[PPT] - Perl Tutorial-Part II CSCI 3136 Principles of Programming and PowerPoint Presentation

SLIDE 1

Perl Tutorial-Part II

CSCI 3136 Principles of Programming and Languages

1 Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/

SLIDE 2

Outline

Run a Perl Program
Data Types
Context
Operators
Flow Control
Subroutines
References
Files
Regular Expressions

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 2

SLIDE 3

Run a Perl Program

on bluenose:

– perl hello.pl – Add “#!/usr/bin/perl” to the first line of hello.pl – chmod a+x hello.pl – ./hello.pl

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 3

SLIDE 4

Data Types

Scalar

– Singles values – Name preceded by a $ character

Arrays

– Multiple ordered values – Name preceded by a @ character

Hashes

– Multiple unordered values – Name preceded by a % character

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 4

SLIDE 5

Context

Context determines how variables and values

are evaluated

Numeric context
String context
Boolean context
Determined by operators

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 5

SLIDE 6

Operators

Numeric

– ++, --, +, -, *, /, +=, *=, >>, <<

String

– Concatenation (.), repetition (x), assignment (.=, x=)

Quoting

– qq (“), q (‘), qx(`), qw, pattern matching

Boolean

– <, >, ==, !=, =>, <= – lt, gt, eq, ne, le, ge – &&, ||, !, and, or, not – ? :

List

– sort, reverse, push/pop, unshift/shift, split/join, grep, map

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 6

SLIDE 7

Flow Control

Flow control is expressive in Perl
Conditional statements

– if – unless

Loop statements

– while – until – for – foreach

Modifiers

– Simple statements may be modified

See perldoc perlsyn for complete syntax

7 Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/

SLIDE 8

Conditional Statements

Conditional statements provide boolean

context

if statement controls the following block

– if, elsif, else – Yes, that does say elsif not else if

unless is opposite of if

– Equivalent to if (not $boolean) – unless, elsif, else – There is no elsunless, thankfully.

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 8

SLIDE 9

Conditional Statements

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 9

my ($a, $b) = (0, 1); if (!$a && !$b) {print "Neither\n";} # Conventional if (not $a and not $b) {print "Neither\n";} # Same, but in English if (not ($a or $b)) {print "Neither\n";} # Same, but parentheses unless ($a or $b) {print "Neither\n";} # Same, but clearest ($a,$b) = (1,0); unless($a) { print “a is not true”; }elsif($b) { print “b is true”; }else{ print “a is true but b is not true”; }

SLIDE 10

Loop Statements

Loop statements provide a boolean context
while

– Loops while boolean is true

until

– Loops until boolean is true – Opposite of while

do

– At least one loop, then depends on while or until

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 10

SLIDE 11

Loop Statements

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 11

my $counter = 10; while ($counter >= 0) { print $counter.”,”; $counter--; } Output: 10,9,8,7,6,5,4,3,2,1,0, $counter = 5; until ($counter < 0) { print $counter.”,” $counter--; } Output: 5,4,3,2,1,0, $counter = 0; do { print $counter.”,” $counter--; } while ($counter >= 0); Output: 0,

SLIDE 12

Loop Statements

for

– Like C, C++, Java: for (initialization; condition; increment)

foreach

– Iterates over a list or array

Good to localize loop variables with my

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 12

SLIDE 13

Loop Statements

Example

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 13

for (my $i = 10; $i >= 0; $i--) { print "$i,"; # Countdown } Output: 10,9,8,7,6,5,4,3,2,1,0, foreach my $i (reverse 0..10) { print "$i,\n"; # Same } Output: 10,9,8,7,6,5,4,3,2,1,0, %hash = (dog => "lazy", fox => "quick"); foreach my $key (keys %hash) { print "The $key is $hash{$key}.\n"; # Print out hash pairs } Output: The fox is quick. The dog is lazy.

