Perl notes Curro Pérez 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). Copyright Notice
Copyright © 2011 by Curro Perez-Bernal
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...... 6 1.6.1 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 Arrays9 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 CONTENTS ii
2.4.4 The reverse Operator...... 13 2.4.5 The sort Operator...... 13 2.4.6 The each Operator...... 13 2.4.7 Array clearing...... 13 2.5 The foreach control structure...... 14 2.5.1 Perl’s 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 1
Chapter 1
Scalar Data
The simplest kind of data in Perl are scalar data, that can be mostly numbers or strings of characters.
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. 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
• 75 % 2
• 70 % 3
1.2 Strings
Strings are character sequences that may contain any possible compination of characters. We may 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" Chapter 1. Scalar Data 3
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
• 70 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 Perl identifier. All the variables that follow are different Chapter 1. Scalar Data 4
$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 that takes 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. Chapter 1. Scalar Data 5
$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 variable names
$job = "student"; print "The book\’s owner is an $student\n"; print "This is the favourite bar of the college ${job}s\n";
1.5 Operator Associativity and Precedence
From the perlop documentation.
Associativity Precedence (highest to lowest) left terms and list operators (leftward) left -> nonassoc ++ -- right ** right ! ~ \ and unary + and - left =~ !~ left * / % x left + - . left << >> nonassoc named unary operators nonassoc < > <= >= lt gt le ge nonassoc == != <=> eq ne cmp ~~ left & left | ^ left && left || // nonassoc .. ... right ?: right = += -= *= etc. left , => nonassoc list operators (rightward) right not left and left or xor Chapter 1. Scalar Data 6
In case of doubt: use parenthesis. . .
1.6 The if Control Structure
1.6.1 Comparison Operators
Comparison operators return a true or false value and are the following
Equal, numeric ==
Equal, string eq
Not Equal, numeric !=
Not Equal, string ne
Less than, numeric <
Less than, string lt
Less than or equal, numeric <=;
Less than or equal, string leq
Greater than, numeric >
Greater than, string gt
Greater than or equal, numeric >=;
Greater than or equal, string geq
Comparison, numeric <=>
Comparison, string comp
The unary not operator (!) give the opposite value of any Boolean value.
1.6.2 Using the if Control Structure
The if control structure defines a block that only executed if its associated condition returns a true value
if ($name gt "Monika") { print "’$name’ is after ’Monika’ in sort order\n"; } Chapter 1. Scalar Data 7
The keyword else allows an alternative choice
if ($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,
$value_1 = 10.0; $value_2 =
This can be done in a single step Chapter 1. Scalar Data 8
$value_1 = 10.0; chomp($value_2 =
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 not executed 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 a numerical value then the assigned value is zero, while in a string value the variable takes the empty string value. This is a standard behavior, though Perl will usally warn the user when unusual uses of the undef value occur. 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 called replicants we execute1:
my @replicants;
The array elements are numbered using sequential integers, starting at zero, and each array element 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 next lower integer.
1In fact the scalar variable $replicants is a different variable, though for the sake of clarity it is better to avoid having arrays and scalar variables with the same name. 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 elements minus 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 "$replicants[-8]\n"; #
2.1.1 List Literals
A list literal is how a list is represented in the code, as a list of comma separated values between parentheses. In this case the range operator (..) can be used.
@replicants = ("zhora", "pris", "leon", "rachel", "roy"); my @numbers = (12, 32, 13, 44, 14, 66); my @elist = (); # Empty list - zero elements my @list_1 = (1..100); my @list_2 = (0..10, 50..100); my @list_3 = ($replicants[1], $replicants[0], 45 + $list_2[3]);
The qw (quoted word) shortcut simplifies the list definition:
my @numbers = qw(12 32 13 44 14 66);
The elements are treated as single-quoted strings and it allows to choose any punctuation char- acter as a delimiter
@numbers = qw!12 32 13 44 14 66!; @numbers = qw/12 32 13 44 14 66/; @numbers = qw<12 32 13 44 14 66>; Chapter 2. Lists and Arrays 11
2.2 List Assignment
You can assign list values to variables and easily swap variables’ values
my ($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 the left 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 the elements 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 $replicants[2].\n"; Chapter 2. Lists and Arrays 12
2.4 Array Operators
2.4.1 Operators pop and push
An array can be considered as an stack of information, where you add and remove from the end of the array using the operators push and pop.
print pop @numbers, "\n"; print "$#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 beginning of the list.
print shift @numbers, "\n"; shift @numbers; print "$numbers[1]\n"; unshift @numbers, 10; print "$numbers[1]\n"; unshift @numbers, 1..60; print "$numbers[1]\n";
2.4.3 The splice Operator
This operator takes a maximum of four arguments and allows to work with sections of an array, deleting or adding elements at any place. The last two arguments are optional. The first argument is the array and the second is the starting 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 of the array. The fourth argument is a replacement list that is added to the array in the position stated 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 element print "\@array = @array\n"; Chapter 2. Lists and Arrays 13
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 the internal 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. Chapter 2. Lists and Arrays 14
@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 executing a 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 the actual list element. For example, if you want to precede every list element by a tab adn add a newline 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 Perl’s default scalar variable $_.
If the control variable is omitted in the definition of the foreach control structure the default variable is used: $_. For example
foreach (1..20) { print; print "$_ ", 2*$_, " ", 3*$_, " ", 4*$_,"\n"; } Chapter 2. Lists and Arrays 15
This is by large the most commonly used default variable in Perl. In many cases, when a variable 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. Considering scalars and lists, when Perl parses a particular expression, it expects either a scalar or a list value. For example
99 + something # Something should be a scalar (scalar context) sort something # Something should be a list (list context)
If something is the same exact sequence of characters, it may give completely different values depending on the evaluation context. For example, an array variable would give the list of elements in a list context while in scalar context it would give the number of elements of the list.
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 context of expressions that are usually used to produce a list. Some expression (e.g. sort) have no scalar-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) { ... $tyrell[something] = something; Chapter 2. Lists and Arrays 16
Some list context examples
@variable = something; ($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 the scalar 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 an end-of-file, each line is a separate list element. For example, reading several lines and removing the end-of-lines:
@input =
Input from a file will be read until the end of the file, while keyboard input should be stopped with the end-of-file key combination, normally CTRL-D in UNIX systems. You can chomp all lines simultaneously
chomp (@input =
Chapter 3
References
Learning Perl. Ed. O’Reilly (http://en.wikipedia.org/wiki/Learning_Perl) perlop: Precedence and Associativity (http://perldoc.perl.org/perlop.html# Operator-Precedence-and-Associativity)