Introduction to Programming the WWW I

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Introduction to Programming the WWW I

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Title: Introduction to Programming the WWW I

1
Introduction to Programming the WWW I

CMSC 10100-1
Winter 2004
Lecture 9

2
Todays Topics

List (array) and hash
Loop statements (contd)
Working with files

3
List Data

A list is an ordered collection of scalar values
Represented as a comma-separated list of values
within parentheses
Example (a,2,3,red)
Use qw() function to generate a list
A list value usually stored in an array variable
An array variable is prefixed with a _at_ symbol

4
Why use array variable?

Using array variables enable programs to
Include a flexible number of list elements. You
can add items to and delete items from lists on
the fly in your program
Examine each element more concisely. Can use
looping constructs (described later) with array
variables to work with list items in a very
concise manner
Use special list operators and functions. Can use
to determine list length, output your entire
list, and sort your list, other things

5
Creating Array Variables

Suppose wanted to create an array variable to
hold 4 student names
Creates array variable _at_students with values
Johnson, Jones, Jackson, and Jefferson

6
Creating Array Variables Of Scalars

Suppose wanted to create an array variable to
hold 4 student grades (numerical values)
_at_grades ( 66, 75, 85, 80 )
Creates array variable _at_grades with values 66,
75, 85, 80.

7
Referencing Array Items

Items within an array variable are referenced by
a set of related scalar variables
For example,
students0, students1, students2, and
students3
Reference in a variable name/subscript pair

8
Referencing Array Items - II

Subscripts can be whole numbers, another
variable, or even expressions enclosed within the
square brackets.
Consider the following example
i3
_at_preferences (ketchup , mustard ,
pickles , lettuce )
print preferencesi preferencesi-1
preferencesi-2 preferences0
Outputs the list in reverse order
lettuce pickles mustard ketchup

9
Changing Items In An Array Variable

Change values in an array variable and use them
in expressions like other scalar variables. For
example
_at_scores ( 75, 65, 85, 90)
scores3 95
average ( scores0 scores1
scores2 scores3 ) / 4
The third line sets average equal to (75 65
85 95 ) / 4, that is, to 80.

10
A Complete Array Example Program

1. !/usr/local/bin/perl
2. _at_menu ('Meat Loaf','Meat Pie','Minced
Meat', 'Meat Surprise')
3. print "What do you want to eat for
dinner?\n"
4. print 1. menu0\n"
5. print 2. menu1\n"
6. print 3. menu2\n"
7. print 4. menu3\n"

http//people.cs.uchicago.edu/hai/hw4/array1.cgi

11
Outputting the Entire Array Variable

Output all of the elements of an array variable
by using the array variable with print
For example,
_at_workWeek ('Monday', 'Tuesday', 'Wednesday',
'Thursday', 'Friday' )
print "My work week is _at_workWeek"
Would output the following
My work week is Monday Tuesday Wednesday Thursday
Friday

12
Getting the Number in an Array Variable

Use Range operator to find last element of list
For example
_at_grades ( 66, 75, 85, 80 )
last_one gradesgrades

grades3
13
Using Range Operator for list length

Ranger operator is always 1 less than the total
number in the list
(since list start counting at 0 rather than 1).
_at_workWeek (Monday, Tuesday, Wednesday,
Thursday, Friday
)
daysLong workWeek 1
print My work week is daysLong days long
Would output the following message
My work week is 5 days long.

14
A Better Way to Get List Length

You can also find the length of an array variable
by assigning the array variable name to a scalar
variable
For example, the following code assigns to size
the number of elements in the array variable
_at_grades
size_at_grades

15
Adding and Removing List Items

shift() and unshift() add/remove elements from
the beginning of a list.
shift() removes an item from the beginning of a
list. For example,
_at_workWeek (Monday, Tuesday,
Wednesday,Thursday, Friday )
dayOff shift(_at_workWeek)
print "dayOff dayOff workWeek_at_workWeek"
Would output the following
dayOff Monday workWeekTuesday Wednesday
Thursday Friday

16
Adding and Removing List Items

unshift() adds an element to the beginning of
the list For example,
_at_workWeek (Monday, Tuesday, Wednesday,
Thursday, Friday )
unshift(_at_workWeek, Sunday)
print workWeek is now _at_workWeek
would output the following
workWeek is now Sunday Monday Tuesday Wednesday
Thursday Friday

