables holding exactly one value that start with a $ (named the sigil) followed by a Perlidentifier. All the variables that follow are different


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$hello

$Hello

$HELLO

$Starting_Value

$quite_long_variable_name

It is important to select meaningful variable names, making use of underscores when possible2.

The Perl assignment operator is the equals sign that takes a variable name in the left side thattakes the value of the expression on the right.

$hello = 5;

$Hello = 4.33;$HELLO = "Good morning!\n";

$Starting_Value = $index - 3;

$quite_long_variable_name = $x * 2;

Binary assigments are shortcuts like the following

$ a = $ a + 3 ;

$a += 3;

$a = $a * 3;$a *= 3;

$string = $string." ";

$string .= " ";

1.4 Basic Output with print

print "This is a message for you.\n";

print "This is ";

print "a message for you.";

print "\n";

print "This is ", "a message for you.";

print "The solution is ", 2*3.125,"\n";

Scalar variables in doubly-quoted string literals are subject to variable interpolation

2More valuable advices in the Perlstyle documentation.


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$op_sys = "GNU/Linux";

print "One of the best operating systems is $op_sys\n";

To print the dollar sign it has to be escaped or between single quotes

print "The \$op_sys variable value is $op_sys\n";

print The \$op_sys variable value , "is $op_sys\n";

The variable name can be located between curly braces to prevent errors delimiting variablenames

$job = "student";

print "The book\s owner is an $student\n";print "This is the favourite bar of the college ${job}s\n";

1.5 Operator Associativity and Precedence

From the perlop documentation.

Associativity Precedence (highest to lowest)

left terms and list operators (leftward)

left ->

nonassoc ++ --

right **right ! ~ \ and unary + and -

left =~ !~

left * / % x

left + - .

left >

nonassoc named unary operators

nonassoc < > = lt gt le ge

nonassoc == != eq ne cmp ~~

left &

left | ^

left &&

left || //

nonassoc .. ...

right ?:

right = += -= *= etc.

left , =>

nonassoc list operators (rightward)

right not

left and

left or xor


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In case of doubt: use parenthesis. . .

1.6 The if Control Structure

1.6.1 Comparison Operators

Comparison operators return a true or false value and are the following

Equal, numeric ==

Equal, string eq

Not Equal, numeric !=

Not Equal, string ne

Less than, numeric =;

Greater than or equal, string geq

Comparison, numeric

Comparison, string comp

The unary not operator (!) give the opposite value of any Boolean value.

1.6.2 Using the if Control Structure

The if control structure defines a block that only executed if its associated condition returns atrue value

if ($name gt "Monika") {

print "$name is after Monika in sort order\n";

}


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The keyword else allows an alternative choice

if ($name gt "Monika") {

print "$name is after Monika in sort order\n";

} else {

print "$name is before Monika in sort order\n";

}

You may use any scalar value in the conditional

$value_1 = 10.0;

$value_2 = 2;

$check = $value_1 > $value_2;

if ($check) {

print "\$value_1 is larger than \$value_2\n";

}

The rules for deciding if a value is true or false are the following:

All numbers are true except 0 (zero).

All strings are true besides the empty string ().

All other cases are converted to a number or a string and the previous rules apply.

1.7 Getting User Input

The simplest way to get a value from the keyboard into the program is the line-input operator,. Every time a program finds an where a scalar value is expected, Perlreads the next complete line from the standard input. The newline character at the end of theline can be removed using the chomp operator.

$value_1 = 10.0;$value_2 = ;

print "\$value_2 = $value_2\n";

chomp($value_2); # newline is removed



if ($check) {


}

This can be done in a single step


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$value_1 = 10.0;

chomp($value_2 = );



if ($check) {


}

1.8 The while Control Structure

This is one of the possible control structures in Perl. It repeats a block of code as long as a

given condition is accomplished:

$counter = 10;

while ($counter > 0) {

print "\$counter = $counter\n";

$counter -= 2;

}

The conditional is evaluated prior to the first iteration, thus it is possible that the block is notexecuted a single time if the condition is initially false.

1.9 The undef Value

Values used before being assigned take the special value undef. If it is expected to take anumerical value then the assigned value is zero, while in a string value the variable takes theempty string value.

This is a standard behavior, though Perl will usally warn the user when unusual uses of theundef value occur.


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9

Chapter 2

Lists and Arrays

A list is defined as an ordered collection of scalars, and an array is a variable that contains a list.Each element is an independent scalar value and a list can hold a mixture of different scalars(numbers and strings).

