Perl Lecture #1

Perl Lecture #1

Scripting Languages

Fall 2004

Perl

•
Practical Extraction and Report Language



•
-
created by Larry Wall
--

mid
–

1980’s

–
needed a quick language

–
didn’t want to resort to C

–
derivative of sed or awk (interpreted language
used on Unix / sed
–

stream editor.

Perl Intro Cont’d

•
fills the gap between C and awk

•
very powerful language / easy to learn

•
used to write small scripting programs as well as
larger applications

•
for the web has been used
–
cgi scripts
–

run
forms etc.

•
also web apps
–

shopping cart applications

•
makes use of regular expressions
–

powerful
sequence of characters

Why Use?

•
Perl is free

•
many Perl ide’s are free
–

works great
integrated in a Unix environment as most
version come with Perl / Mod
-
Perl and
Emb Perl

•
Works well in a Windows environment as
well

•
CPAN
–

comprehensive Perl Archive
Network

#!/usr/bin/perl

#Author: Lori N

#Description: First program

#Date: Today’s Date

$string="Top 10";

$number=10.0;

print "Number is 10.0 and string is 'Top 10'
\
n
\
n";

$add = $number + $string;

print "Adding a number and a string: $add
\
n";

$concatenate = $number . $string;

print "Concatenating a number and a string: $concatenate
\
n";

$add2 = $concatenate + $add;

print "Adding the previous two results: $add2
\
n
\
n";

$undefAdd = 10 + $undefNumber;

print "Adding 10 to an undefined variable: $undefAdd
\
n";

print "Printing an undefined variable: $undefVariable(end)
\
n";

$Scalar

•
When we have just one of something we
have a scalar

–
simplest kind of data that Perl Manipulates.

–
either a number or a string of characters

–
Perl uses them interchangeably

–
no need to declare a variable

–
Perl will figure it out by its usage

Numbers

•
int and floating pt numbers

•
Perl computes with double
-
precision fp values

•
Literal

–
is not a result of calculation or I/O op
–

data written
directly into the source code

–
0

–
2001

–
-
4

–
also use Octal ( base 8 ) , hexadecimal ( base 16)

Strings

•
seq of characters

•
they have a literal representation
–

‘single
quoted’ and “double quoted”

•
Single quoted Literals



-

‘string’



-

‘string
\
’s’



-

‘hello
\
n’
–

no newline

Double Quoted Strings

-
Double quoted Literals



--
“string”


--
“string
\
n”
–

newline


--
“string
\
””

String operators:

•
“hello” . “world” = helloworld

•
“hello” . ‘ ‘ . “world” = hello world

String repetition operator

–
x
–

takes its left operand ( a string ) and
makes as many concatenated copies as you
specify

–
“string” x 3
--

stringstringstring

–
5 x 4
--

5555

Automatic Conversions

•
Perl automatically performs conversions
between Numbers and Strings.


–
by the operator used or they way you attempt
to use them in your script



--

be careful this might not work out
logically like you’d like it to

Warnings

•
Perl’s Built in Warnings.

–
Command line
–

perl
–
w myfile.pl

–
Or add it to your code #!/usr/bin/perl
–
w


–
use man perldiag to see more useful
troubleshooting flags

–
also see man perllexwarn man page for
warnings that can be turned on and off.

Scalar Variables

•
variable
–

all should be familiar
–

they hold
values

•
a scalar variable holds a single scalar
value

•
they all begin with $Perl_identifier

•
can’t start with a digit

•
they are also referenced with the leading $

Scalar Assignment

•
--
assignment


•

--
$income = ‘tolittle’;

•

--
$tax_amount = 1000;

•

--
$miles = 100;

•

--
$distance = $miles * 5;

•
Similar binary operators as C

•
--
+= , *= , .= (string concatenator)

Output

•
--
print “Hello World
\
n”;

•
--
print ( )

•
--
in a series separated by comma’s

•
--
print “My income is “, 0 * 10000 , “
.
or
null
\
n”;

Interpolation of Scalar variables
into Strings

•
$income = “not much”;

•
$expenses = “quite a bit”;

•
$lifesavings = “My income is $income but
my expenditures are $expenses”;

•
or $lifesavings = ‘My income is ‘ . $income
. ‘but my expenditures are ‘ . $expenses;

•
Book has table on page 32
–

Operator
Precedence and Associativity

Comparison Operators

•
< <= == >= > !=

•
Strings
–

eq , ne , lt , gt , le , ge

if Control Structure

•
if ( $variable <= $anothervariable) {

•

Print this;

•
}

•
Curly braces are required

No Boolean Data Type

•
No Boolean data type
–

used simple rules:

•
--
the undef value

•

--
what if you use a scalar value before you give it a value?

•

--
Perl gives it a undef value
–

neither a string or a number

•

--
acts like zero
–

or an empty string

•


•
--
for Boolean process uses simple rules

•

--
undef is false

•

--
Zero is false
–

all else true

•

--
empty string ‘ ‘ is false
–

all else true

•

--
The one exception
–

since numbers and strings are
equivalent, the string form of zero, ‘0’ has the same value as its
numeric form
–

false

User Input

•
--
line
-
input operator <STDIN>

•
--
Perl reads the next complete line of text from standard
input ( up to the first newline)

•
--
uses it as the value of <STDIN>

•
--
its string value has a newline character on the end of it
:

•

$line = <STDIN>

•

if ($line eq “
\
n”) {

•


print “That was just a blank line!
\
n”;

•

}else {

•


print “That line of input was : $line”;

•

}

Chomp Operator

•
--
works on a variable

•
--
variable has to hold a string

•
--
if the string ends in a newline
–

it removes it

•
--
take input from <STDIN>
--

chomp removes
\
n and

•
provides us with just the string.

•
One step:

•

chomp($variable = <STDIN>)

•
--
chomps return value is the number of characters
removed
–

1

while Control Structure

•
--
same as C++

Tutorial:

Simple Perl program:

#!/usr/bin/perl
–
w

#

#Name: Add Name

#Date: Today’s Date

#Description: first.pl Ask and Display name

print “Please enter you name “;

$name = <STDIN>;

chomp ($name);

print “Your name is $name”;

Execute Your Code

•
Has to be executable:


•
chmod 755 first.pl


•
To run two ways:

•
perl first.pl

•
or ./first.pl