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Perl notes
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 representation.

Examples of floating-point literals[1]

Examples of integer literals

1.1.1 Numeric Operators

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"

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

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 

     $hello
     $Hello
     $HELLO
     $Starting_Value
     $quite_long_variable_name

It is important to select meaningful variable names, making use of underscores when possible[2].

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 

     $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

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";
     }

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:

1.7 Getting User Input

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

     $value_1 = 10.0;
     $value_2 = <STDIN>;
     print "\$value_2 = $value_2\n";
     chomp($value_2); # newline is removed
     print "\$value_2 = $value_2\n";
     $check = $value_1 > $value_2;
     if ($check) {
           print "\$value_1 is larger than \$value_2\n";
     }

This can be done in a single step 

     $value_1 = 10.0;
     chomp($value_2 = <STDIN>);
     print "\$value_2 = $value_2\n";
     $check = $value_1 > $value_2;
     if ($check) {
           print "\$value_1 is larger than \$value_2\n";
     }

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.

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Perl notes

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

Curro Pérez Bernal francisco.perez@dfaie.uhu.es