SLIDE 14

Modifiers

Simple statements can take single modifiers

– Places emphasis on the statement, not the control – Can make programs more legible – Parentheses usually not needed – Good for setting default values – Valid modifiers are if, unless, while, until, foreach

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 14

SLIDE 15

Modifiers

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 15

my $default = 0; my $a = $default unless defined $a; my $b = 3; $b = $default unless defined $b; print “$a,$b\n”; Output: 0,3 my $balance = 5000; $balance += $deposit if $deposit; print “$balance\n”; Output: 5000 my withdrawal = 300; $balance -= $withdrawal if $withdrawal and $withdrawal <= $balance; Print “$balance\n”; Output: 4700

SLIDE 16

Subroutines

Subs group related statements into a single

task

Perl allows both declared and anonymous

subs

Perl allows various ways of handling

arguments

Perl allows various ways of calling subs
perldoc perlsub gives complete description

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 16

SLIDE 17

Declaring Subroutines

Subroutines are declared with the sub keyword
Subroutines return values

– Explicitly with the return command – Implicitly as the value of the last executed statement

Return values can be a scalar or a flat list

– wantarray describes what context was used – Unused values are just lost

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 17

SLIDE 18

Declaring Subroutines

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 18

sub ten { @array = (1..10); return wantarray() ? @array : 100; } @ten = ten(); # (1, 2, 3, 4, 5, 6, 7, 8, 9, 10) $ten = ten(); # 100 ($ten) = ten(); # (1) ($one, $two) = ten(); # (1, 2)

sub ten{

@array = (1..10); return @array; } @ten = ten(); # (1, 2, 3, 4, 5, 6, 7, 8, 9, 10) $ten = ten(); # 10

SLIDE 19

Handling Arguments

Two common means of passing arguments to

subs

– Pass by value – Pass by reference – Perl allows either

Arguments are passed into the @_ array

– @_ is the "fill in the blanks" array – Usually should copy @_ into local variables

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 19

SLIDE 20

Handling Arguments

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 20

sub add_one { # Like pass by value my ($n) = @_; # Copy first argument return ($n + 1); # Return 1 more than argument } sub plus_plus { # Like pass by reference $_[0] = $_[0] + 1; # Modify first argument } # no return statement my ($a, $b) = (10, 0); add_one($a); # Return value is lost, nothing changes $b = add_one($a); # $a is 10, $b is 11 plus_plus($a); # Return value lost, but a now is 11 $b = plus_plus($a); # $a and $b are 12

SLIDE 21

Calling Subroutines

Subroutine calls usually have arguments in

parentheses

– Parentheses are not needed if sub is declared first – But using parentheses is often good style

Subroutine calls may be recursive
Subroutines are another data type

– Name may be preceded by an & character – & is not needed when calling subs

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 21

SLIDE 22

Calling Subroutines

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 22

print factorial(5) . "\n"; # Parentheses required sub factorial { my ($n) = @_; return $n if $n <= 2; $n * factorial($n - 1); } print ((factorial 5) . "\n"); # Parentheses around argument not required, # but need to ensure there are no extra arguments print &factorial(5) . "\n"; # Neither () nor & required

SLIDE 23

Subroutines

Example:

– Declare subroutine – Copy arguments – Check arguments – Perform computation – Return results

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 23

SLIDE 24

Subroutines

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 24

sub fibonacci { my ($n) = @_; die "Number must be positive" if $n <= 0; return 1 if $n <= 2; return (fibonacci($n-1) + fibonacci($n-2)); } foreach my $i (1..5) { my $fib = fibonacci($i); print "fibonacci($i) is $fib\n"; } fibonacci(1) is 1 fibonacci(2) is 1 fibonacci(3) is 2 fibonacci(4) is 3 fibonacci(5) is 5

SLIDE 25

References

References indirectly refer to other data
Dereferencing yields the data
References allow you to create anonymous

data

References allow you to build hierarchical data

structures

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 25

my @fruit = qw(apple banana cherry); my $fruitref = \@fruit;

SLIDE 26

Dereferencing Data

Dereferencing yields the data

– Appropriate symbol dereferences original data – Arrow operator (->) dereferences items

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 26

my @fruit = qw(apple banana cherry); my $fruitref = \@fruit; print "I have these fruits: @$fruitref.\n"; print "I want a $fruitref->[1].\n"; Output: I have these fruits: apple banana cherry. I want a banana.