17
Adding and Removing List Items

pop() and push() add/remove elements from the
end of a list.
pop() removes an item from the end of a list.
For example,
_at_workWeek (Monday, Tuesday, Wednesday,
Thursday, Friday )
dayOff pop(_at_workWeek)
Would output the following
dayOff Friday workWeekMonday Tuesday Wednesday
Thursday

18
Adding and Removing List Items

push() adds an element to the end of the list
For example,
_at_workWeek (Monday, Tuesday, Wednesday,
Thursday, Friday )
push(_at_workWeek, Saturday)
print workWeek is now _at_workWeek
would output the following
workWeek is now Monday Tuesday Wednesday
Thursday Friday Saturday

19
splice() function remove or replace array items

splice(_at_array, offsetindex, length, list)
_at_arraythe array that will have elements removed
or replaced
offset the index of the first element to be
removed or replaced
length the number of elements to remove
list a list of elements to replace the removed
elements
If no length is given, everything beginning with
the offset element will be removed
If list is given, the removed elements will be
replaced by the list
http//www.classes.cs.uchicago.edu/classes/archive
/2004/winter/10100-1/02/perl/perl_splice.html

20
Extracting Multiple List Values

If you use multiple subscripts for a list
variable, you will extract a sub-list with the
matching list items.
For example,
_at_myList ( 'hot dogs, 'ketchup', 'lettuce',
'celery')
_at_essentials _at_myList 2, 3
print "essentials_at_essentials"
The output of this code is
essentialslettuce celery

21
Lists of Lists (or multidimensional lists)

Some data are best represented by a list of lists

22
Accessing Individual Items

Use multiple subsripts to access individual items
The first subscript indicates the row in which
the item appears, and
the second subscript identifies the column where
it is found.
In the preceding example,
Inventory00 points to AC1000,
Inventory10 points to AC1001, and
Inventory20 points to AC1002

23
A Partial Example ...

_at_Inventory (
'AC1000', 'Hammer', 122, 12.50 ,
'AC1001', 'Wrench', 344, 5.50 ,
'AC1002', 'Hand Saw', 150, 10.00
)
numHammers Inventory02
firstPartNo Inventory00
Inventory03 15
print numHammers, firstPartNo,Inventory03
This would output
122, AC1000, 15

24
Review for loop and while loop

for loop
sum 0
for (count1 countlt6 count)
sum count

while loop sum 0 count1 while (count lt
6) sum count count
25
The until Loop

Operates just like the while loop except that it
loops as long as its test condition is false and
continues until it is true

26
Example Program

Calculation of sum from 1 to 5
1. !/usr/local/bin/perl
2. print 'The sum from 1 to 5 is '
3. sum 0 count1
4. do
sum count
count
7. until (count gt 6)
8. print "sum\n"

until loop must end with a
27
The foreach Loop

The foreach loop is typically used to repeat a
set of statements for each item in an array
If _at_items_array (A, B, C)
Then item would A then B then C.

28
foreach Example

1. !/usr/local/bin/perl
2. _at_secretNums ( 3, 6, 9 )
3.uinput 6
4. ctr0 found 0
5. foreach item ( _at_secretNums )
6. ctrctr1
7. if ( item uinput )
print "Number item. Item found was number
ctrltBRgt"
found1
last
10.
11.
12.if (!found)
print Could not find the number/n

The last statement will force an exit of the loop
29
Would Output The Following ...
30
Loop Control

last statement
To end a loop before the initial test expression
evaluates to false
while (expression)
last if (input eq "Run for your life!")
statement block
next statement
To skip an iteration of a loop (loop NOT end!)
for (ctrl_var10 ctrl_var gt 0 --ctrl_var)
next if (input eq "You don\'t need this.")
statement block

31
Logical Operators(Compound Conditions)

logical conditional operators can test more than
one test condition at once when used with if
statements, while loops, and until loops
For example,
while ( x gt max found ne TRUE )
will test if x greater than max AND found is
not equal to TRUE

32
Some Basic Logical Operators

the AND operator - True if both tests are
true
while ( ctr lt max flag 0 )
the OR operator. True if any test is true
if ( name eq SAM name eq MITCH )
! the NOT operator. True if test is false
if ( !(FLAG 0) )

33
Consider the following ...

1. !/usr/local/bin/perl
2. _at_safe (1, 7)
3. print ('My Personal Safe')
4. in1 1
5. in2 6
6. if (( in1 safe0 ) ( in2
safe1))
7. print "Congrats you got the combo"
8. elsif(( in1 safe0 ) ( in2
safe1))
9. print You got half the combo"
10.else
11. print "Sorry you are wrong! "
12. print "You guessed in1 and in2 "
13.

