Chapter 6 Hashes (Associative Arrays), Environmental Variables and Functions

2. Chapter 6 Hashes (Associative Arrays), Environmental Variables and Functions

3. Chapter Objectives • Describe how to use hash lists and hash tables in Perl • Look at Perl Environmental Variables • Learn how to use subroutines in a programs • Combine form generation and processing into a single program

4. Chapter Objectives • Describe how to use hash lists and hash tables in Perl • Look at Perl Environmental Variables • Learn how to use subroutines in a programs • Combine form generation and processing into a single program

5. Hash Lists (or associated arrays) • A type of array that uses string indices instead of sequential numbers. • 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.

6. 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).

7. Using Hash Lists • General Format to define a hash list: • Alternate syntax %months = ( "Jan" => 31, "Feb" => 28, "Mar" => 31, "Apr" => 30, "May" => 31, "Jun" => 30, "Jul" => 31, "Aug" => 31, "Sep" => 30, "Oct" => 31,"Nov" => 30, "Dec" => 31 );

8. Accessing Items From A Hash List

9. Accessing Items From a Hash List • When access an individual item, use the following syntax: • Note: You Cannot Fetch Keys by Data Values. This is NOT valid: $mon = $months{ 28 };

10. Complete Example. . . • Consider a Web Application that • Allows you select a start month and day • Outputs number of days left in month • Calling form: • <FORM ACTION=“http://65.108.8.8/cgi-bin/C6/drivehash1.cgi”> <BR> <SELECT NAME=“startmon" SIZE=3 > <OPTION selected>Jan <OPTION>Feb <OPTION>Mar . . . </SELECT> . . .

11. Would Output The Following ...

12. Example Hash List Program 1. #!/usr/bin/perl 2. use CGI ':standard'; 3. print header, start_html( 'Month Calc' ); 4. %months = ( "Jan", 31, "Feb", 28, "Mar", 31, "Apr", 30, "May", 31, "Jun", 30, "Jul", 31, "Aug", 31, "Sep", 30,"Oct", 31, "Nov", 30, "Dec", 31 ); 5. $startm = param('startmon'); 6. $startd = param('startday'); 7. $mdays=$months{ "$startm" }; 8. if ( $startd <= $mdays ) { 9. $daysLeft = $mdays - $startd; 10. print "There are $daysLeft days left in $startm."; 11. print br, "$startm has $mdays total days "; 12. } else { 13. print "Hmmm, there aren't $startd day in $startm"; 14. } 15. print end_html;

13. Hash Keys and Values Access Functions • The keys() function returns a list of all keys in the hash list. For example, %Inventory = ( 'Nuts', 33, 'Bolts', 55, 'Screws', 12); @keyitems = keys(%Inventory); print “keyitems= @keyitems”; • Perl outputs hash keys and values according to how they are stored internally. So, 1 possible output order: keyitems= Screws Bolts Nuts

14. 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 $item ( (keys %Inventory) ) { print "Item=$item Value=$Inventory{$item} "; } • The following is one possible output order: Item=Screws Value=12 Item=Bolts Value=55 Item=Nuts Value=33

15. 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); $Inventory{‘Nuts’} = 34; • This line changes the value of the value associated with Nuts to 34.

16. 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); $Inventory{‘Nails’} = 23; • These lines add the key Nails with a value of 23 to the hash list.

17. 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 $Inventory{‘Bolts’}; • These lines delete the Bolts key and its value of 55 from the hash list.

18. Quick Case Study • Consider the program at • http://hawk.depaul.edu/cgi-bin/cgiwrap/dlash/drive_distance.cgi • It is a front-end form to calc distances. • It calls the program at • http://hawk.depaul.edu/cgi-bin/cgiwrap/dlash/distance.cgi

19. Verifying an Element’s Existence • Use the exists() function verifies if a particular key exists in the hash list. • It returns true (1) if the key exists. (False if not). For example: %Inventory = ( 'Nuts', 33, 'Bolts', 55, 'Screws', 12); if ( exists( $Inventory{‘Nuts’} ) ) { print ( “Nuts are in the list” ); } else { print ( “No Nuts in this list” ); } • This code outputs Nuts are in the list.

