Perl Practical Extraction and Report Language Tutorial

1
Perl (Practical Extraction and Report Language)
Tutorial

• Based on NCSA tutorial at
• http//archive.ncsa.uiuc.edu/General/Training/Perl
  Intro/

2
Introduction

• Interpreted programming language
• Combines the syntax of Bourne shell, csh, awk,
  sed, grep, and C

3
Starting Perl

• perl scriptname or
• cat scriptname perl or
• within an executable text file the first line
  should be
• !/usr/bin/perl

4
Basic Syntax

• Perl is free form - whitespace doesn't matter
• All perl statements end with a
• Comments begin with sign
• Variables need not be declared (not strongly
  typed)

5
Basic Syntax Contd.

• Variable names (identifiers) can be composed of
  lower or upper case letters, numbers and the
  underscore ( _ )
• Scalars (a single value, either a string or a
  numeric) are accessed by prefixing an identifier
  with ''.
• Scalars are assigned to using '
• scalar expression

6
Strings

• (apostrophe)
• a pair of apostrophes is interpreted literally -
  no variable expansion takes place
• instrument 'saxophone'
• littleInstrument 'soprano instrument'
• the string value of littleInstrument includes
  the text "instrument"

7
Strings Contd.

• " (double quotes)
• The double quote expands variables between the
  pair of quotes
• instrument "saxophone"
• littleInstrument "soprano instrument"
• the string value of littleInstrument is "soprano
  saxophone"

8
Strings Contd.

• (backtick)
• This quote performs as it does in the UNIX shells
  - the text inside the backticks is executed as a
  separate process
• To include an apostrophe or a double quote or a
  backtick in the string, you must escape it with a
  backslash '\'.

9
Perl Functions

• Perl has many built-in functions. A list of
  functions is available at
• http//www.perldoc.com/perl5.6/pod/perlfunc.html
  
• The function's arguments are supplied as a comma
  separated list in parenthesis.
• The commas are necessary, the parentheses are
  often not.
• print("length " ,length("hello world"), \n)
• prints the same thing as
• print "length ", length hello world, \n

10
Lists

• A list is an ordered collection of scalars.
• Space for lists are dynamically allocated and
  removed from the program's memory.
• Each element can be addressed by its integer
  position in the list. Lists are 0-indexed the
  first element is called "0".

11
Lists Contd.

• Typical operators include
• ( )
• The list constructor
• ,
• The comma is used to separate elements of the
  list
• (brackets)
• Brackets are used to take elements of the list
• An Example of a list (cat", dog", tiger",
  elephant")
• largestAnimal (cat", dog", tiger",
  elephant")3

12
Some functions manipulating lists

• sort (LIST)
• sort ((a, d, b, c, e)) gives a, b, c, d, e
• reverse (LIST)
• reverse ((a, b, c, d, e)) gives e, d, c, b,
  a
• join (EXPR, LIST)
• join ( , (a, b, c, d, e)) gives a b
  c d e
• split (PATTERN, EXPR)
• info a b c d
• split (//, info) gives abcd

13
Arrays

• An array is a named list.
• As with lists, its space is dynamically allocated
  and removed, it is 0-indexed
• _at_stringInstruments ("violin","viola","cello","b
  ass")

14
Some functions manipulating arrays

• push (_at_arr, LIST)
• _at_arr (a, b, c)
• push (_at_arr, x, y, z) gives an array of
  a, b, c, x, y, z
• pop (_at_arr)
• _at_arr (a, b, c)
• pop (_at_arr) removes and returns the last
  element c
• shift (_at_array)
• _at_arr (a, b, c)
• shift (_at_arr) removes and returns the first
  element a, shifts the other elements forward
• unshift (_at_array, LIST)
• _at_arr (x, y, z)
• unshift (_at_arr, a, b, c) gives a, b, c,
  x, y, z
• scalar (_at_array)
• _at_arr (a, b, c)
• scalar (_at_arr) gives 3 the number of
  elements in the array

15
Associative Arrays (Hashes)

• Arrays that are indexed not on ordered integers,
  but on arbitrary string values.
• Elements of an associative array are referred to
  as "key" and "value" pairs - the "key" is used to
  find the element of the array that has the
  "value
• (percent sign)
• Refers to the entire array
• (braces)
• Used to denote the key
• When used with , this is the value of the
  array element indexed on the key.