34
Hash Lists (or associated arrays)

Items not stored sequentially but stored in pairs
of values
the first item the key, the second the data
The key is used to look up or provide a
cross-reference to the data value.
Instead of using sequential subscripts to refer
to data in a list, you use keys.

35
Advantages of Hash Lists

Need to cross-reference one piece of data with
another.
Perl supports some convenient functions that use
hash lists for this purpose. (E.g,. You cross
reference a part number with a product
description.)
Concerned about the access time required for
looking up data.
Hash lists provide quicker access to
cross-referenced data. E.g, have a large list of
product numbers and product description, cost,
and size, pictures).

36
Using Hash Lists

General Format to define a hash list
Alternate syntax
months ( "Jan" gt 31, "Feb" gt 28, "Mar" gt
31, "Apr" gt 30, "May" gt 31, "Jun" gt 30, "Jul"
gt 31, "Aug" gt 31, "Sep" gt 30, "Oct" gt
31,"Nov" gt 30, "Dec" gt 31 )

37
Accessing Hash List Item

When access an individual item, use the following
syntax
Note You Cannot Fetch Keys by Data Values. This
is NOT valid
mon months 28

38
Hash Keys and Values Access Functions

The keys() function returns a list of all keys in
the hash list
Inventory ( 'Nuts', 33, 'Bolts', 55,
'Screws', 12)
_at_keyitems keys(Inventory)
print keyitems _at_keyitems
The values() function returns a list of all
values Example,
Inventory ( 'Nuts', 33, 'Bolts', 55,
'Screws', 12)
_at_keyitems keys(Inventory)
print keyitems _at_keyitems
Perl outputs hash keys and values according to
how they are stored internally. So, a possible
output order
keyitems Screws Bolts Nuts

39
Using keys() and values()

Keys() values() are often used to output the
contents of a hash list. For example,
Inventory ( 'Nuts', 33, 'Bolts', 55, 'Screws',
12)
foreach item ( keys (Inventory) )
print "Itemitem ValueInventoryitem
"
The following is one possible output order
ItemScrews Value12 ItemBolts Value55
ItemNuts Value33

40
Changing a Hash Element

You can change the value of a hash list item by
giving it a new value in an assignment statement.
For example,
Inventory ( 'Nuts', 33, 'Bolts', 55, 'Screws',
12)
InventoryNuts 34
This line changes the value of the value
associated with Nuts to 34.

41
Adding a Hash Element

You can add items to the hash list by assigning a
new key a value. For example,
Inventory ( 'Nuts', 33, 'Bolts', 55, 'Screws',
12)
InventoryNails 23
These lines add the key Nails with a value of 23
to the hash list.

42
Deleting a Hash Element

You can delete an item from the hash list by
using the delete() function. For example,
Inventory ( 'Nuts', 33, 'Bolts', 55, 'Screws',
12)
delete InventoryBolts
These lines delete the Bolts key and its value of
55 from the hash list.

43
Verifying an Elements Existence

Use the exists() function verifies if a
particular key exists in the hash list.
It returns true ( or 1) if the key exists.
(False if not). For example
Inventory ( 'Nuts', 33, 'Bolts', 55, 'Screws',
12)
if ( exists( InventoryNuts ) )
print ( Nuts are in the list )
else
print ( No Nuts in this list )
This code outputs
Nuts are in the list.

44
Environmental Hash Lists

When your Perl program starts from a Web browser,
a special environmental hash list is made
available to it.
Comprises a hash list that describe the
environment (state) when your program was called.
(called the ENV hash).
Can be used just like any other hash lists. For
example,

45
Some environmental variables

HTTP_USER_AGENT. defines the browser name,
browser version, and computer platform of the
user who is starting your program.
For example, its value might be Mozilla/4.7
en (Win 98, I) for Netscape.
You may find this value useful if you need to
output browser-specific HTML code.