2.1 Defining and Accessing an Array

When using the strict pragma it is necessary to declare an array before it is first used. The

character that defines a variable as an array variable is @. Thus, to define an array calledreplicants we execute1:

my @replicants;

The array elements are numbered using sequential integers, starting at zero, and each arrayelement behaves as an scalar variable

my @replicants;

#

$replicants[0] = "roy";$replicants[1] = "leon";

$replicants[2] = "pris";

$replicants[3] = "zhora";

#

print "$replicants[1]\n";

#

my $index = 3;

print $replicants[$index-1],"\n"; # floating-point indexes truncate to the ne

1In fact the scalar variable $replicants is a different variable, though for the sake of clarity it is better to avoidhaving arrays and scalar variables with the same name.


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Chapter 2. Lists and Arrays 10

The storage of an array element beyond the end of the array extends the array size, with inter-vening elements created as undef values.

#

$replicants[10] = "rachel"; # six undef elements

#

The last index of the array replicants is $#replicants, which is the number of elementsminus one

#

my $end = $#replicants;

my $number_of_replicants = $end + 1;

print "$replicants[$end]\n";

#

To extract elements from the end of the list a negative index can be used.

#

print "$replicants[-1]\n";

print "$replicants[-8]\n";

#

2.1.1 List Literals

A list literal is how a list is represented in the code, as a list of comma separated values betweenparentheses. In this case the range operator (..) can be used.

@replicants = ("zhora", "pris", "leon", "rachel", "roy");

my @numbers = (12, 32, 13, 44, 14, 66);

my @elist = (); # Empty list - zero elements

my @list_1 = (1..100);

my @list_2 = (0..10, 50..100);

my @list_3 = ($replicants[1], $replicants[0], 45 + $list_2[3]);

The qw (quoted word) shortcut simplifies the list definition:

my @numbers = qw(12 32 13 44 14 66);

The elements are treated as single-quoted strings and it allows to choose any punctuation char-acter as a delimiter

@numbers = qw!12 32 13 44 14 66!;

@numbers = qw/12 32 13 44 14 66/;

@numbers = qw;


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2.2 List Assignment

You can assign list values to variables and easily swap variables values

my ($var_1, $var_2, $var_3) = ("one", "two", "three");

@replicants = ("zhora", "pris");

@replicants = ("pris", "zhora",);

If there are extra values in the right side they are ignored, and if there are extra values in theleft side they are given the undef value. You can mix arrays and scalars

my @characters = (@replicants,"deckard","gaff");

And you can easily copy an array into another array

my @copy_arr = @characters;

2.3 Interpolating Arrays

An array into a double-quoted string is interpolated, their values expanded, separating theelements by spaces.

print "The replicants are @replicants\n";

Thus, it is important to be careful when including the character @ in a double-quoted string.For example, to define a variable containing an email address once should do it in one of these

two alternative ways

my $email;

$email = "sebastian\@tyrell.com";

$email = [email protected];

A single element array interpolates into its value

print "The 3rd element of the array \@replicants is $replicants[2].\n";


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2.4 Array Operators

2.4.1 Operators pop and push

An array can be considered as an stackof information, where you add and remove from theend of the array using the operators push and pop.

print pop @numbers, "\n";

print "$#numbers\n";

push @numbers, 20;

push @numbers, 1..60;

2.4.2 Operators shift and unshift

This is equivalen to the previous case but the programs take and add elements to the beginningof the list.

print shift @numbers, "\n";

shift @numbers;

print "$numbers[1]\n";

unshift @numbers, 10;


unshift @numbers, 1..60;


2.4.3 The splice Operator

This operator takes a maximum of four arguments and allows to work with sections of anarray, deleting or adding elements at any place.

The last two arguments are optional. The first argument is the array and the second is thestarting position. If only this two arguments are used Perl removes all the elements from the

starting position to the end of the array and returns them to you.

The third argument is a length making possible to remove some elements from the middle ofthe array. The fourth argument is a replacement list that is added to the array in the positionstated by the second argument. Thus you can remove and add elements in a single statement.If no elements should be deleted, then argument three is made equal to zero.

As with scalars, array values between double quotes are interpolated.

print "\@characters = @characters\n";

my @array = splice @characters, 2; # remove everything after the third array

print "\@array = @array\n";


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print "\@characters = @characters\n";

my @removed_1 = splice @numbers, 3, 6;

my @new_array = splice @numbers, 3, 0, 1..5;

2.4.4 The reverse Operator

This operator takes a list of values as an argument and returns the list in the opposite order.

my @num_range = 1..10;

print "@num_range\n";

my @reversed_num_range = reverse @num_range;

print "Original array = @num_range\n";print "Reversed array = @reversed_num_range\n";

2.4.5 The sort Operator

This operator takes a list of values as an argument and returns the list sorted according to theinternal character values (code point order).

my @sorted_characters = sort @characters;

print "Original array = @characters\n";print "Sorted array = @sorted_characters\n";

my @sorted_num_range = sort @num_range;

print "Original array = @num_range\n";

print "Sorted array = @sorted_num_range\n";

2.4.6 The each Operator

This operator2 takes an array as an argument and each time it is called returns a pair of values(index, array_value):

while (my ($index_value, $array_element) = each @characters) {

print "Index: $index_value\tElement: $array_element\n";

}

2.4.7 Array clearing

The correct way to clear an array is to assign the array to an empty list

2Only valid for Perl 5.12 and later versions.


