introduction to perl pawel sirotkin 28.11-01.12.2008, riga

Introduction to Perl

Pawel Sirotkin28.11-01.12.2008, Riga

Overview

Introduction to Perl, NLL Riga 2008, by Pawel Sirotkin

2

About programming Why Perl?

How to write, how to run Variables Operations Basic input and output Conditionals and loops Regular expressions

About programming


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Working with algorithms Program needs to contain exact commands

(Mostly) not: Go buy some bread But: Put on your coat and shoes, open the door, go

through it, close the door, go down the stairs… Has a certain input Processes it Produces a certain output

Why Perl?


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Easy to learn Simple syntax Good at manipulating text

Good at dealing with regular expressions

How to write a Perl program


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Perl programs can be written in any text editor Notepad, vim, even Word… Recommended: A simple text editor with syntax

highlighting

Write the program code Save the file as xxx.pl

.pl extension not necessary, but useful

What is a Perl program like?


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# This *very* simple program prints "Hello World!“

print "Hello World!";



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The content of a line after the # is commentary. It is ignored by the program

What are commentaries for, then? They are for you, and others who will have to read

the code Imaging looking at a complex program in a few

months and trying to figure out what it does Write as much commentary as you can





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This is a Perl command In this case, for printing text on the screen

Every command should start at a new line Not a Perl requirement, but crucial for readability

Every command should end with a semicolon; Many commands take arguments

Here: “Hello World!”



What to do with the program?


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Perl works from the command line Windows: „Start“ „Run…“ Go to the directory where you saved the

program E.g.: cd C:\Perl\MyPrograms

Run the program: perl myprogram.pl

See the results of your labours!

Exercise (1)


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Create a folder for your Perl programs Open the editor of your choice and write the

„Hello World“ program The command is print „Hello World!“; Don‘t forget the commentary!

Save the program Run it! What happens if you misprint the print

command?

Variables


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The „Hello World“ program always has the same output Not a very useful program, as such

We need to be able to change the output Variables are objects that can hold different

values

Defining variables


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To define a variable, write a dollar sign followed by the variable’s name Names should consist of letters, numbers and the

underscore They should start with a letter Variable names are case-sensitive!

$a and $A are different variables! Generally, a variable’s name should tell you what

the variable does

# We define a variable „a“ and assign it a value of „42“

$a = 42;

Defining variables


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Variables can be assigned values String: text (character sequence) in quotes/double

quotes Numbers

$a = 42; $a = “some text”;

# We define a variable „a“ and assign it a value of „42“

$a = 42;

Changing variables


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Arithmetic operations $a = 42 / 2; # division $a = 42 + 5; # addition $a = $b * 2;# multiplication $a = $a - $b; # subtraction

Also useful: $a += 42; # the same as $a = $a + 42; The same for +, -, /

String operations $a = “some“ . “ text“; # concatenation $a = $a . “ more text“;

Basic output


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We have already seen an output command print “text“; print $a; print “text $a“; print “text “ . $a+$b . “ more text.“; Special characters:

\n – new line \t – tabulator

Exercise (2)


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Define a variable Assign it a value of 15 Print it Double the value Print it again Define another variable with the string

„apples“ Print both variables Change the first variable to its square and the

second to „pears“ Print both variables

Basic input


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The <> operator returns input from the standard source (usually, the keyboard)

Syntax: $a = <>;

Don’t forget to tell the user what he’s supposed to enter!

Try the following program:# This program asks the user for his name and greets him

print "What is your name? ";$name = <>;print "Hello $name!";

Input, output and new lines


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As the user input is followed by the [Enter] key, the string in $name ends in a new line

The chomp function deletes the new line at the end of a string

Try the following, modified program:

# This program asks the user for his name and greets him

print "What is your name? ";$name = <>;chomp($name);print "Hello $name!";

Exercise (3)


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Let the user enter the radius of a circle Tell him the diameter (2r), circumference (2πr)

and area (πr²) of the circle Try doing this using one variable for each measure Try doing this using only one variable

If, else


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Until now, the course the program runs is fixed The if clause allows us to take different actions

in different circumstances

# Let‘s try out a conditional clause

print "Please enter password: ";$password = <>;if ($password == 42) {

print "Correct password! Welcome.";} else {

print "Wrong password! Access denied.";}

If, else


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Note: = is the assignment operator, == is the comparison operator

Else is an optional operator triggering if the if condition fails

# Let‘s try out a conditional clause

print "Please enter password: ";$password = <>;if ($password == 42) {

print "Correct password! Welcome.";} else {

print "Wrong password! Access denied.";}

Exercise (4)


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Try out the password program. Why doesn‘t it work correctly? Fix it. Tell the user if the number he entered is too large

or too small Hint: The comparison operators you’ll need are < and >

Ask the user for a geometrical form (circle or square), and then for a radius or side length. Return the area and perimeter.

While


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What if we want to do checks until something happens?

The while loop repeats commands until its criteria are met Note: in the example below, $password has no value,

so it specifically doesn’t have the value 42

# Now on to a "while" loopwhile ($password != 42) {

print "Access denied.\n";print "Please enter password: ";$password = <>;chomp($password);

}print "Correct password! Welcome.";

Exercise (5)


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Write a small game: take a number, and make the user guess it. Tell him if it‘s too high or too low. If the user gets it right, the program terminates. If you like, you can take a random number:

$random = int (rand(10) );

Perl regular expressions


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Regular expressions very useful for text processing Perl matching character: =~

Perl non-matching character: !~ The regular expression must be in backslashes:

/regex/ The program below accepts any password that

contains the characters „42“ anywhere# A "while" loop with regular expressionswhile ($password !~ /42/) { # While the entered line doesn’t contain “42”

print "Access denied.\n";print "Please enter password: ";$password = <>;chomp($password);

}print "Correct password! Welcome.";



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Simple string: some text One of a number of symbols: [aA]

Matches a or A Also possible: [tT]he, matching the or The

One of a continuous string of symbols: [a-h][1-8] Matches any two-character string from a1 to h8

Special characters ^ matches the beginning of a line $ matches the end of a line



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More special characters Wildcard: the dot . Matches any single character

b.d matches bad, bed, bid, bud… Don‘t forget: it also matches forbid, badly…

+ matches one or more of the previous character re+d matches red and reed (and also reeed and so on!)

