Introduction to Perl

Lecture 7: Perl Introduction
Programming Tools and
Environments

Perl
"Practical Extraction and Reporting Language"
written by Larry Wall and first released in 1987
Perl has become a very large system of
modules
name came first, then the acronym
designed to be a "glue" language to fill the gap
between compiled programs (output of "gcc",
etc.) and scripting languages
"Perl is a language for easily manipulating text,
files and processes": originally aimed at
systems administrators and developers

What is Perl?
Perl is a High-level Scripting language
Faster than sh or csh, slower than C
No need for sed, awk, head, wc, tr, …
Compiles at run-time
Available for Unix, PC, Mac
Best Regular Expressions on Earth

What’s Perl Good For?
Quick scripts, complex scripts
Parsing & restructuring data files
CGI-BIN scripts
High-level programming
 Networking libraries
 Graphics libraries
 Database interface libraries

What’s Perl Bad For?
Compute-intensive applications (use C)
Hardware interfacing (device drivers…)

Executing Perl scripts
"bang path" convention for scripts:
 can invoke Perl at the command line, or
 add #!/public/bin/perl at the beginning of the script
 exact value of path depends upon your platform (use
"which perl" to find the path)
one execution method:
% perl
print "Hello, World!n";
CTRL-D
Hello, World!
preferred method: set bang-path and ensure
executable flag is set on the script file

Perl Basics
Comment lines begin with: #
File Naming Scheme
 filename.pl (programs)
 filename.pm (modules)
Example prog: print “Hello, World!n”;

Perl Basics
Statements must end with semicolon
 $a = 0;
Should call exit() function when finished
 Exit value of zero means success
 exit (0); # successful
 Exit value non-zero means failure
 exit (2); # failure

Data Types
Integer
 25 750000 1_000_000_000
 8#100 16#FFFF0000
Floating Point
 1.25 50.0 6.02e23 -1.6E-8
String
 ‘hi there’ “hi there, $name” qq(tin can)
 print “Text Utility, version $vern”;

Data Types
Boolean
 0 0.0 “” "0" represent False
 all other values represent True

Variable Types
Scalar
 $num = 14;
 $fullname = “John H. Smith”;
 Variable Names are Case Sensitive
 Underlines Allowed: $Program_Version = 1.0;

Scalars
usage of scalars:
print ("pi is equal to: $pin");
print "pi is still equal to: ", $pi, "n";
$c = $a + $b
important! A scalar variable can be "used" before it is
first assigned a value
 result depends on context
 either a blank string ("") or a zero (0)
 this is a source of very subtle bugs
 if variable name is mispelled — what should be the
result?
 do not let yourself get caught by this – use the "-w"
flag in the bang path:
#!/public/bin/perl -w

Variable Types
List (one-dimensional array)
 @memory = (16, 32, 48, 64);
 @people = (“Alice”, “Alex”, “Albert”);
 First element numbered 0
 Single elements are scalar: $names[0] = “Fred”;
 Slices are ranges of elements
 @guys = @people[1..2];
 How big is my list?
 print “Number of people: “, scalar @people, “ n”;

Variable Types
Hash (associative array)
 %var = { “name” => “paul”, “age” => 33
};
 Single elements are scalar
 print $var{“name”}; $var{age}++;
 How many elements are in my hash?
 @allkeys = keys(%var);
 $num = @allkeys;

Operators
Math
 The usual suspects: + - * / %
 $total = $subtotal * (1 + $tax / 100.0);
 Exponentiation: **
 $cube = $value ** 3;
 $cuberoot = $value ** (1.0/3);
 Bit-level Operations
 left-shift: << $val = $bits << 1;
 right-shift: >> $val = $bits >> 8;

Operators
Assignments
 As usual: = += -= *= /= **= <<=
>>=
 $value *= 5;
 $longword <<= 16;
 Increment: ++
 $counter++ ++$counter
 Decrement: --
 $num_tries-- --$num_tries

Arithmetic
Perl operators are the same as in C and Java
these are only good for numbers
 but beware:
$b = "3" + "5";
print $b, "n"; # prints the number 8
 if a string can be interpreted as a number given
arithmetic operators, it will be
 what is the value of $b?:
$b = "3" + "five" + 6?
 Perl semantics can be tricky to completely understand

Operators
Boolean (against bits in each byte)
 Usual operators: & |
 Exclusive-or:^
 Bitwise Negation: ~
 $picture = $backgnd & ~$mask | $image;
Boolean Assignment
 &= |= ^=
 $picture &= $mask;

Operators
Logical (expressions)
 && And operator
 | | Or operator
 ! Not operator