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@replicants = ("leon", "pris", "zhora");

.

.

.

@replicants = (); # The array is emptied

Note that this is different from

@replicants = ("leon", "pris", "zhora");

.

.

.

@replicants = undef; # The array is the one-element list (undef)

2.5 The foreach control structure

This is a useful control structure to process every element of a list, one at a time, and executinga block of instructions each iteration. For example

print "Contents of \@characters:\n";

foreach my $chtr (@characters) {

print "$chtr\n";

}

Be careful because if you modify the control variable ($chtr in the example) you modify theactual list element. For example, if you want to precede every list element by a tab adn add anewline character after the list element

print "Contents of \@characters: @characters\n";

foreach my $chtr (@characters) {

$chtr = "\t$chtr";

$chtr .= "\n";

}

print "Contents of \@characters: @characters\n";

2.5.1 Perls default scalar variable $_.

If the control variable is omitted in the definition of the foreach control structure the defaultvariable is used: $_. For example

foreach (1..20) {

print;

print "$_ ", 2*$_, " ", 3*$_, " ", 4*$_,"\n";

}


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This is by large the most commonly used default variable in Perl. In many cases, when avariable is needed and no name is provided, Perl will use $_as a replacement.

2.6 Scalar and List context

In Perl a given expression can have a different meaning according to the context, thus accord-ing where it appears and how it is used. This is common in natural languages. Consideringscalars and lists, when Perl parses a particular expression, it expects either a scalar or a listvalue. For example

99 + something # Something should be a scalar (scalar context)

sort something # Something should be a list (list context)

Ifsomething is the same exact sequence of characters, it may give completely different valuesdepending on the evaluation context. For example, an array variable would give the list ofelements in a list context while in scalar context it would give the number of elements of thelist.

my @characters = ("leon","deckard","gaff");

@sorted = sort @characters; # list context :: deckard gaff leon

my $i = 22 + @characters; # scalar context :: i = 22 + 3 = 25

Each expression can have different output according to the evaluation context.

2.6.1 List-producing expressions in scalar context.

In principle you should check the documentation to see what is the output in scalar contextof expressions that are usually used to produce a list. Some expression (e.g. sort) have noscalar-context value. Others have a different value according to the context, like reverse

my @characters = ("leon","deckard","gaff");

@reversed = reverse @characters; # list context :: gaff deckard leon$reversed = reverse @characters; # scalar context :: noeldrakcedffag

Some scalar context examples

$variable = something;

$tyrell[3] = something;

1234 + something;

if (something) { ...

while (something) { ...

$tyrell[something] = something;


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Some list context examples

@variable = something;

($tyrell,$sebastian) = something;

($tyrell) = something;

push @tyrell, something;

foreach (something) { ...

sort something;

print something;

2.6.2 Scalar-producing expressions in list context.

The use of a scalar-producing expression in list context always results in the promotion of thescalar to a one-element list.

@output = 6*12; # One-element list (72)

2.6.3 STDIN in list context.

The line-input operator STDIN in list context returns a list with all the remaining lines up to anend-of-file, each line is a separate list element. For example, reading several lines and removing

the end-of-lines:

@input = ;

chomp(@input);

Input from a file will be read until the end of the file, while keyboard input should be stoppedwith the end-of-file key combination, normally CTRL-D in UNIX systems. You can chomp alllines simultaneously

chomp (@input = );


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17

Chapter 3

References

Learning Perl. Ed. OReilly (http://en.wikipedia.org/wiki/Learning_Perl)

perlop: Precedence and Associativity (http://perldoc.perl.org/perlop.html#Operator-Precedence-and-Associativity)
http://en.wikipedia.org/wiki/Learning_Perlhttp://en.wikipedia.org/wiki/Learning_Perlhttp://perldoc.perl.org/perlop.html#Operator-Precedence-and-Associativityhttp://perldoc.perl.org/perlop.html#Operator-Precedence-and-Associativityhttp://perldoc.perl.org/perlop.html#Operator-Precedence-and-Associativityhttp://perldoc.perl.org/perlop.html#Operator-Precedence-and-Associativityhttp://en.wikipedia.org/wiki/Learning_Perl

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Perl_notesby Rajeev 2013