Introduction to Programming the WWW I

1
Introduction to Programming the WWW I

• CMSC 10100-1
• Summer 2004
• Lecture 12

2
Todays Topics

• Working with files
• Subroutines(functions)

3
Working with Files

• So far programs cannot store data values
  in-between times when they are started.
• Working with files enable programs to store data,
  which can then be used at some future time.
• Will describe ways to work with files in CGI/Perl
  programs, including
• opening files,
• closing files,
• and reading from and writing to files

4
Using the open() Function

• Use to connect a program to a physical file on a
  Web server. It has the following format
• file handle - Starts with a letter or numbernot
  with , _at_, or . Specify in all capital
  letters by Perl convention
• filename - name of file to connect to. If resides
  in the same file system directory then just
  specify the filename (and not the entire full
  file path).

5
More On open() function

• open() returns 1 (true) when it successfully
  opens and returns 0 (false) when this attempt
  fails.
• A common way to use open()
• infile mydata.txt
• open (INFILE, infile ) die Cannot open
  infile !

Perl special variable, containing the error string
Connect to mydata.txt.
Execute die only when open fails
Output system message
6
Specifying Filenames

• So far need to keep file in same directory
• You can specify a full directory path name for
  the file to be opened.

7
File Handles

• Use the file handle to refer to an opened file
• Combine with the file handle with the file input
  operator (ltgt) to read a file into your program
• Perl automatically opens 3 file handles upon
  starting a program
• STDIN -- standard in, usually the keyboard
• Empty input operator (ltgt) is the same as ltSTDINgt
• STDOUT -- standard out, usually the monitor
  screen
• print() function prints to STDOUT by default
• STDERR -- standard error, usually the monitor
  screen

8
Using the File Handle to Read Files

• You can read all the content of a file into an
  array variable
• Each line turns to an array item
• Or you can read the file line by line
• Using the special variable of Perl, _
• The mydata.txt file used in the following 2
  examples
• Apples are red
• Bananas are yellow
• Carrots are orange
• Dates are brown

9
Reading File into Array

• infile"mydata.txt"
• open (INFILE, infile ) die "Cannot
• open infile !"
• _at_infile ltINFILEgt
• print infile0
• print infile2
• close (INFILE)
• Then the output of this program would be
• Apples are red
• Carrots are orange
• http//people.cs.uchicago.edu/hai/hw4/readFile1.c
  gi

10
Reading One Line At a Time

• Reading a very large file into the list variable
  _at_infile consumes a lot of computer memory.
• Better is to read one line at a time. For example
  the following would print each line of the input
  file.
• infilemydata.txt
• open (INFILE, infile ) die Cannot open
  infile !
• while ( ltINFILEgt )
• inline _
• print inline
• close (INFILE)

Automatically set to the next input line.
http//people.cs.uchicago.edu/hai/hw4/readFile2.c
gi
11
Two String Functions

• split(/pattern/, string)
• Search the string for occurrence of the pattern.
  it encounters one, it puts the string section
  before the pattern into an array and continues to
  search
• Return the array
• Example (run in command line) http//people.cs.uc
  hicago.edu/hai/hw4/split.cgi
• join(pattern, array)
• Taking an array or list of values and putting the
  into a single string separated by the pattern
• Return the string
• Reverse of split()
• Example http//people.cs.uchicago.edu/hai/hw4/jo
  in.cgi

12
Working with split() Function

• Data usually stored as records in a file
• Each line is a record
• Multiple record members usually separated by a
  certain delimiter
• The record in the following input file part.txt
  has the format
• part_nopart_namestock_numberprice
• AC1000Hammers12212
• AC1001Wrenches3445
• AC1002Hand Saws15010
• AC1003Screw Drivers2223

13
Example Program

• infile"infile.txt"
• open (INFILE, infile ) die "Cannot open
• infile!"
• while ( ltINFILEgt )
• inline_
• (ptno, ptname, num, price )
• split ( //, inline )
• print "We have num ptname (ptno). "
• print "The cost is price dollars.\n"
• close (INFILE)

http//people.cs.uchicago.edu/hai/hw4/readFile3.c
gi
14
Using Data Files

• Do not store your data files in a location that
  is viewable by people over the Internet
• too much potential for tampering by people you do
  not know.
• Make sure permissions are set correctly (644)

15
Open Modes

• Three open modes are commonly used to open a
  file
• read-only (default mode) To explicitly specify
  it, put the character lt before the filename.
• open(INFILE, ltmyfile.txt) die Cannot open
  !
• write-only-overwrite allows you to write to a
  file.
• If the file exists, it overwrites the existing
  file with the output of the program.
• To specify this mode, use gt at the beginning of
  the filename used in the open() function. For
  example,
• open(INFILE, gtmyfile.txt) die Cannot open
  !