Operators
 Short Circuit Operators
 expr1 && expr2
 expr1 is evaluated.
 expr2 is only evaluated if expr1 was true.
 expr1 || expr2
 expr1 is evaluated.
 expr2 is only evaluated if expr1 was false.
 Examples
 open (…) || die “couldn’t open file”;
 $debug && print “user’s name is $namen”;

Operators
Modulo: %
 $a = 123 % 10; ($a is 3)
Multiplier: x
 print “ride on the ”, “choo-”x2, “train”;
(prints “ride on the choo-choo-train”)
 $stars = “*” x 80;
Assignment: %= x=

Operators
String Concatenation: . .=
 $name = “Uncle” . $space . “Sam”;
 $cost = 34.99;
 $price = “Hope Diamond, now only $”;
 $price .= “$cost”;

Conditionals
numeric string
Equal: == eq
Less/Greater Than: < > lt gt
Less/Greater or equal: <= >= le ge
Zero and empty-string means False
All other values equate to True

Conditionals
numeric string
Comparison: <=> cmp
 Results in a value of -1, 0, or 1
Logical Not: !
 if (! $done) {
print “keep going”;
}

numeric vs. string comparisons
#!/usr/bin/perl
$a = "123";
$b = "1234";
$c = "124";
if ($b > $c) {
print "$b > $cn";
} else {
print "$b <= $cn";
}
if ($b gt $c) {
print "$b gt $cn";
} else {
print "$b le $cn";
}
1234 > 124
1234 le 124

Control Structures
“if” statement - first style
 if ($porridge_temp < 40) {
print “too hot.n”;
}
elsif ($porridge_temp > 150) {
print “too cold.n”;
}
else {
print “just rightn”;
}

Control Structures
“if” statement - second style
 statement if condition;
 print “$index is $index” if $DEBUG;
 Single statements only
 Simple expressions only
“unless” is a reverse “if”
 statement unless condition;
 print “millenium is here!” unless $year < 2000;

Control Structures
“for” loop - first style
 for (initial; condition; increment) { code }
 for ($i=0; $i<10; $i++) {
print “hellon”;
}
“for” loop - second style
 for [variable] (range) { code }
 for $name (@employees) {
print “$name is an employee.n”;
}

Control Structures
“for” loop with default loop variable
 for (@employees) {
print “$_ is an employeen”;
print; # this prints “$_”
}
Foreach and For are actually the same.

Control Structures
“while” loop
 while (condition) { code }
 $cars = 7;
while ($cars > 0) {
print “cars left: ”, $cars--, “n”;
}
 while ($game_not_over) {…}

Control Structures
“until” loop is opposite of “while”
 until (condition) { code }
 $cars = 7;
until ($cars <= 0) {
print “cars left: ”, $cars--, “n”;
}
 while ($game_not_over) {…}

Control Structures
Bottom-check Loops
 do { code } while (condition);
 do { code } until (condition);
 $value = 0;
do {
print “Enter Value: ”;
$value = <STDIN>;
} until ($value > 0);

No Switch Statement?!?
Perl needs no Switch (Case) statement.
Use if/else combinations instead
 if (cond1) { … }
elsif (cond2) { … }
elsif…
else…
This will be optimized at compile time

Subroutines (Functions)
Defining a Subroutine
 sub name { code }
 Arguments passed in via “@_” list
 sub multiply {
my ($a, $b) = @_;
return $a * $b;
}
 Last value processed is the return value
(could have left out word “return”, above)

Calling a Subroutine
 &subname; # no args, no return value
 &subname (args);
 retval = &subname (args);
 The “&” is optional so long as…
 subname is not a reserved word
 subroutine was defined before being called

Passing Arguments
 Passes the value
 Lists are expanded
 @a = (5,10,15);
@b = (20,25);
&mysub(@a,@b);
 this passes five arguments: 5,10,15,20,25
 mysub can receive them as 5 scalars, or one array

Examples
 sub good1 {
my($a,$b,$c) = @_;
}
&good1 (@triplet);
 sub good2 {
my(@a) = @_;
}
&good2 ($one, $two, $three);

Examples
 sub good3 {
my($a,$b,@c) = @_;
}
&good3 ($name, $phone, @address);
 sub bad1 {
my(@a,$b) = @_;
}
 @a will absorb all args, $b will have nothing.