SLIDE 27

Anonymous Data

References allow you to create anonymous

data

– Referencing arrays and hashes are common – Build unnamed arrays with brackets ([]) – Build unnamed hashes with braces ({})

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 27

SLIDE 28

Anonymous Data

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 28

my $fruits = ["apple", "bananas", "cherries"]; #my @fruitarr = ("apple", "bananas", "cherries"); #my $fruits = \@fruits my $wheels = {unicycle => 1, bike => 2, tricycle => 3, car => 4, semi => 18}; #my %wheelhash = (unicycle => 1, bike => 2, tricycle => 3, car => 4, semi => 18); #my $wheels = \$wheelhash; print "A car has $wheels->{car} wheels.\n"; Output: A car has 4 wheels.

SLIDE 29

Hierarchical Data

References allow you to build hierarchical data

structures

– Arrays and hashes can only contain scalars – But a reference is a scalar, even if it refers to an array or a hash – Don't even need the arrow operator for these structures

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 29

SLIDE 30

Hierarchical Data

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 30

my $fruits = ["apple", "bananas", "cherries"]; my $veggies = ["spinach", "turnips"]; my $grains = ["rice", "corn"]; my @shopping_list = ($fruits, $veggies, $grains); print "I should remember to get $shopping_list[2]->[1].\n"; print "I should remember to get $shopping_list[0][2].\n"; Output: I should remember to get corn. I should remember to get cherries.

SLIDE 31

Files

Access to files is similar to shell redirection

– < for input, > for output, >> for append

Standard files

– STDIN, STDOUT, STDERR

Reading from files
Writing to files
Pipes
File checks

– exist, read, write, etc.

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 31

SLIDE 32

File Access

Access to files is similar to shell redirection

– open allows access to the file – Redirect characters (<, >) define access type

Can read, write, append, read & write, etc.

– Filehandle refers to opened file

Variable without any preceded character
Capital characters by convention

– close stops access to the file – $! contains IO error messages – perldoc perlopentut has complete description

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 32

SLIDE 33

File Access

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 33

pen INPUT, "< datafile" or die "Can't open input file: $!";
pen OUTPUT, "> outfile " or die "Can't open output file: $!";
pen LOG, ">> logfile " or die "Can't open log file: $!";
pen RWFILE, "+< myfile " or die "Can't open file: $!";

close INPUT;

SLIDE 34

Standard Files

Standard files are opened automatically

– STDIN is standard input – STDOUT is standard output – STDERR is standard error output – Can re-open these for special handling – <> uses standard input by default – print uses standard output by default – die and warn use standard error by default

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 34

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Standard Files

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 35

print STDOUT "Hello, world.\n"; # STDOUT not needed

pen STDERR, ">> logfile" or die "Can't redirect errors to log: $!";

print STDERR "Oh no, here's an error message.\n"; warn "Oh no, here's another error message.\n"; close STDERR;

SLIDE 36

Reading from Files

Reading from files

– Input operator <> reads one line from the file, including the newline character – <> reads from STDIN by default – chomp will remove newline if you want – Can modify input recorder separator $/ to read characters, words, paragraphs, records, etc.

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 36

SLIDE 37

Reading from Files

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 37

print "What type of pet do you have? "; my $pet = <STDIN>; # Read a line from STDIN, say parrot #$pet is “parrot\n” chomp $pet; # Remove newline, $pet contains “parrot” print "Enter your pet's name: "; my $name = <>; # STDIN is optional chomp $name; print "Your pet $pet is named $name.\n"; Output: What type of pet do you have? parrot Enter your pet's name: Polly Your pet parrot is named Polly.