16
Open Modes(contd)

• write-only-append allows you to write and append
  data to the end of a file.
• If the file exists, it will write to the end of
  the existing file. Otherwise will create it.
• To specify this mode, use gtgt before the filename
  in the open() function.
• open(OFILE, gtgtmyfile.txt) die Cannot open
  !
• Example way to write to
• print OFILE My program was here

17
Locking Before Writing

• If two programs try to write to the same file at
  the same time, can corrupt a file.
• an unintelligible, usefully mixture of file
  contents
• provides a flock() function that ensures only one
  Perl program at a time can write data.
• flock(OFILE, 2)

Change 2 to 8 To unlock the file
Exclusive access to file.
File handle.
http//www.icewalkers.com/Perl/5.8.0/pod/func/floc
k.html
18
Example of flock()

• outfile"gtgt/home/perlpgm/data/mydata.txt"
• open (OFILE, outfile ) die "Cannot open
  outfile !"
• flock( OFILE, 2 )
• print OFILE "AC1003Screw Drivers2223\n"
• close (OFILE)
• Appends the following to part.txt
• AC1003Screw Drivers2223

19
Reading and Writing Files

• !/usr/bin/perl
• print "Content-type text/html\n\n"
• print qq
• lthtmlgtltheadgtlttitlegtMy Pagelt/titlegtlt/headgt
• ltbodygtltFONT SIZE5gtWELCOME TO MY SITE lt/FONTgt
• ctfile"counter.txt"
• open (CTFILE, "lt" . ctfile ) die "Cannot open
  infile !"
• _at_inline ltCTFILEgt
• countinline0 1
• close (CTFILE)
• open (CTFILE, "gtctfile" ) die "Cannot open
  infile !"
• flock (CTFILE, 2)
• print CTFILE "count"
• close (CTFILE)
• print qq
• ltbrgtltFONT COLORBLUEgtYou Are Visitor count
  lt/FONTgt
• lt/bodygtlt/htmlgt

counter.txt permission must be RW
http//people.cs.uchicago.edu/hai/hw4/readWriteFi
le1.cgi
20
Read Directory Content

• Not much different from reading a file
• Use opendir(), readdir(), and closedir()
  functions
• Example
• dir "/home/hai/html/hw4/"
• opendir(DIR, dir)
• _at_content readdir(DIR)
• foreach entry (_at_content)
• print entry . "\n"
• closedir(DIR)

http//people.cs.uchicago.edu/hai/hw4/readDir1.cg
i
21
File Test

• Example of Perl file testing functions
• -e file exists
• -z file zero file
• -s file non-zero file, returns size
• -r file readable
• -w file write-able
• -x file executable
• -f file plain file
• -d file directory
• -T file text file
• -B file binary file
• -M file age in days since modified
• -A file age in days since last accessed

http//www.netadmintools.com/html/1perlfunc.man.ht
mlixABE
22
File Test

• _at_content readdir(DIR)
• if (-d entry ) test if it is a directory
• print entry . " is a directory\n"
• elsif (-f entry ) test if it is a file
• print entry . " is a "
• if (-T entry ) test if it is a text file
• print "text file,"
• elsif (-B entry ) test if it is a
  binary file
• print "binary file,"
• else
• print "file of unknown format,"
• print " . (-s entry) . " bytes\n" get
  the file size

http//people.cs.uchicago.edu/hai/hw4/readDir2.cg
i
23
Subroutines (functions)

• Subroutines provide a way for programmers to
  group a set of statements, set them aside, and
  turn them into mini-programs within a larger
  program.
• These mini-programs can be executed several times
  from different places in the overall program
• Library functions vs. User-defined functions

24
Subroutine Advantages

• Smaller overall program size. Can place
  statements executed several times into a
  subroutine and just call them when needed.
• Programs that are easier to understand and
  change. Subroutines can make complex and long
  programs easier to understand and change. (e.g.,
  Divide into logical sections).
• Reusable program sections. You might find that
  some subroutines are useful to other programs.
  (E.g, Common page footer). common page footer.

