Perl for People

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Perl for People

• Singer XJ Wang
• Learning Center

Available At http//www.cs.dal.ca/swang/4173/pfp
.ppt
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Perl Variables

• Perl is a type-less language (no explicit types
  like integers, strings, doubles etc)
• Main data categories (types) are
• Scalars
• Lists
• Hashes
• Variables are automatically created when you
  first use them however to be safe you should
  declare them first
• Declaring a variable makes them valid for the
  block of code they are in
• Putting the following in the top of the Perl
  script enforces you to declare variables ? use
  strict
• my scalarVariable
• my _at_listVariable
• my hashVariable

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Scalar Variables

• Either a number or a string of characters
  (doesnt matter)
• Scalar variables always begin with
• No limitation on size of a scalar variable
• un-initialized values are set to undef which is
  either 0 or (empty string) depending on
  context
• 0 and are both considered false in
  conditionals
• All standard operators are provided like
• Arithmetic Operations ? , -, /,
• String Length ? length( scalar )
• Concatenation ? newScalar front . back
• Substitution is allowed for scalars
• In case below b would contain the value hello
  ted
• a ted b hello a

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Scalar Variables (cont)

• Scalar Contexts
• a 5 b 4A
• c a b d a . b
• Numerical Context c ? since addition is number
  operation, b is internally converted to a number
  and added to a the result is 9
• String Context d ? since . is an string
  operation a is internally converted to a string
  and appended to b the result is 54A
• the value of a and b are not changed
• Comparison Operators
• Numerical Context ? lt, lt, , !, gt, gt
• if ( a lt b )
• numerical comparison is made
• String Context ? lt, le, eq, ne, ge, gt
• if ( a eq b )
• alphabetical comparison is made

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List Variables

• Similar to Vectors or Linked Lists
• can also be used like a stack and queue
• Elements are indexed like an array starting at
  0
• Each element in the list must be a scalar
• No fixed size up to limits of system memory
• List variables names always begin with _at_
• Any position in list that hasnt been explicitly
  defined has the value undef
• _at_ refers to the list as a whole, refers to
  elements
• _at_array (elementA, elementB, elementC)
• array2 is the 3rd element with value
  elementC
• array5 ted ? sets the 6th element in the
  list to ted
• Copying is done through assignment (each element
  is copied)
• _at_copy _at_original

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List Variables (cont)

• Sorting
• Alphabetically ? _at_sorted sort _at_unsorted
• Numerical ? _at_sorted sort a ltgt b _at_unsorted
• Reversing
• _at_ reversed reverse _at_normal
• Accessing Element _at_list
• scalarVariable list4
• scalarVariable listindex
• Setting Element _at_list
• list5 cat
• listindex nameOfPerson
• list8 5
• Getting Number of Elements in a List
• numElements scalar _at_list

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List Variables (cont)

• Operation at Head of List (low index)
• Insertion ? unshift _at_list, scalar
• Deletion --gt scalar shift _at_list
• Operation at Tail of List (high index)
• Insertion ? push _at_list, scalar
• Deletion ? scalar pop _at_list
• Combining Lists and Scalars
• _at_newList (_at_listA, scalar)
• _at_newList (scalar, _at_listA)
• Combinining Multiple Lists
• _at_newList (_at_listA, _at_listB)
• _at_newList (_at_listB, _at_listA)
• NOTE does not form multi-dimensional lists,
  instead creates one list with elements from both
  parts

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Hash Variables

• A hash table data structure Like a STL Map in
  C, HashMap or HashSet in Java
• Key, Value pairs both must be scalars
• No internal order is maintained
• Keys must be unique no two element can have the
  same key
• Attempting to insert a second value for the same
  key, then the first value gets overwritten
• Hash variables names always begin with
• refers to hash as a whole, refers to elements
• hash
• hashkey value
• hash5 egg
• hashted bacon

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Hash Variables (cont)

• Copying is done through assignment (each element
  is copied)
• copy original
• Lists of Keys/Value
• List of Keys ? _at_keys keys hash
• List of Values ? _at_values values hash
• Insertion/Deletion/Access hash
• Insertion ? hashkey value
• Deletion ? delete hashkey
• Access ? scalarValue hashkey
• If no value exists for the given key, the value
  undef is returned
• Reversing
• reverseHash reverse normalHash
• Swaps the keys and values
• If it has two values that are the same, then the
  result is not deterministic

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Hash Variables (cont)

• Hash from List
• hash _at_list
• Every even element becomes key for odd element
  (the odd element after the even one)
• If two even elements (potential keys) are the
  same, then the result is not deterministic
• If the list contains an odd number of elements,
  the last element (a potential key) is ignored
• Hash from List
• _at_list hash
• Every even element becomes key for odd element
  (the odd element after the even one)
• Order of the elments are not gauarneteed

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Control Structures

• Control structures in Perl are very much like C,
  C, Java with the following exceptions
• No switch or case statement
• else if is written as elsif
• Additional unless control which works exactly the
  opposite of the if control not used that often
• curly braces are always required ?

