Introduce to perl slide

Your first Perl script#!/usr/bin/perl #This script prints a friendly greeting to the screen print “Hello World\n”; • Scripts are first “compiled” and then “executed” in the order in whic

Introduction to Perl

What is Perl?

• Practical Extraction and Report Language

• A scripting language which is both relatively simple to learn and yet remarkably powerful.

Introduction to Perl

Perl is often described as a cross between shell

programming and the C programming language

C (numbers)

Shell programming (text)

Smalltalk (objects)

C++

(numbers, objects)

Perl (text, numbers) Java

Introduction to Perl

• A “glue” language Ideal for connecting things

together, such as a GUI to a number cruncher, or a

database to a web server

• Has replaced shell programming as the most popular

programming language for text processing and Unix

system administration

• Runs under all operating systems (including Windows)

• Open source, many libraries available (e.g database,

internet)

• Extremely popular for CGI and GUI programming

Why use Perl ?

• It is easy to gain a basic understanding of the language and start writing useful programs

Why use Perl?

• Perl is free and available on all computing

platforms.

– Unix/Linux, Windows, Macintosh, Palm OS

• There are many freely available additions to Perl (‘Modules’).

• Most importantly, Perl is designed to

understand and manipulate text.

Where to find help!

• http://www.perl.com

• http://www.perl.org

Your first Perl script

#!/usr/bin/perl

#This script prints a friendly greeting to the screen

print “Hello World\n”;

• Scripts are first “compiled” and then “executed” in the order in which the lines of code appear

• You can write a script with any text editor The only rule is that it must be saved as plain text

Running Perl Scripts

• Perl 5 is installed on our CS system

• Run from the command line:

palazzi% which perl /usr/bin/perl

palazzi$ perl hello.pl Hello world!

• You can run the script directly if you make the script executable, and the first line uses ‘hash-bang’ notation:

palazzi% chmod +x hello.pl palazzi% hello.pl

#!/usr/bin/perl -w print "Hello world!\n";

Basic Syntax

• The -w option tells Perl to produce extra warning messages about potential dangers Always use

this option- there is never (ok, rarely) a good

reason not to.

#!/usr/bin/perl -w

• White space doesn't matter in Perl (like C++),

except for #!/usr/bin/perl -w which must start from column 1 on line 1.

Basic Syntax

• All Perl statements end in a semicolon ; (like C)

• In Perl, comments begin with # (like shell

scripts)

– everything after the # to the end of the line is

ignored

– # need not be at the beginning of the line

– there are no C-like multi-line comments: /* */

Perl Example

• Back to our “Hello World” program:

palazzi% hello.pl

#!/usr/bin/perl -w

# This is a simple Hello World! Program.

print "Hello world!\n";

– The print command sends the string to the screen, and “\n“ adds a new line

– You can optionally add parentheses:

print(Hello world!\n);

First Script Line by Line

# This script prints a friendly greeting to the screen

• This is a Perl ‘comment’ Anything you type after a pound sign (#) is not interpreted by the compiler

These are notes to yourself or a future reader

Comments start at the ‘#’ and end at a carriage return

• #!/usr/bin/perl is NOT a comment (note this exception)

First Script Line by Line

print “Hello World!\n”;

• This is a Perl ‘statement’, or line of code

• ‘print’ is a function - one of many

• “Hello World!\n” is a string of characters

– note the ‘\n’ is read as a single character

meaning ‘newline’

• The semicolon ‘;’ tells the interpreter that this line of code is complete.

Many ways to do it!

# welcome.pl

print ( "1 Welcome to Perl!\n" );

print "2 Welcome to Perl!\n" ;

print "3 Welcome ", "to ", "Perl!\n";

print "4 Welcome ";

print "to Perl!\n";

print "5 Welcome to Perl!\n";

print "6 Welcome\n to\n\n Perl!\n";

System Calls

• You can use Perl to execute shell

commands, just as if you were typing them

on the command line.