25
More Library Functions

• abs() return the absolute value of a number
• rand() generate a random number from 0 to the
  input number
• Use int() with rand()
• Example http//people.cs.uchicago.edu/hai/hw4/ra
  nd.cgi
• localtime() used with time() function to
  determine the current date and time
• (sec, min, hr, day, mon, yr, wkday,
  DayNumOfYr, TZ ) localtime(time())
• Example http//people.cs.uchicago.edu/hai/hw4/lo
  caltime.cgi

26
String Functions

• changing case
• lc(), lcfirst(), uc(), ucfirst()
• length and substrings
• length()
• Example len length(Hello)
• substr(string, index, length)
• Example part substr(Hello, 2, 2)
• chomp() and chop()
• chop() will remove the last char of the string
  and return it
• Example char chop (Hello)

27
String Functions

• split(/pattern/, string)
• Search the string for occurrence of the pattern.
  it encounters one, it puts the string section
  before the pattern into an array and continues to
  search
• Return the array
• Example (run in command line) http//people.cs.uc
  hicago.edu/hai/hw4/split.cgi
• join(pattern, array)
• Taking an array or list of values and putting the
  into a single string separated by the pattern
• Return the string
• Reverse of split()
• Example http//people.cs.uchicago.edu/hai/hw4/jo
  in.cgi

28
Define Your Own Subroutines

• You can create a subroutine by placing a group of
  statements into the following format
• sub subroutine_name
• set of statements
• sub subroutine_name (parameter1, parameter2,)
  
• set of statements
• For example a outputTableRow subroutine
• sub outputTableRow
• print ltTRgtltTDgtOnelt/TDgtltTDgtTwolt/TDgtlt/TRgt

29
Rules to Execute a Function

• Function name with parentheses
• myFunction( argument1, argument2, ... )
• Prepended function symbol, , function name with
  parentheses
• myFunction( argument1, argument2, ... )
• outputTableRow() or outputTableRow
• Parentheses may be omitted in certain conditions
  
• myFunction, myFunction, or myFunction parameter
• Perl's Calling Conventions of Functions
• http//www.classes.cs.uchicago.edu/classes/archive
  /2004/winter/10100-1/02/perl/perl_function_calls.h
  tml

30
Subroutine Example Program

• !/usr/local/bin/perl
• print "Content-type text/html\n\n"
• print qq
• lthtmlgt
• ltheadgt
• lttitlegtSubroutine Examplelt/titlegt
• lt/headgt
• ltbodygt
• Here is simple table ltbrgt
• ltTABLE BORDER1gt
• outputTableRow() outputTableRow()
  outputTableRow()
• print qq
• lt/TABLEgt
• lt/bodygt
• lt/htmlgt
• sub outputTableRow

31
Would Output The Following
http//people.cs.uchicago.edu/hai/hw4/sub1.cgi
32
Passing Arguments to Subroutines

• Generalize a subroutine using input variables
  (called parameters or arguments).
• outputTableRow( A First Cell, A Second Cell
  )
• Use a special array variable _at__ to access
  arguments
• _0 as the variable name for the first
  argument,
• _0 would be set to A First Cell and
• _1 as the variable name for the second
  argument,
• _1 would be set to A Second Cell
• _n as the variable name for the nth argument.

33
Full Subroutine Example

• !/usr/local/bin/perl
• print "Content-type text/html\n\n"
• print qq
• lthtmlgt
• ltheadgtlttitlegtSubroutine Examplelt/titlegtlt/headgt
• ltbodygt
• Here is simple table ltbrgt
• ltTABLE BORDER1gt
• for ( i1 ilt3 i )
• outputTableRow( "Row i Col 1", "Row i Col
  2")
• print qq
• lt/TABLEgt
• lt/bodygtlt/htmlgt
• sub outputTableRow
• print ltTRgtltTDgt_0lt/TDgtltTDgt_1lt/TDgtlt/TRgt
  