Wrong Right
if (a 5 ) print hello world else print world hello if (a 5 ) print hello world else print world hello
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Control Structures (cont)

• foreach control structure
• Goes through list and set the scalar to
  successive values in the list in order (low index
  to high) ?
• my scal
• foreach scal ( _at_list )
• print element -gt scal \n
• To print all keys and values in a hash ?
• my scal
• foreach scal ( keys hash )
• print scal -gt hashscal\n
• To sort all the keys alphabetically use this line
  instead
• foreach scal ( sorted keys hash )

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Input and Output

• Reading from STDIN
• ltSTDINgt is variable for standard in
• Reads one line and assign it to variable a
• chomp removes the new line at the end
• a ltSTDINgt
• chomp( a )
• Printing to STDOUT/STDERR
• Use print to send a line to standard out
• Variables are subisuted when necessary
• print Hello world\n
• printf() like C/C is also available, but slower
  then print
• To print to STDERR print STDERR Hello
  world\n

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Input and Output (cont)

• Printing Large Documents
• the end marker (EOF) must immediately follow the
  ltlt
• the second end marker (EOF) must be at the
  beginning of the line
• the semi-colon must be on a new line
• variables are substituted when appropriate
• print ltltEOF
• blah blah variable
• blah blah blah text
• EOF

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Input and Output (cont)

• Opening/Closing Files
• Reading ? open(FILE, file.txt)
• Writing ? open(FILE, gtfile.txt)
• Appending ? open(FILE, gtgtfile)
• Closing ? close(FILE)
• FILE is a file handle, can be any other name
• Separate from , , _at_ variables
• File I/O
• Read from file line ltFILEgt
• Write to file print FILE hello wrold
• Sample Program
• open(FILE, "file.txt") open(FILE2, "gtout.txt")
• my line
• while( line ltFILEgt )
• print FILE2 line
• close(FILE) close(FILE2)

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Warn and Die

• Warn and Die can be used to convey error messages
  when statement fails to execute properly
• warn ? prints error message and continues
  execution
• die ? print error messages and halts execution
• the predefined variable ! contains the error
  message the system provides
• Syntax
• open(FILE, in.txt) die not found !\n
• open(FILE, in2.txt) warn not found !\n

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Subroutines

• Subroutines are like functions/procedures in
  C/C/Java
• Subroutines are identified only by the name when
  called, the arguments do not matter by default
• Passing of parameters to subroutines are done by
  copying (like C)
• All parameters to a subroutine are put into a
  special list _at__
• when parsing arguments in the subroutine the list
  or hash will grab all remaining elements in the
  list _at__, thus only one list or hash can be passed
  in as arguments the last argument
• except when using References (later)
• Return value from a subroutine can be a scalar or
  an list/hash

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Subroutines (cont)

• sub sample1 _at_l1 grabs all remaining arguments,
  _at_l2 empty
• my (var1, var2, _at_l1, _at_l2) _at__
• ..
• sub sample2 _at_rest grabs rest of arguments if
  any
• my (var1, var2, _at_rest) _at__
• ..
• my _at_list1 (a, b, c)
• my _at_list2 (c, d, e)
• my v1 alpha my v2 beta
• sample1(v1, v2, _at_list1, _at_list2) 1
• sample1(v1, v2, _at_list1) 2
• sample2(v1, v2, _at_list1, _at_list2) 3
• sample2(v1, v2, _at_list1) 4
• the first list that is part of the argument list
  in the subroutine takes all remaining arguments
  the second list is empty

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References

• References are like pointers in C/C
• the operator \ gives you reference to a variable,
  the reference itsself is a scalar
• scalarRef \scalar
• listRef \_at_list
• hashRef \hash
• Accessing the Variable being References
• Scalar ? scalarRef
• List ? listRef-gtindex
• Hash ? hashRef-gtkey
• References provides
• way to pass multiple lists/hashes into an
  subroutines
• a way for subroutines to change a list/hash
  passed into it

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References (cont)

• sub changeL
• my listRef _0
• listRef-gt0 ABA
• my _at_list (BAB)
• my listR \_at_list
• print list0\n
• changeL(listR)
• print list0\n
• Results when Run ?
• BAB
• ABA

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Sample Program

• !/usr/bin/perl -w
• this is a comment
• use strict
• sub printHello
• my (name, number) _at__
• print "Hello name, welcome to World!\n"
• return (number2, number4)
• print "Enter Your Name "
• my nA ltSTDINgt
• chomp(nA)
• print "Enter Your Age "
• my nB ltSTDINgt
• chomp(nB)
• my (returnA, returnB) printHello(nA, nB)
• print "returnA is first and returnB is
  second\n"

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CGI - Scripts Made Easier

• Perl Modules for CGI and Web Pages
• Helps with the following
• Setting context types
• Accessing and parsing values submitted by forms
• Setting and retrieving cookies
• generate forms and HTML Pages
• setting and retrieving session data
• Two ways of accessing it
• Traditional functions
• Object Oriented style

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CGI (cont)