Dealing with Hashes
keys( ) - get an array of all keys
 foreach (keys (%hash)) { … }
values( ) - get an array of all values
 @array = values (%hash);
each( ) - get key/value pairs
 while (@pair = each(%hash)) {
print “element $pair[0] has $pair[1]n”;
}

Dealing with Hashes
exists( ) - check if element exists
 if (exists $ARRAY{$key}) { … }
delete( ) - delete one element
 delete $ARRAY{$key};

Launching External Programs
The “system” library call
 example: system (“ls -la”);
 Returns exit status of program it launched
 a shell actually runs, so you can use:
 pipes (“|”)
 redirection (“<”, “>”)

Launching External Programs
Run a command, insert output inline
 $numlines = `wc -l < /etc/passwd`;

Command Line Args
$0 = program name
@ARGV array of arguments to program
zero-based index (default for all arrays)
Example
 yourprog -a somefile
 $0 is “yourprog”
 $ARGV[0] is “-a”
 $ARGV[1] is “somefile”

Basic File I/O
Reading a File
 open (FILEHANDLE, “$filename”) || die
“open of $filename failed: $!”;
while (<FILEHANDLE>) {
chomp $_; # or just: chomp;
print “$_n”;
}
close FILEHANDLE;

Basic File I/O
Writing a File
 open (FILEHANDLE, “>$filename”) || die
“open of $filename failed: $!”;
while (@data) {
print FILEHANDLE “$_n”;
# note, no comma!
}
close FILEHANDLE;

Basic File I/O
Predefined File Handles
 <STDIN> input
 <STDOUT> output
 <STDERR> output
 print STDERR “big bad error occurredn”;
 <> ARGV or STDIN

Basic File I/O
How does <> work?
 opens each ARGV filename for reading
 if no ARGV’s, reads from stdin
 great for writing filters, here’s “cat”:
 while (<>) {
print; # same as print “$_”;
}

Basic File I/O
Reading from a Pipe
 open (FILEHANDLE, “ps aux |”) || die
“launch of ‘ps’ failed: $!”;
while (<FILEHANDLE>) {
chomp;
print “$_n”;
}
close FILEHANDLE;

Basic File I/O
Writing to a Pipe
 open (FILEHANDLE, “| mail frank”) || die
“launch of ‘mail’ failed: $!”;
while (@data) {
print FILEHANDLE “$_n”;
}
close FILEHANDLE;

Common mistakes
putting comma after filehandle in print statement
using == instead of eq, and != instead of ne
leaving $ off the front of a variable on th left side of
an assignment
forgetting the & on a subroutine call
leaving $ off of the loop variable of foreach
using else if or elif instead of elsif
forgetting trailing semicolon
forgetting the @ or @ on the front of variables
saying @foo[1] when you mean $foo[1]

Debugging in Perl
-w option is great!
 #!/bin/perl -w
 tells you about…
 misused variables
 using uninitialized data/varables
 identifiers that are used only once
 and more

Debugging in Perl
Debug mode: perl -d filename [args]
 Display Commands
 h Extended help
 h h Abbreviated help
 l (lowercase-L) list lines of code
 l sub list subroutine sub
 l 5 list line 5
 l 3-6 list lines 3 through 6 inclusive
 l list next window of lines

Debugging in Perl
 Display Commands (continued)
 w list window around current location
 S list all subroutines w/ packages
 V [pkg] list all variables [in package pkg]
 X [vars] list variables in this package
 /pat search forwards for pattern pat
(regular expressions legal)
 ?pat search backwards for pattern pat
(regular expressions legal)

Debugging in Perl
 Display Commands (continued)
 - (hyphen) list previous window
 p expr prints Perl expression expr
 = alias value set a command alias
 H [-num] history (list previous cmds)
 !num redo a debug
command
 command execute any Perl command
or line of code you want
 q quit

Debugging in Perl
 Stepping & Tracing
 s step -- execute 1 line of code.
Steps “over” subroutines.
 n next -- execute 1 line of code.
Steps “into” subroutines.
 [RETURN] repeats previous “step” or “next”
 r return from function
 t toggle trace mode

Debugging in Perl
 Breakpoints
 b line set a breakpoint at line line
 b sub set a breakpoint at subroutine sub
 d line delete breakpoint
 D delete all breakpoints
 c [line] continue running until next breakpt
[set a temporary breakpoint at line]
 L List all breakpoints

Debugging in Perl
 Actions
 a line cmd executes perl code cmd
each time line is reached
 A delete all line actions
 < cmd set action right before the
debugging prompt
 > cmd set action right after the
debugging prompt

Debugging in Perl
How to Learn More
 man perldebug
 just try it!

CPAN
Comprehensive Perl Archive Network
 Hundreds of reusable Perl modules written
by people on the Internet
 Archive sites
 ftp://ftp.cs.colorado.edu/pub/perl/CPAN/
 ftp://ftp.cdrom.com/pub/perl/CPAN/
 ftp://ftp.orst.edu/pub/packages/CPAN/

How to Learn More
 On-line man pages
 man perl -- starting point
man perldoc -- documentation
man perlmod -- modules
 perldoc command gives manpage-like info
 example: perldoc Getopt::Std

Introduction to Perl

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