20. Quick Case Study • Consider the program at • http://hawk.depaul.edu/cgi-bin/cgiwrap/dlash/drive_distance.cgi • It is a front-end form to calc distances. • It calls the program at • http://hawk.depaul.edu/cgi-bin/cgiwrap/dlash/distance.cgi

21. Complete Example. . . • Consider a Web Application that • Allows you add an item to a hash list. • Allows “add” or “Delete” option but only implement “add” • Calling form: • <FORM ACTION=“http://65.108.8.8/cgi-bin/C6/hashadd.cgi”> <BR> Operation? <INPUT RADIO NAME=“Action” Value=“Add” > Add <INPUT RADIO NAME=“Action” Value=“Unkonwn” > Unknown <BR> <INPUT TEXT NAME=“Key” > <INPUT TEXT NAME=“Value” > </FORM>

22. Would Output The Following ..

23. Example Program 1. #!/usr/bin/perl 2. use CGI ':standard'; 3. print header, start_html( 'Inventory' ); 4. %invent = ( "Nuts", 44, "Nails", 34, "Bolts", 31 ); 5. $action = param('Action'); 6. $whatkey = param('Key'); 7. $whatvalue = param('Value'); 8. if ( $action eq "Add" ) { 9. if ( exists( $invent{ "$whatkey" } ) ) { 10. print "Sorry already exists $whatkey <BR>"; 11. } else { 12. $invent{"$whatkey"} = $whatvalue; 13. print "Adding key=$whatkey val=$whatvalue <BR>"; 14. print "-----<BR>"; 15. foreach $key ( ( keys %invent ) ) { 16. print "$invent{$key} key=$key <BR>"; 17. } 18. } 19. } else { print "Sorry No Such Action=$action<BR>"; } 20. print end_html;

24. Chapter Objectives • Describe how to use hash lists and hash tables in Perl • Look at Perl Environmental Variables • Learn how to use subroutines in a programs • Combine form generation and processing into a single program

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

26. Some environmental variables • HTTP_REFFERER - defines the URL of the Web page that the user accessed prior to accessing your page. • For example, a value of HTTP_REFFERER might be “http://www.mypage.com”. • Set when someone links to or reaches your site via the submit button. • Useful to check if your program was called from the your form-generating program. (Might reject anyone accessing from somewhere else)

27. Some environmental variables • REQUEST_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.

28. 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_ADDRESS - indicates the TCP/IP address of the computer thatis 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

29. 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 address—for example, www.yahoo.com. • It is empty if the Web server cannot translate the TCP/IP address into a domain name. • There are many other environmental variables.

30. Complete example … • Consider a CGI/Perl program that checks the language. • Output one message when english • Another when not. • Need to check how both IE and Netscape set • $ENV{'HTTP_ACCEPT_LANGUAGE' • 'en' or 'en-us'

31. Example: Checking Language 1. #!/usr/bin/perl 2. use CGI ':standard'; 3. print header, start_html('Check Environment'); 4. $lang=$ENV{'HTTP_ACCEPT_LANGUAGE'}; 5. if ( $lang eq 'en' || $lang eq 'en-us' ) { 6. print "Language=$ENV{'HTTP_ACCEPT_LANGUAGE'}"; 7. print "<BR>Browser= $ENV{'HTTP_USER_AGENT'}"; 8. } 9. else { 10. print 'Sorry I do not speak your language'; 11. } 12. print end_html;

32. Would Output The Following ...

33. Checking Browser Types $browser=$ENV{'HTTP_USER_AGENT'} ; if ( $browser =~ m/MSIE/ ) { print "Got Internet Explorer Browser=$browser"; } else { print "browser=$browser"; }

34. Chapter Objectives • Describe how to use hash lists and hash tables in Perl • Look at Perl Environmental Variables • Learn how to use subroutines in a programs • Combine form generation and processing into a single program

35. Chapter Objectives • Describe how to use hash lists and hash tables in Perl • Look at Perl Environmental Variables • Learn how to use subroutines in a programs • Combine form generation and processing into a single program

36. Using Subroutines • 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

37. 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.