• Running Perl-CGI on Borg/Torch/Locutus
• ensure that your web page is accessible and has
  correct permissions
• http//help.cs.dal.ca/faq/tutorials/web_sites/basi
  c_site.php
• Setup the scripts as following
• http//help.cs.dal.ca/faq/tutorials/web_sites/cgi_
  script.php
• First you will need to create a cgi-bin bin
  directory in your public_html directory mkdir
  /public_html/cgi-bin
• Next you must grant world read and execute
  permissions on your cgi-bin directory chmod arx
  /public_html/cgi-bin or chmod 755
  /public_html/cgi-bin
• Next you must place your CGI scripts in the
  cgi-bin directory and grant world execute
  permissions on each script chomd ax script.cgi
  or chmod 711 script.cgi
• Your scripts can now be referenced at
  http//www.cs.dal.ca/username/cgi-bin/script.cgi

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CGI (cont)

• Must include the CGI module in the script
• use CGI qw/standard/
• Printing the context type for the browser
• print header()
• outputs something like ? Content-Type text/html
• To call a Perl-CGI script from a form
• set the script as the action of the form
• GET or POST does not matter
• ensure that each form element has a unique name
• for check boxes, choice boxes, selects give each
  option an unique value

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CGI (cont)

• ltform methodGET actionsample.cgigt
• ltinput type"text" name"name" size"12"gt
• ltinput value"Submit Form!"gt
• ltinput type"reset" value"Clear Form!"gt
• to get the value in the form element, simply call
  the param function with the element name as
  argument
• the return type depends on the form element, ones
  that allow multiple choices return a list
• Notes
• calls for check boxes return checked or undef
• radio buttons return the value of the one chosen
• value param(name)
• _at_values param(other)

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DBI - Database Interface

• Database Independent Driver for Perl
• Can talk to any major database like
• MySQL
• Oracle
• DB2
• Microsoft SQL Server
• PostgreSQL
• You can create your tables through DBI but for
  convince you typically want to do it through
  SQLPlus (Oracle)
• Include the module DBI in the script ? use DBI
• Major Objects
• dbh ? database handle
• sth ? statement handle

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DBI (cont)

• Connecting to DB (Oracle) and Disconnecting
• ENV'ORACLE_HOME'"/opt/oracle/9.0.1"
• my db "TRCH"
• my user "username"
• my pass "password"
• my dbh DBI-gtconnect("dbiOracledb",user,
  pass) die "ERROR DBIerrstr "
• ... ... ... ... ...
• ... ... ... ... ...
• dbh-gtdisconnect
• Executing SQL Statements
• Select Statements
• Non-Select Statements

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DBI Non-Select Statement

• sth dbh-gtprepare(insert into person
  values(?))
• rv sth-gtexecute( baz )
• sth-gtfinish
• The return value of the call rv
• rv is number of rows changed
• need to call finish on the statement handle when
  it is no longer needed
• execute fills in the question marks (?) in the
  prepare statement with the arguments
• automatically escapes any character that may be
  needed such as or \

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DBI Select Statements

• sth dbh-gtprepare("SELECT foo, bar FROM table
  WHEREbaz?")
• sth-gtexecute( baz )
• my _at_row
• while ( _at_row sth-gtfetchrow_array )
• print "foo is row0 and bar is row1\n"
• sth-gtfinish
• the while loop fetches the results from the
  select into a list one row at a time
• Can also get number of rows returned by select
  statement via
• sth-gtrows

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CGI/DBI Example

• Simple Forms
• user submits name and it is put into a database
• HTML Page with Form for user to input name
• Perl-CGI Script to process the form and insert
  the data into the Oracle Database also print out
  message
• Uses both CGI and DBI
• display all names in the database
• Perl-CGI script to extract data from Oracle
  Database and Display to User
• uses both CGI and DBI

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CGI/DBI Example HTML File

• User Form
• input.html
• http//torch.cs.dal.ca/swang/4173/input.html
• Process User Input
• input.cgi
• http//torch.cs.dal.ca/swang/4173/input.txt
• Display All Daqta
• disp.cgi
• http//torch.cs.dal.ca/swang/4173/disp.txt

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CGI/DBI Example Input Script

• Filename input.cgi (input.cgi.txt)
• lthtmlgtltheadgt
• lttitlegtCSCI 4173lt/titlegt
• lt/headgtltbodygt
• Input Name
• ltform method"GET" action"cgi-bin/input.cgi"gt
• ltinput type"text" name"name" size"12"gt
• ltinput type"submit" value"Submit Form!"gt
• ltinput type"reset" value"Clear Form!"gt
• lt/bodygt

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Other Resources

• Perl Documentation perldoc
• For modules ? perldoc moduleName
• perldoc DBI
• perldoc CGI
• For functions ? perldoc -f functionName
• perldoc -f printf
• perldoc -f chomp
• CS Online Perl Books
• http//www.library.dal.ca/subjects/csci_ebks.htm
• Learning Perl
• Perl and CGI for the World Wide Web
• Advanced Perl Programming