• Syntax:

– `command` # note that ` is the ‘backtick’

character, not the single quote ‘

A script which uses a system call

• Note we are now using a ‘variable’ to hold the results of our system call

#!/usr/bin/perl

$directory_listing = `ls -l `;

print $directory_listing;

Perl Variables and Truth

What is a variable?

• A named container for a single value

– can be text or number

– sometimes called a ‘scalar’

• A scalar variable has the following rules

– Must start with a dollar sign ($)

– Must not start with a number

– Must not contain any spaces

– May contain ‘a’ through ‘Z’, any number character, or the ‘_’ character

• $msg=“My name is $name”;

• In most cases, Perl determines the type (numeric

vs string) on its own, and will convert

automatically, depending on context (eg, printing

vs multiplying)

Scalar variable names

• These are valid names

Variable name tips

• Use descriptive names

– $sequence is much more informative than $x

– $sequence1 is ok $sequence_one is fine too

• Avoid using names that look like functions

– $print is probably bad (it will work!)

• Try to avoid single letter variable names

– $a and $b are used for something else

– Experienced programmers will often use $i and

$j as ‘counters’ for historical reasons

Operator Description Example Result

. String concatenate 'Teddy' 'Bear' TeddyBear

= Assignment $bear = 'Teddy' $bear variable contains 'Teddy'

A Perl calculator

#!/usr/bin/perl

$value_one = shift; #Takes the first argument from the command line

$value_two = shift; #Takes the next argument from the command line

$sum = $value_one + $value_two;

$difference = $value_one - $value_two;

$product = $value_one * $value_two;

$ratio = $value_one / $value_two;

$power = $value_one ** $value_two;

print "The sum is: $sum\n";

print "The difference is: $difference\n";

print "The product is: $product\n";

print "The ratio is: $ratio\n";

print "The first number raised to the power of the second number is: $power\n";

print ("I could have also written the sum as:", $value_one + $value_two, "\n”);

• Special chars: $,@,%,&,”

• Use single quotes to avoid interpolation:

– print ‘My email is bhecker@acm.org Please send me $’;

– (Now you need to escape single quotes.)

• Another quoting mechanism: qq() and q()

– print qq(She said “Nortel cost \$$cost \@ $time”.);

– print q(My email is bhecker@acm.org Please send me $);

– Useful for strings full of quotes

Backquotes: Command Substitution

• You can use command substitution in Perl like in shell

scripts:

$ whoami bhecker

Hi bhecker! There are 6 users logged on

• Command substitution will usually include a new line, so

$nword = `wc -w $big | cut -c6-8 `;

chomp($nword);

$nchar = `wc -c $big | cut -c6-8 `;

chomp($nchar);

print "The biggest file in $dir is $big.\n";

print "$big has $nline lines, $nword words, $nchar characters.\n " ;

$ big1

The biggest file in /homes/horner/111/perl is big1.

Quotes and more Quotes - Recap

• There is a fine distinction between double quoted

strings and single quoted strings:

– print “$variable\n” # prints the contents of $variable

and then a newline

– print ‘$variable\n’ # prints the string $variable\n to the

screen

• Single quotes treat all characters as literal (no

characters are special)

• You can always specify a character to be treated

literally in a double quoted string:

– print “I really want to print a \$ character\n”;

Even more options

• the qq operator

– print qq[She said “Hi there, $stranger”.\n] ; #same as

– print “She said \”Hi there, $stranger\”.\n” ;

• qq means change the character used to denote the

string

– Almost any non-letter character can be used, best to pick

one not in your string

• print qq$I can print this string\n$;

• print qq^Or I can print this string\n^;

• print qq &Or this one\n&;

– perl thinks that if you use a ‘(‘, ‘[‘, or ‘{‘ to open the

string, you mean to use a ‘)’, ‘]’, or ‘}’ to close it

What is Truth?