38. Working with Subroutines • You can create a subroutine by placing a group of statements into the following format: sub subroutine_name { set of statements } • For example a outputTableRow subroutine sub outputTableRow { print ‘<TR><TD>One</TD><TD>Two</TD></TR>’; } • Execute the statements in this subroutine, by preceding the name by an ampersand: &outputTableRow;

39. Subroutine Example Program 1. #!/usr/bin/perl 2. use CGI ':standard'; 3. print header, start_html( 'Subroutine Example' ); 4. print 'Here is simple table <TABLE BORDER=1>'; 5. &outputTableRow; 6. &outputTableRow; 7. &outputTableRow; 8. print '</TABLE>', end_html; 9. sub outputTableRow { 10. print '<TR><TD>One</TD><TD>Two</TD></TR>'; 11.} Call subroutine Three times. Subroutine definition

40. Would Output The Following …

41. PassingArguments to Subroutines • Generalize a subroutine using input variables (called arguments to the subroutine). &outputTableRow( ‘A First Cell’, ‘A Second Cell’ ); • Use @_ (underbar array) to access arguments: $_[0] as the variable name for the first argument, $_[1] as the variable name for the second argument, $_[2] as the variable name for the third argument, . . . $_[n] as the variable name for the nth argument. Array name

42. PassingArguments to Subroutines • Suppose outputTableRow subroutine was called as shown below: &outputTableRow( ‘A First Cell’, ‘A Second Cell’ ); • Then • $_[0] would be set to ‘A First Cell’and • $_[1] would be set to ‘A Second Cell’: First argument Second argument

43. Full Subroutine Example 1. #!/usr/bin/perl 2. use CGI ':standard'; 3. print header, start_html( 'Subroutine with arguments' ); 4. print 'Here is simple table: <TABLE BORDER=1>'; 5.for ( $i=1; $i<=3; $i++ ) { 6. &outputTableRow( "Row $i Col 1", "Row $i Col 2"); 7. } 8. print '</TABLE>', end_html; 9. sub outputTableRow { 10. print "<TR><TD>$_[0]</TD><TD>$_[1]</TD></TR>"; 11. } arguments First Argument Second Argument

44. Would Output The Following ...

45. 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 @_ variable directly. For example, • $numargs = @_; More Commonly used

46. Returning Values • Subroutines can return values to the calling program. (Auseful way to send the results of a computation) • $result = sqrt(144); • To Return a value within a subroutine: • Stops the subroutine execution and return the specified value to the calling program. Returns 12 to $result

47. Example Subroutine Return Value $largest = &simple_calc( $num1, $num2 ); 1. sub simple_calc { 2. # PURPOSE: returns largest of 2 numbers 3. # ARGUMENTS $[0] – 1st number, $_[1] – 2nd number 4. if ( $[0] > $[1] ) { 5. return( $[0] ); 6. } else { 7. return( $[1] ); 8. } 9. } Return arg1 if bigger Return arg2 If bigger

48. Abbreviated way to check for 0 returned • Can check for 0 returned very concisely if ( myfunction(4) ) { stuff to do if not 0 } else { stuff to do if 0 } Or Alternatively if ( !myfunction(4) ) { stuff to do if 0 } else { stuff to do if not 0 } Perhaps Output an error

49. For example, ... Ask for Input. Not allow words like “mud”

50. Example Program 1. #!/usr/bin/perl 2. use CGI ':standard'; 3. print header, start_html('Input Check '); 4. @dirty=('mud', 'dirt', 'slime', 'grease'); 5. $input=param('uinput'); 6. if ( &CheckInput($input ) ) { 7. print "Input received: $input"; 8. } else { 9. print "Sorry I cannot accept the input=$input"; 10. } 11. sub CheckInput { 12. foreach $item ( @dirty ) { 13. if ( $_[0] =~ m/$item/ ) { 14. print "Hey $item is not clean!", br; 15. return 0; 16. } 17. } 18. return 1; 19. } If 0 returned OK Else not OK Matches if $item is found Return 0 if Found it.