46
Some Environmental Variables

HTTP_ACCEPT_LANGUAGE - defines the language that
the browser is using.
E.g., might be en for English for Netscape or
en-us for English for Internet Explorer.
REMOTE_ADDR - indicates the TCP/IP address of the
computer that is accessing your site.
(ie., the physical network addresses of computers
on the Internet for example, 65.186.8.8.)
May have value if log visitor information

47
Some environmental variables

REMOTE_HOST - set to the domain name of the
computer connecting to your Web site.
It is a logical name that maps to a TCP/IP
addressfor example, www.yahoo.com.
It is empty if the Web server cannot translate
the TCP/IP address into a domain name.
Display all environmental variables
http//people.cs.uchicago.edu/hai/hw4/env.cgi

48
Example Checking Language

!/usr/local/bin/perl
print "content-type text/html\n\n"
print "lthtmlgt\nltheadgt\n"
print "lttitlegtCheck Environmentlt/titlegt"
print "lt/headgt\nltbodygt\n"
langENV'HTTP_ACCEPT_LANGUAGE'
if ( lang eq 'en' lang eq 'en-us' )
print "LanguageENV'HTTP_ACCEPT_LANGUAGE'"
print "ltBRgtBrowser ENV'HTTP_USER_AGENT'"
else
print 'Sorry I do not speak your language'
print "\nlt/bodygt\nlt/htmlgt"
http//people.cs.uchicago.edu/hai/hw4/check_lan.c
gi

49
Printing Text Blocks

In Perl there are easier ways to print out large
blocks of code instead of line-by-line
Solution 1
print ltlt"CodeBlock" some word(s) to indicate
the beginning of a
block(no " " are needed)
if it's a simple word
..
..
CodeBlock same word(s) to mark the end of
block.
Notice no quotes or semicolon here!
Solution 2
print qq qq followed by some character not used
in
the following text to start the block
..
..
the same character to mark the end of the
block.
Notice semicolon here!
Examples old_version solution1 solution2

50
Creating a Hash List of List Items

Hash tables use a key to cross-reference the key
with a list of values (instead of using simple
key/value pairs).
For example consider the following table

51
Creating Hash Tables

Access items from a hash table much like you
access a hash list
InventoryAC10000 is Hammer,
InventoryAC10010 is Wrench,
InventoryAC10021 is 150.

52
Changing Hash Table Items

Changing Hash table items
Inventory (
AC1000 gt 'Hammer', 122, 12, 'hammer.gif',
AC1001 gt 'Wrench', 344, 5, 'wrench.gif',
AC1002 gt 'Hand Saw', 150, 10, 'saw.gif'
)
numHammers InventoryAC10001
InventoryAC10011 InventoryAC10011
1
partName InventoryAC10020
print numHammers, partName, InventoryAC1001
1
This code would output
122, Hand Saw, 345

53
Adding/Deleting Hash Table Items

When you want to add a hash table row, you must
specify a key and list of items.
For example, the following item adds an entry
line to the Inventory hash table
Inventory (
AC1000 gt 'Hammers', 122, 12,
'hammer.gif',
AC1001 gt 'Wrenches', 344, 5,
'wrench.gif',
AC1002 gt 'Hand Saws', 150, 10,
'saw.gif'
)
InventoryAC1003 Screw Drivers, 222, 3,
sdriver.gif

54
Adding/Deleting Hash Table Items

Because the exists() and delete() hash functions
both work on a single key,specify them just as
you did before
For example, the following code checks whether a
key exists before deleting it
if ( exists InventoryAC1003 )
delete InventoryAC1003
else
print Sorry we do not have the key
Check if the record exists before you try to
delete it.

55
Working with Files (contd in Lecture 10)

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

56
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).

57
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
58
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.

59
File Handles

Use the file handle to refer to the file once
opened
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() by default it print to STDOUT
STDERR -- standard error, usually the monitor
screen

60
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

61
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

62
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
63
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

64
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

65
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.", br
close (INFILE)

http//people.cs.uchicago.edu/hai/hw4/readFile3.c
gi
66
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)

67
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
!

68
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

69
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
70
Example of flock()