* matches zero or more occurrences of the previous character bel* matches be, bel and bell (and belll…)

? matches zero or one occurrences of the previous character soo?n Matches son or soon



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Character classes \d: digits

Rule \d+ matches Rule 1, Rule 2, ..., Rule 334... \w: “word characters” – letters, digits, _

\w \w – any two “words” separated by a blank \s: any whitespace (blanks, tabs)

^\s+\d – any line where the first character is a digit Capitalize the symbols to get the opposite

\S is anything but whitespace, \D are non-digits…

Exercise (6)


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Write a program which asks the user for his e-mail address.

Check if the address is syntactically correct. Possible rules:

Must contain an @ character At least one symbol before it Must contain a dot At least two symbols between @ and . At least two symbols after . No fancy symbols like {§*

Do you accept addresses with more than one dot?



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Switches Tell Perl how to deal with the regular expression /regex/i: ignore lower/upper case

/wiebke/i matches Wiebke and wiebke s/regex/regex2/: substitute regex with regex2

$text =~ s/Mark/Euro/ /regex/g: repeat match until end of the line

# What the //g switch does

$text = “The meat costs 10 Mark, the fish costs 15 Mark.”;$text2 = $text1;$text =~ s/Mark/Euro/; # “The meat costs 10 Euro, the fish costs 15 Mark.”$text2 =~ s/Mark/Euro/g; # “The meat costs 10 Euro, the fish costs 15 Euro.”



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Grouping Allows us to use matched string /(text)/ matches text and stores it in a variable

The first group is stored in $1, the second in $2...

# Substitution and grouping

$sum = 0; # initializing the variable with zero$text = “The meat costs 10 Mark, the fish costs 15 Mark.”while ($text =~ s/(\d+) Mark/$1 Euro/) { # numbers-spaces-”Mark”

$sum = $sum + $1; # adding amount to $sum value}print “Substituted $sum Mark for Euro!”;

Reading files


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What if we want to have input from a file, not from the user?

Open file for reading: open(INPUT, "<file.ext");

Read a line: $line = <SOURCE>; $line = <>; # is just a special case

Writing files


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What if we want to print to a file, not to the screen?

Open file for writing: open(OUTPUT, “>file.ext");

Write: print OUTPUT “Some text...”;

Reading files


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A program for testing e-mail addresses Note: If we want to use a special character literally,

we need to escape it with a backslash In strings : " In regular expressions: . + * ^ $ and the backslash \ itself

open(INPUT, "<test.txt");while ($line = <INPUT>) {

chomp($line);if ($line =~ /^.+@..+\...+$/) { # testing for e-mail:

[email protected] "\"$line\" is a valid e-mail address.\n";

} else {print "E-mail address \" $line\" not valid.\n";

}}

Exercise (7)


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Make a text file and fill it with a Wikipedia article Count the number of definite and indefinite

articles Count the number of numbers and digits Insert a <number!> tag before every number

Arrays


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Arrays contain lists of variables Syntax:

@days = [“Monday“, “Tuesday“, “Friday“]; $days[0] = “Saturday“; $day = $days[2];

Useful for storing linear sequences of variables

Note: @ for whole lists, $ for single variables

Arrays


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Useful array commands push(@array, “element“);

Adds a new element to the end of the array Creates the array if necessary

$element = pop(@array); Moves the last value of @array to $element

# Trying out arrays

@tags = (“N”, “V”, “Adj”);$tag1 = pop(@tags); # $tag1 is now “Adj”, @tags is (“N”, “V”)$tag2 = pop(@tags); # $tag2 is now “V”, @tags is (“N”)Push(@tags, „V“, $tag2); # @tags is now again (“N”, “V”, “Adj”)

Hashes


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Hashes are associative arrays They are lists where the elements are not

ordered, but identified by a „name“ Syntax:

%probability = (”verb“, 0.32,“adjective“, 0.02,“adverb“, 0);

$probability{“noun”} = 0.52;

Exercise (7)


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What happens if you try to print an array? What about a hash?

What happens if you convert an array into a hash, or the other way round?

Practical: Tokenizer


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Take a Wikipedia article and put it into a text file Clean it up if necessary

Tokenize it! We only want one word per line Insert a „sentence boundary“ symbol where

appropriate The output should be another file Think about what choices you make and why!

Practical: Tagger


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Take the POS-annotated corpus from treebank.txt

Clean and tokenize it Count the tag-token probabilities Count the transition probabilities

For the first time, I strongly recommend bigrams Apply the Viterbi algorithm and tag an input

file of your choice!

Practical: Tagger++


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If it‘s still too easy, or if you want a long-term aim: Implement smoothing: words can have tags you

haven‘t seen them with, or appear in contexts you never saw them before

Try to figure out a way to guess the tags for unknown words better

Write a program to train on 9/10 of the corpus, and test it on the rest. Compare your results to the actual annotations Do this 10 times for every 9/10

Still too easy? Implement trigrams and compare the results.

introduction to perl pawel sirotkin 28.11-01.12.2008, riga

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