34
Would Output The Following ...
http//people.cs.uchicago.edu/hai/hw4/sub2.cgi
35
Subroutine Argument Pitfalls

• _at_array(1,2,3)
• foo42
• something(foo, _at_array)
• print foo _at_array\n
• sub something
• _013
• _24
• print _at__\n
• will print
• 13 1 4 3
• 13 1 4 3

• _at_array(1,2,3)
• foo42
• something(foo, _at_array)
• print foo _at_array\n
• sub something
• _at_copy_at__
• copy013
• copy24
• print _at_copy\n
• will print
• 13 1 4 3
• 42 1 2 3

Make a private copy of the arguments to keep the
original values intact
36
Getting the Number of Arguments

• There are at least two different ways
• The range operator is set to the last element in
  a list variable. Therefore, within a subroutine
  the number of arguments is
• numargs _ 1
• Second, you can use the _at__ variable directly. For
  example,
• numargs _at__

37
Returning Values

• Subroutines can return values (can return list)
  to the calling program.
• result sqrt(144)
• To Return a value within a subroutine
• Stops the subroutine execution and return the
  specified value to the calling program.

38
Example Subroutine Return Value

• bigger calc_bigger( num1, num2 )
• 1. sub calc_bigger
• 2. PURPOSE returns the bigger of 2 numbers
• 3. ARGUMENTS 0 1st number, _1 2nd
  number
• 4. if ( 0 gt 1 )
• 5. return( 0 )
• 6. else
• 7. return( 1 )
• 8.
• 9.

39
Scope

• Global, lexical(private) and dynamic scopes
• Default function variables are global
• A variable can be made lexical(private) by
  preceding with the keyword my
• my myVar 23.65
• A variable can be made dynamic by preceding with
  the keyword local
• local myHash (California gt "Sacramento",
  Wisconsin gt "Madison")
• Examples http//people.cs.uchicago.edu/hai/hw4/s
  cope.cgi

40
Using Subroutines in External Files

• It is sometimes useful to place sub-routines in
  external separate files to promote their reuse
• Other programs can use them without creating
  separate copies
• To store in external file
• 1. Move subroutine lines to a new file. For
  example, move outputTableRow
• sub outputTableRow
• print 'ltTRgtltTDgtOnelt/TDgtltTDgtTwolt/TDgtlt/TRgt'

41
Using Subroutines in External Files

• 2. Place a 1 at the end of the new file. This
  step provides a return value of 1, which helps
  Perl recognize that the subroutine executed
  successfully.
• 3. Name the subroutine file. Usually, this file
  has a .lib suffix, which indicates that it is a
  library file of subroutines. For example, you
  might call the file startdoc.lib.

42
Using Subroutines in External Files

• 4. Place the file in the same directory as the
  program file. (Can reside in another directory
  but For now assume in same.)
• 5. Include an additional require line in the
  calling program. Before a program can use the
  subroutine library file, it must add a line that
  indicates where to look for that file. This line
  has the following format
• require library_filename

43
Example External Subroutine File

• Content of file 'htmlhelper1.lib
• 1. sub outputTableRow
• 2. PURPOSE outputs a table row with 2 cols
• 3. ARGUMENTS uses _0 for first col
• 4. uses _1 for second col
• 5. print "ltTRgtltTDgt_0lt/TDgtltTDgt_1lt/TDgtlt/TRgt
  "
• 6.
• 7. sub specialLine
• 8. PURPOSE Output a line with varible
  color
• 9. ARGUMENTS _0 is the line to output
• 10. _1 is the line color.
• 11. print "ltBgtltFont COLOR_1
  FACE\"Helvetica\"gt _0 lt/FONTgtlt/Bgt"
• 12.
• 13. 1

44
Example Calling Program

• 1. !/usr/local/bin/perl
• 2. print "Content-type text/html\n\n"
• 3. require 'htmlhelper1.lib'
• 4.
• 5.specialLine('Here is a simple table', 'RED')
• 6.print 'ltTABLE BORDER1gt'
• 7.print 'ltTHgt Num lt/THgt ltTHgt Num Cubed lt/THgt'
• 8.for ( i0 ilt3 i )
• 9. outputTableRow(i, i3)
• 10.
• 11.

45
Would Output The Following ...
http//people.cs.uchicago.edu/hai/hw4/sub3.cgi