Perl Intro 01

8/3/2019 Perl Intro 01

1/29

Getting started in Perl:

Intro to Perl for programmers

Matthew Heusser xndev.com - [email protected]

Presented to the West Michigan Perl Users Group


2/29

Why use Perl?

Discuss


3/29

Getting Perl

www.activestate.com

The easy way to get started in Windows

www.cpan.org (Comprehensive Perl ArchiveNetwork)

Ifyou have Linux, chances are you have Perl

Both support modules for almost everythingimaginable.


4/29

My first Perl script

#!/usr/bin/perl -wuse strict;print "Hello, world\n";


5/29

Variables in Perl

Scalars start with a $ ex: $foo

Arrays start with a @ ex: @foo

Hashs (Associative arrays) start with % ex:%foo

By convention file handles are UPPERCASE

(no funny character). my gives variable lexical (local) scope. Use

this unless you have reason for some otherscope.


6/29

Scalars

A Scalar begins with a $

A Scalarholds a single string, number or

reference.

Unitialized scalars are undef.

Examples:

my $num = 3.1412;

$str = "This is a string";


7/29

Type Safe

Perl is a loosely-typed language

To make a string, evaluate the scalar in

string context Example: if ($a eq Fifty) { do_stuff($a); }

To make a number, evaluate as a number

Example: if ($a == 50) { do_stuff($a); }

Conversion is like ATOI

To guarantee output, use prinft or sprintf


8/29

Some control structures

Use {} (Curlies) to declare what your controlstrucuture is working with. Like BEGIN/END

in PL/SQL or Pascal. Usual if-elsif (note missing e), while, and

for structures.

Also supports unless (!if), until (!while),and foreach strucutures.

next; is like continue; in C.

last; is like break; in C.


9/29

Boolean context

The scalars: (null string), 0, 0, and undefevaluate to false, everything else evalutates

to true. Lists are put into scalar context, then

evaluated for truth value. Zero length lists,and undef arrays evalute to false, all other

lists are true.


10/29

Some operators

Perl supports the usual +,-,*,/,%,++,-- (add,substract, multiply, divide, modulo,

increment, decrement). . (period) concatenates strings.

C style +=, .= etc. are supported.

Use ==, !=, >, >=,


11/29

Basic I/O STDIN is a file

To read from a file, do this:

my $str = ;

Control-Z is the EOF symbol on windows

To re-direct STDIN from a file, do this:

UNIX: perl scriptname.pl < in.txt

Win: type in.txt | perl scriptname.pl

To loop until EOF, do this:

while (my $str = ) {

}


12/29

Exercise

1) Write a program to add two scalar variablestogether and print the total.

3) Write a program to:

Read three numbers from the command line

Add them up

Print the total

2) Mod the program to also print an average


13/29

Arrays

Array names begin with a @

Arrays are composites of scalars that areindexed with numbers beginning with 0.

Arrays are named Lists. Subtle differencesexist between Arrays and Lists.

Examples:

my @stuff = (1,2,3); # use @ for thecomposite

$stuff[0] = 10; # use $ for a single element(scalar)

$stuff[99] = Numbers and strings can be mixed;


14/29

Iterating over a list

for (my $i=0; $i< scalar(@a);$i++) {

do_something($a[$i]);

}

for my $val (@a) {

do_something($val);

}


15/29

Push and Pop

my @a;

push (@a, 2);

push (@a, 3);

my $var = pop(@a);

Remember, lists are automatically managed

A stack using arrays is now trivial

I sure wish I had this for the AP computer science Atest!


16/29

Exercises

1) Write a program to:

Read three numbers from the commandline

Into a list

Loop and re-print the numbers

Print the total

2) Print the numbers in reverse

Its ok to use a c-style for loop


17/29

Hashes

Hash names begin with %

Hashes are composites of scalars that are indexedwith scalars.

Hashes are unordered.

Example:

my %employee;

$employee{"name"} = "Bill Day";$employee{"SSN"} = "353-27-7625";

print $employee{"name"}, $employee{"SSN"}

- Outputs: Bill Day353-27-7625


18/29

Scalar vs List Context

In an assignment, context is determined byleft side of equal sign.

An array in scalar context evaluates to lengthof the array: $len = @stuff;

(parenthesis) will put a scalar into list

context: ($thing) = @stuff; # assigns $stuff[0]to $thing.

Psudeo-function scalar can be used to toforce a list into a scalar. Example:

scalar(@stuff);


19/29

Quotes in Perl

'Ordinary quotes'

"Interpolated quotes - $vars exapanded\n", This ismy favorite..

`execute a shell` command and return the resultas a string.

my $var = " test ";

print " $var\n",'$var\n

';

Outputs:

test

$var\n


20/29

Iterating over a hash

#!/usr/bin/perl -w

use strict;

...

my $key, $value, %hash;

...

while (($key, $value) = each %hash) {

print $key $value "\n";

}


21/29

Pronouns in Perl

$_ is the default variable Example:#!/usr/bin/perl -w

use strict;

my @array = ("a", "b", "c");

# the following 2 loops are equivalent

foreach my $element (@array) {

print $element;}

foreach (@array) {

print;

}


22/29

Regular Expressions

Much like SED

if (/Bill Day/) # evaluate $_, true if it contains

string. if ($var =~ /Bill Day/) # evalute $var for string.

$var =~ s/Bill/William/; # substitue the 1st occuranceof Bill with William in $var.

Lots of special characters: ., ?, *, +, (, ), [, ], | ^, \, {,},

One of the most powerful features of Perl.

Unfortunately beyond the scope of this talk.


23/29

I/O in Perl

open IN, "name"; # open name for reading.

open HANDLE, "output; # create or truncatefile for output.

open LOG, ">>logfile "; # append or create

file for output. close HANDLE; # When done with file.

All the POSIX C style stuff works.


24/29

I/O in Perl Continued

# (diamond operator) to read aline from the file.

print HANDLE "string"; # prints to file. Note:no comma between HANDLE and string.

with no handle reads each file given onthe command line, else if command lineblank STDIN. Just like you want yourstandard Unix utility to do.


25/29

Errors and warnings

die "meltdown in progress"; # message toSTDERR for fatal errors (exits program).

warn "your shoe is untied."; # message toSTDERR for non-fatal warnings.


26/29

Putting it all together#!/usr/bin/perl -w

use strict;

open ORIGINAL, "


27/29

Where to get help

In shell: perldoc perl

In shell: perldoc perlre

www.perl.com

www.cpan.org

www.activestate.com

grand-rapids.pm.org


28/29

Review

Perl is the premier open source highperformance cross platform enterprise class

object oriented language that holds togetherthe world wide web.

There's more than one way to do it TMTOWTDI (Pronounced Tim-Toady)

Perl makes Easy things easy, and hardthings possible.

Questions?


29/29

Exercises:

1) Create a hash; write a program to loop through a hash

and print all hash pairs.

2) Write a program to read some numbers in from STDIN.stopping when the user types in 'quit', and print thetotal.

3) Write a program to read in some words from thecommand line, replacing Matthew withMatt, and re-print the words back onto the command line.

Documents

Perl Intro 01