Session 2 Variables in PerlRA M N SA N GWA N

WWW. R N SA NGWA N.COM

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Agenda

• Variables in Perl

• Scalars

• Scalars: Declaration

• Scalars: Numbers

• Usage of scalars

• List : one-dimensional array

• Data Types

• Numbers and Strings

• User Input

• Special Variables

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Variables in Perl

• You DO NOT have to declare variables in Perl.

◦ Unless you force it to force you to declare variables.

• Three basic types of variables:

◦ Scalar variables : $a, $b, $c

◦ Array variables : @array

◦ Hash variables : %hash

• Variable type (int, char, ...) is decided at run time$a = 5; # now an integer

$a = “perl”; # now a string

Scalars

• Scalar variables store “single values”.

• This “single value” can be any of the following types:◦ int, float, double, char, string, or references.

• In Perl these various types fall into one of threecategories:◦ numbers, strings, and references.

• When you first use (declare) a variable use the mykeyword to indicate the variable’s scope◦ Not necessary but good programming practice

Scalars: Declaration

• All scalar variables begin with a $.◦ $ is NOT part of the variable!

$num = 14;

$fullname = “John H. Smith”;

◦ Variable Names are Case Sensitive

$Program_Version = 1.0;

• Next character is either a letter or ‘_’.

• Remaining characters are a mix of letters, numbers,and ‘_’.

• Correct: $x, $myvar123, $avg_height

• Wrong: $2values, $avg-height, $good$day.

Scalars: Numbers

• Unlike C/C++, all numeric data in Perl are stored asdouble precision floating points.◦ Ex: 37 3.7 .37 37. 3E7 3e7 3E-7

◦ Hex: 0x2aff, 0xAA3, 0XFFF

◦ Octal: 077 0276

◦ Underscore: 3_212_567 → 3,212,567.

• Again, you do not need to specify the type.

Usage of scalars

• All of the following statements are valid

print ("pi is equal to: $pi\n");

print "pi is still equal to: ", $pi, "\n";$c = $a + $b

• Important! A scalar variable can be "used" before it isfirst assigned a value◦ result depends on context

◦ either a blank string ("") or a zero (0)

◦ this is a source bugs

◦ if variable name is mispelled — what should be theresult?

◦ do not let yourself get caught by this – use the "-w"flag in the bang path:#!/public/bin/perl -w

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List : one-dimensional array

• Declaration/Usage

@memory = (16, 32, 48, 64);

@people = (“Alice”, “Alex”, “Albert”);

• First element numbered 0

• Single elements are scalar: $names[0] = “Fred”;

• Slices are ranges of elements

@guys = @people[1..2];

• How big is my list?

print “Number of people: “, scalar @people, “ \n”;

Data Types

• Integer25 750000 1_000_000_000

8#100 16#FFFF0000

• Floating Point1.25 50.0 6.02e23 -1.6E-8

• String‘hi there’ “hi there, $name” qq(tin can)

print “Text Utility, version $ver\n”;

• Boolean0 0.0 “” "0" represent False

all other values represent True

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Numbers and Strings

• Numbers and Strings are Interchangeable

• If a scalar variable looks like a number and Perl needsa number, it will use it as a number$a = 4; # a number

print $a + 18; # prints 22

$b = “50”; # looks like a string, but ...

print $b – 10; # will print 40!

User Input

• Entering a string and printing it:$text = <STDIN>;print "You just entered $text";

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• $[

• Determines which index in a list is the first, the default is 0.

my @mylist = (Larry, Moe, Curly);

print $mylist[1];

$[ = 1;

print $mylist[1];

Moe

Larry

Special Variables

• $& Entire pattern from most recent match

• $/ Input record separator, default is \nundef $/;

open(FILE,<input.txt);

$buffer=<INFILE>;

$buffer contains the entire file

Special Variables

• $. Current line number

• $, Default separator used when a list is printed,default is ‘’.$,=‘ ‘; will add a space between each item if you print out a list.

• $\ Default record separator, default is ‘’

$\ = “\n”; will add a blank line after each printstatement.

Special Variables

• $^T time the perl program was executed

• $| autoflush

• $$ process ID number for Perl

• $0 name of perl script executed

• %ENV hash containing environmental variables.

Special Variables

@ARGV is a list that old all the arguments passed tothe Perl script.

@_ is a list of all the variables passed to thecurrent subroutine

Special Variables

$_ is a variable that hold the current topic.

e.g.;

while (<FILE1>){

print “line $. $_”

}

Special Variables

This is the current line number

This is the current line being

processed in FILE1

Thankyou

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