SLIDE 38

Reading from Files

Reading from files

– Easy to loop over entire file – Loops will assign to $_ by default – Be sure that the file is open for reading first

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 38

SLIDE 39

Reading from Files

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 39

pen CUSTOMERS, "< mailing_list" or die "Can't open input file: $!";

while (my $line = <CUSTOMERS>) { my @fields = split(":", $line); # Fields separated by colons print "$fields[1] $fields[0]\n"; # Display selected fields print "$fields[3], $fields[4]\n"; print "$fields[5], $fields[6] $fields[7]\n"; } print while <>; # cat utility in Unix/Linux print STDOUT $_ while ($_ = <STDIN>); # same, but more verbose Output: Last name:First name:Age:Address:Apartment:City:State:ZIP Smith:Al:18:123 Apple St.:Apt. #1:Cambridge:MA:02139 Al Smith 123 Apple St., Apt. #1 Cambridge, MA 02139

SLIDE 40

Writing to Files

Writing to files

– print writes to a file – print writes to a STDOUT by default – Be sure that the file is open for writing first

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 40

SLIDE 41

Writing to Files

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 41

pen CUSTOMERS, "< mailing_list" or die "Can't open input file: $!";
pen LABELS, "> labels" or die "Can't open output file: $!";

while (my $line = <CUSTOMERS>) { my @fields = split(":", $line); print LABELS "$fields[1] $fields[0]\n"; print LABELS "$fields[3], $fields[4]\n"; print LABELS "$fields[5], $fields[6] $fields[7]\n"; }

SLIDE 42

Pipes

Pipes

– Pipes redirect input from or output to another process – Just like shell redirection, pipes act like normal files

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 42

# Use another process as input

pen INPUT, "ps aux |" or die "Can't open input command: $!";

# Print labels to printer instead of to a file

pen LABELS, "| lpr" or die "Can't open lpr command: $!";

SLIDE 43

File Checks

File checks

– File test operators check if a file exists, is readable or writable, etc. – -e tests if file is exists – -r tests if file is readable – -w tests if file is writable – -x tests if file is executable – -l tests if file is a symlink – -T tests if file is a text file – perldoc perlfunc lists more

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 43

SLIDE 44

File Checks

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 44

my $filename = "pie_recipe"; if (-r $filename) {

pen INPUT, “< $filename" or die "Can't open $filename: $!";

}else { print "The file $filename is not readable.\n"; }

my $filename = “log_file";

if (-w $filename) {

pen INPUT, “> $filename" or die "Can't open $filename: $!";

}else { print "The file $filename is not writable.\n"; }

SLIDE 45

Regular Expressions

Regexes perform textual pattern matching
Regexes may be quoted in several ways
Regexes are their own mini-language

– Match letters, numbers, other characters – Exclude certain characters from matches – Match boundaries between types of characters – Group subpatterns – Match repetitions

Perl has several regex operators

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 45

SLIDE 46

Regular Expressons

Regexes perform textual pattern matching

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 46

my $string = "Did the fox jump over the dogs?"; #contain the letters "dog" in order? print "1: $string\n" if $string =~ /dog/; # matches #not contain the letter "z"? print "2: $string\n" if $string !~ /z/; # matches #begin with the letters "Y" or "y"? print "3: $string\n" if $string =~ /^[Yy]/; #end with a question mark? print "4: $string\n" if $string =~ /\?$/; # matches #contain only letters? print "5: $string\n" if $string =~ /^[a-zA-Z]*$/; #contain only digits? print "6: $string\n" if $string =~ /^\d*$/;

SLIDE 47

Regular Expressions

Regexes may be quoted in several ways

– Regex quotes are usually slashes (/regex/) – May use other quotes with the match operator m – Use other quotes when matching slashes (m[/]) – Comparison operators (=~, !~) test for matches