• A question debated by man since before cave art.

• A very defined thing in PERL.

– Something is FALSE if:

• a) it evaluates to zero

• b) it evaluates to ‘’ (empty string)

• c) it evaluates to an empty list (@array = “”)

• d) the value is undefined (ie uninitialized variable)

– Everything else is TRUE

Numeric Comparison Operators

Operator Description Example Result

<= Greater Than or Equal 3<=2 TRUE

>= Less Than or Equal 3>=2 FALSE

<=> Comparison 3 <=> 2 1

" 2 <=> 3 -1

" 3 <=> 3 0

• Do not confuse ‘=‘ with ‘==‘ !!!!

•<=> is really only useful when using the ‘sort’ function

String (Text) Comparison

Operators

•cmp is really only useful when using the ‘sort’ function

Operator Description Example Result

eq Equality 'cat' eq 'cat' TRUE

ne Non Equality 'cat' ne 'cat' FALSE

gt Greater Than 'data' gt 'cat' TRUE

lt Less Than 'data' lt 'cat' FALSE

ge Greater Than or Equal 'data' ge 'cat' TRUE

le Less Than or Equal 'data' le 'cat' FALSE

cmp Comparison 'data' cmp 'cat' 1

" 'cat' cmp 'data' -1

" 'cat' cmp 'cat' 0

What did you mean?

• To make your life ‘easier’, Perl has only one

data type for both strings (characters) and numbers.

• When you use something in numeric context,

Perl treats it like a number

– $y = ‘2.0’ + ‘1’; # $y contains ‘3’

– $y = ‘cat’ + 1; # $y contains ‘1’

• When you use something in string context,

perl treats it like a string.

– $y = ‘2.0’ ‘1’; # $y contains ‘2.01’

• In short, be careful what you ask for!!

• Functions are little bundles of Perl code

with names They exist to make it easy to

do routine operations

• Most functions do what you think they do,

to find out how they work type:

– perldoc -f function_name

A Perl Idiom - if

• if is a function which does something if a

condition is true.

– print “Number is 2” if ($number == 2);

• Of course, there is also a function that does the opposite - unless

– print “Number isn’t 2” unless ($number == 2);

• You don’t ever need to use unless, unless

you want to

– print “Number isn’t 2” if ($number != 2);

More about if

• A frequent Perl construction is the

if/elsif/else construct

– if (something){ do something }

– elsif (something else) { do something }

– else { do the default thing }

• The block of code associated with the first

true condition is executed.

• Note: elsif, not elseif

print “Class at four.\n”;

} elsif ($today eq “Friday”) {

print “See you at the bar.\n”;

} else {

print “What’s on TV?\n”;

}

Control flow

You’ve already seen a while loop.

for loops are just like C:

for ($i=0; $i<10; $i++) {

print “i is $I\n”;

}

Getting at your data (Input and Output)

A brief Diversion

• Get into the habit of using the -w flag

– mnemonic (Warn me when weird)

• Enables more strict error checking

– Will warn you when you try to compare strings numerically, for example

• Usage

– command line: ‘perl -w script.pl’

• even more diversion: ‘perl -c script.pl’ compiles but

does not run script.pl

– Or line: #!/usr/bin/perl -w

Data flow

• Unless you say otherwise:

– Data comes in through STDIN (Standard IN)

– Data goes out through STDOUT (Standard Out) – Errors go to STDERR (Standard Error)

• Error code contained in a ‘magic’ variable $!