16
Associative Arrays Contd.

• Examples
• principalkey1" v1"
• principalkey2" v2"
• principalkey3" v3"
• principalkey4" v4"
• _at_Keys keys(principal)
• _at_Values values(principal)

17
Conditionals

• To do comparisons on different values, perl
  provides two sets of conditionals - one for
  numbers and one for strings.
• Comparison Strings Numbers
• Equality eq
• Inequality ne !
• Greater than gt
• Greater than or equal to ge
  
• Less than lt
• Less than or equal to le
• Comparison returns -1,0,1 cmp
  

18
Conditionals Contd.

• It's important to use the right conditional -
  there are no "syntax" errors
• That is if you use a variable in the wrong
  context, there could be unexpected results
• You can build up conditionals using (AND) and
  (OR) operations

19
IF ELSE

• if (EXPRESSION )
• STATEMENTS
• if (EXPRESSION )
• executed if true STATEMENTS
• else
• executed if false STATEMENTS
• if (EXPRESSION )
• STATEMENTS
• elsif
• STATEMENTS optional additional ELSIF's
• else
• STATEMENTS
• Unlike C, the braces are not optional - require
  braces

20
DO / WHILE LOOPS

• while ( EXPRESSION )
• STATEMENTS
• If the condition is initially false, the body of
  the loop will never execute.
• do
• STATEMENTS
• while (EXPRESSION)
• The loop will execute at least once.

21
Loops

• All loops support the following two control
  statements
• last
• Declare that this is the last statement in the
  loop completely exit the loop even if the
  condition is still true, ignoring all statements
  up to the loop's closing brace.
• next
• Start a new iteration of the loop
• An Example one way to count to 10
• number 0
• while(1)
• number
• if (number 10 ) last

22
FOR LOOPS

• for ( INITIAL_EXPR COND_EXPR LOOP_EXPR )
  STATEMENTS
• The loop is initialized by evaluating
  INITIAL_EXPR, iterates while COND_EXPR is true,
  and evaluates LOOP_EXPR before beginning each
  subsequent loop.
• The loop flow can be modified using next and
  last, as in the DO loop.
• Unlike C, the braces around the block of
  STATEMETNTS are mandatory

23
FOR LOOPS Contd.

• An Example countdown
• for ( i10 i0 i- -)
• print("i\n")

24
FOREACH LOOP

• The syntax is
• foreach SCALAR ( LIST )
• STATEMENTS
• Each time through the loop, SCALAR is assigned
  the next element in the LIST, and the STATEMENTS
  are executed.
• The loop flow can be modified using next and
  last, as in the DO loop.
• The braces are mandatory.

25
FOREACH LOOP Contd.

• An Example
• foreach number(one",two",three",four")
• print(number," is smaller than a \n")
• print(five\n")

26
Subroutines

• Perl subroutine definitions take the following
  form
• sub NAME STATEMENTS
• In perl, subroutines are invoked differently than
  the built-ins
• NAME(arg1,arg2,etc)
• or if the subroutine takes no arguments
• NAME
• return values are used just like any other value
  - always return the last thing evaluated
• loudestInstrument measureLoudest(_at_allInstrume
  nts)

27
Subroutines Contd.

• Examples
• sub printargs
• print "_at__\n"
• printargs("perly", "king") prints "perly
  king
• printargs("frog", "and", "toad") Prints
  "frog and toad"
• Just like any other list array the individual
  elements of _at__ can be accessed with the square
  bracket notation
• sub printfirsttwo
• print "Your first argument was _at__0\n" print
  "and _at__1 was your second\n"

28
Subroutines Contd.

• sub maximum
• if (_at__0 _1)
• _at__0
• else
• _at__1
• biggest maximum(37, 24) Now biggest is
  37

29
File Test Operators

• Some file test operators
• -r readable
• -x executable
• -e exists
• -d is a directory
• -t is a tty
• -T text file

30
File Management

• open(FILEHANDLE,FILENAME)
• close(FILEHANDLE)
• FILEHANDLEs aren't denoted by a special character
  like _at_ , and are not strings,
• the FILENAME is a string, special formats are
  below.
• open returns 1 on success, 0 on failure

31
File Management Contd.

• Conventions for opening files
• "FILE" open FILE for input (also
• " FILE" open FILE for output, creating if
  necessary
• " FILE" open FILE for appending

32
File Management Contd.

• When we have a filehandle, we need to read or
  write to it. The read operator is , as in
• read one line
• line
• read the entire file
• _at_lines
• To print to a FILEHANDLE, print takes an
  optional first argument which is the filehandle.
• print takes a list as an argument .The first
  argument does not have a comma.
• print(FILEHANDLE "your","list","here")

33
File Management Contd.

• Reading Example
• file '/etc/passwd' Name the file
• open(INFO,
• _at_lines Read it into an array
• close(INFO) Close the file print
• print _at_lines Print the array
• Writing Example
• open(FHANDLE, NewFile) Open the file
• print (FHANDLE Your,List, Here) Write to
  the file
• close(FHANDLE) Close the file