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 47

my $string = "Did the fox jump over the dogs?"; print "1: $string\n" if $string =~ /dog/; # matches print "2: $string\n" if $string =~ m/dog/; # matches print "3: $string\n" if $string =~ m(dog); # matches print "4: $string\n" if $string =~ m|dog|; # matches

SLIDE 48

Regular Expressions

Regexes are their own mini-language

– Match letters, numbers, other characters – Exclude certain characters from matches – Character classes (in []) denote a possible set of characters to match – Negate a character classes with a carat ([^abc]) – Provided character classes include:

\d, \D for digits, non-digits, \d is the same as [1-9]
\w, \W for word and non-word characters (letters, digits,

underscore), \w is the same as [a-zA-Z1-9_]

\s, \S for white-space and non-space characters
. for any character except newline

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 48

SLIDE 49

Regular Expressions

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 49

my $string = "Did the fox jump over the dogs?"; print "1: $string\n" if $string =~ m/[bdl]og/; # bog, dog, log print "2: $string\n" if $string =~ m/dog[^s]/; # no match print "3: $string\n" if $string =~ m/\s\w\w\wp\s/; # matches

SLIDE 50

Regular Expressions

Match boundaries between types of

characters

– ^ matches the start of the string – $ matches the end of the string – \b matches a word boundary

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 50

my $string = "Did the fox jump over the dogs?"; print "1: $string\n" if $string =~ m/^[Yy]/; # no match print "2: $string\n" if $string =~ m/\?$/; # match print "3: $string\n" if $string =~ m/the\b/; # match

SLIDE 51

Regular Expressions

Quantify matches

– * matches the preceding character 0 or more times – + matches the preceding character 1 or more times – ? matches the preceding character 0 or once – {4} matches exactly 4 times – {3,6} matches 3 to 6 times

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 51

SLIDE 52

Regular Expressions

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 52

my $string = "Did the fox jump over the dogs?"; print "1: $string\n" if $string =~ m/z*/; # matches print "2: $string\n" if $string =~ m/z+/; # no match print "3: $string\n" if $string =~ m/\b\w{4}\b/; # matches "jump“ print “4: $string\n” if $string =~ m/H?/; # matches print “5: $string\n” if $string =~ m/\b\w{2,6}\b/; # matches, # any word whose length is between 2 and 6

SLIDE 53

Regular Expressions

Group subpatterns

– () group a subpattern

Match repetitions

– \1, \2 refer to the first and second groups

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 53

my $string = "Did the fox jump over the dogs?"; print "1: $string\n" if $string =~ m/(fox){2}/; # "foxfox" print "2: $string\n" if $string =~ m/(the\s).*\1/; # matches

SLIDE 54

Regular Expressions

Perl has several regex operators

– m just matches, providing boolean context – s/match/replacement/ substitutes – tr/class/replacement/ transliterates

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 54

my $string = "Did the fox jump over the dogs?"; $string =~ s/dog/cat/; # substitute "cat" for "dog" $string =~ s(fox)(kangaroo); # substitute "kangaroo" for "fox" print "$string\n"; $string =~ tr/a-z/n-za-m/; # Rot13 print "$string\n"; Output: Did the kangaroo jump over the cats? Dvq gur xnatnebb whzc bire gur pngf?

SLIDE 55

Regular Expressions

Perl has several regex modifiers

– g global: allows for multiple substitutions – i case insensitive matching – s treat string as one line – m treat string as multiple lines

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 55

SLIDE 56

Regular Expresssions

Example:

Slides mainly taken from Perl Programming, Quentin Smith http://stuff.mit.edu/iap/perl/ 56

my $breakfast = 'Lox Lox Lox lox and bagels'; $breakfast =~ s/Lox //g; print "$breakfast\n"; my $paragraph = "First Line\nSecond Line\nThird Line\n"; my ($first, $second) = ($paragraph =~ /(^.*$)\n(^.*$)/m); print "$first, $second\n"; Output: lox and bagels First Line, Second Line