• A shortcut to do both operations in one line is:

chomp($name = <STDIN>);

$area = $height * $width;

print "The area of the rectangle is $area\n";

$ test.pl

Enter height of rectangle: 10

Enter width of rectangle: 5

The area of the rectangle is 50

$ test.pl

Enter height of rectangle: 10.1

• The most common use of while is for

reading and acting on lines of data from a file

#while_count.pl while ($val < 5){

Shortcut operators

• Sometimes called auto operators (auto-increment, auto-decrement)

• Optimized for speed and efficiency

Operator Usage Read as:

++ $i++ $i = $i + 1 $i $i = $i - 1 += $i += 20 $i = $i + 20 -= $i -= 5 $i = $i - 5

*= $i *= 2 $i = $i * 2 /= $i /= 2 $i = $i / 2

.= $i = 'foo' $i = $i 'foo'

• A filehandle is a way to interact with input or output

– ‘<>’ interacts with files on the command line

• filehandle names are simple strings with no symbols

– I usually use all caps (SEQFILE), but that isn’t

necessary

• You must open your filehandle before using it

Opening Filehandles

• Open a file for reading

– open NAME, “<filename”;

• This is default behavior, so you don’t actually need the ‘<‘

• Open file for writing

– open NAME, “>filename”; #open new file

• Warning: If filename already exists, it is overwritten!!

– open NAME, “>>filename”; # append to old file

• Flexible coding

– I want to specify the file to open on the

command line, rather than hard coding it

$in_name = shift;

$out_name = shift;

open FILE, “<$in_name” or die “Couldn’t open $in_name for reading: $!\n”; open OUT, “>$out_name” || die“Couldn’t open $out_name for reading: $!\n”; while ($line = <FILE>){

chomp $line;

print OUT “Something about $line\n }

• Usage: <$> myscript.pl inputfile outputfile

When do I use a filehandle?

• You can get away with not using them, mostly

– STDIN is fine (<>) and you can always capture your

STDOUT to a file with a redirect (>) on the command line – <$> myscript.pl file_in > file_out

• If you are using two input files for different purposes

or want more than one output file, you need

filehandles

– <> will slurp all the input files on command line!

– > on the command line will put all output to one file

Perl as Duct Tape (the force that

glues the universe together)

• The STDOUT of one script can serve as the STDIN of another script

– use the pipe (‘|’) symbol to chain scripts together

• Nothing goes to the screen in between scripts

– instead, what would normally go to the screen is redirected and made the STDIN of the next script

Lists and More Lists

(Perl Arrays)

– or: my $variable = ‘value’;

• my sets the ‘scope’ of the variable Variable exists

only within the current block of code

• use strict and my both help you to debug errors,

and help prevent mistakes

What is an array?

• A named container for a list of values

– can be text or number, or mix

– An array is an ordered list

• Array names follow the same rules as scalar variables

– No spaces

– a-Z 0-9 and ‘_’ only

– Cannot start with a number

Making an array

• @my_array = (1,15,’cat’, 23, ‘blue’);

– Note this is a comma separated list, enclosed in parentheses The parentheses are very important!!

• A tricky way:

– @my_array = qw (1 15 cat blue);

• mnemonic: qw - ‘Quote Words’

• Remember no commas if you use qw!

A picture might help

• @my_array = (1,15,’cat’, 23, ‘blue’);

• @my_array

0 1 2 3 4

1 15

‘cat’

23

‘blue’

Element # Contents

Getting at the Array Elements

• @my_array = (5, ‘boo’, ‘16’, ‘hoo’);

• $my_array[1] contains ‘boo’

– Pay attention! The way this is written is important

• An array element is a single (scalar) value

• Starts with the $ sign (just like a scalar) not the @ sign

• Square braces indicate the array position (index, or

element number)

• Perl counts from zero!! First element is $my_array[0]

Manipulating Array Elements

• You can do anything to an array element that you can do to a scalar.

– $my_array[2] = ‘scary’;

• Of course you can do an assignment (=)

• list now is (5, ‘boo’, ‘scary’, ‘hoo’)

– $string = $my_array[2].$my_array[1]

• $string contains ‘scaryboo’

– $my_array[5] = ‘16’;

• list now (5, ‘boo’, ‘scary’, ‘hoo’, ‘’, ’16’)

• your list is as long as it needs to be!