Similarly, example of memory address of array and hash are: ARRAY(0x1a31d44) and HASH(0x80f6c6c) respectively.

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1 Lecture #9 What is reference? Perl Reference A Perl reference is a scalar value that holds the location of another value which could be scalar, arrays, hashes, or even a subroutine. In other words, a reference is a variable that points the memory address of another existing data; therefore, a reference is simply a pointer to something. In Perl, a reference is declared as a variable; however, its value can be a variable, array, hash, or subroutine prefixed with a backslash. $reference = \$data The term memory of a computer is actually a series of memory registers, each register typically has one byte (or two bytes) in size, and each register is assigned a unique address. A memory address is a unique identifier used by a device or CPU for data tracking. Memory addresses are normally written not as one but as two hex numbers. For example, 0x9FFF:000F. However, in Perl, a memory address of a scalar is expressed as: A sample is: SCALAR(0xnnnnnnnn) SCALAR(0x1a9359c) Similarly, example of memory address of array and hash are: ARRAY(0x1a31d44) and HASH(0x80f6c6c) respectively. By the way, in Perl, programmers can create a reference to an anonymous array or hash by using square brackets for an array and braces for a hash: $x = ['apple', 'banana', 'orange']; #array $y = PA => "Pennsylvania", CA => "California", WA => "Washington" ; #hash Reference for variables A Perl variable is string (or name) that represents an area of memory which stores scalar value. In the following example, $x is a Perl variable with an initial value 5, $p is a reference that will be assigned the memory address where the value 5 is stored. That s being said $p will store the value of a memory address. $x = 15; # variable $p = \$x; # reference print $x. " ". $p; 252

2 The following is a sample output. According to this output, 5 is stored in a memory address 0xla9359c. 15 SCALAR(0x1a9359c) The following is another example that illustrates how to reference to a string variable. $name = "Jennifer Lopez"; # variable $p = \$name; # reference print $name. " ". $p; Interestingly, $$p (must have two $ signs) means the value stored in the reference $p In other words, $$p can retrieve the actual value stored in the memory address pointed by $p. Therefore, $$p equals to $x. In Perl, using $$reference to retrieve the actual value is known as dereferencing. De-referencing returns the value a reference points to the location. $x = 15; # variable $p = \$x; # reference The output looks: print $x. " ". $$p; #deference By the same token, the output of the following sample is a two-fold of Jennifer Lopez. $name = "Jennifer Lopez"; # variable $p = \$name; # reference print $name. " ". $$p; Another way to de-reference a reference is: $$p. 253

3 $x = "apple"; $p = \$x; print $$p; Reference for arrays An array is a series of objects all of which are the same size and type. Each object in an array is called an element. The following is the syntax to reference an array in Perl, where $reference is a variable must represent an existing Perl array. Be sure to declare the array before creating the reference. $reference = \@arrayname The following demonstrates how to reference an array. The variable $p is the reference of x array. The value of $p is the memory address that stores the x = (4, 7, 3, 5, 8, 9); # array $p = \@x; # reference $size for ($i = 0; $i < $size; $i++) print $x[$i]. " "; (must is equivalent they both represent an array. In other In the following, the instructor to when finding the size The instructor also uses $$p[$i] to replace $x[$i] because elements in array can be denoted as $x[$i]. As discussed previously, $$p is equivalent to = (4, 7, 3, 5, 8, 9); # array $p = \@x; # reference $size for ($i = 0; $i < $size; $i++) print $$p[$i]. " "; 254

4 The following is another example with a string array. It also demonstrates how to to retrieve the elements of = ("apple", "orange", "banana"); # array $p = \@x; # reference foreach (@$p) print $_. " "; The following is another example that demonstrates how to use the quote word function, qw(), to generate a list of words. It also uses the $$p[$i] = qw(apple orange banana); print " "; $p = \@x; print "\$p[0]: ", $$p[0], " "; print "\$p[1]: ", $$p[1], " "; print "\$p[2]: ", $$p[2], " "; Instead of the $$p[$i] format, $p->[$i] is another option. The following demonstrates how to use the $p->[$i] = qw(apple orange banana); print " "; $p = \@x; print $p->[0], " "; print $p->[1], " "; print $p->[2], " "; 255

5 Another way to de-reference a reference The following is an = (4, 7, 3, 5, 8, 9); # array $p = \@x; # reference = ("apple", "orange", "banana"); # array $p = \@x; # reference " "; References apply to two dimensional arrays. The following is a sample code that demonstrates how a two-dimensional array = ( [1, 2, 3, 4], ["one", "two", "three", "four"], ['ichi', 'nii', 'san', 'shi'], ); $s for ($i = 0; $i < $s; $i++) $s1 for ($j = 0; $j < $s1; $j++) print $x[$i][$j]. " "; The following example creates a reference, $p, and uses it to represent the array. Again $$p is equivalent to = ( [1, 2, 3, 4], ["one", "two", "three", "four"], ['ichi', 'nii', 'san', 'shi'], ); $p = \@x; $s 256

6 for ($i = 0; $i < $s; $i++) $s1 for ($j = 0; $j < $s1; $j++) print $$p[$i][$j]. " "; Here is another = localtime(); # assign date time values to dt array $p = \@dt; foreach (@$p) print $_. " "; Reference for hashes As discussed in a previous lecture, a Perl hash is special type of array whose elements must consist of a key and a value. The following is the syntax to reference a hash in Perl, where $reference is a variable must represent an existing Perl array. Be sure to declare the hash before creating the reference. $reference = \@hashname The following illustrates how to reference to a Perl hash named month. The variable $p is the reference of month hash. The value of $p is the memory address that stores the month hash. 01 is the key of Jan, and Jan is the value. The month hash has 12 elements. #hash %month = ( "01", "Jan", "02", "Feb", "03", "Mar", "04", "Apr", "05", "May", "06", "Jun", "07", "Jul", "08", "Aug", "09", "Sep", "10", "Oct", "11", "Nov", "12", "Dec"); $p = \%month; # reference foreach (keys %month) print $_. " : ". $month$_. " "; 257

7 In the following example, the instructor uses %$p to de-reference the month hash. Since each element of the month has can be denoted as $monthkey, the instructor uses $$pkey instead. #hash %month = ( "01", "Jan", "02", "Feb", "03", "Mar", "04", "Apr", "05", "May", "06", "Jun", "07", "Jul", "08", "Aug", "09", "Sep", "10", "Oct", "11", "Nov", "12", "Dec"); $p = \%month; # reference foreach (keys %$p) print $_. " : ". $$p$_. " "; The following example demonstrates how to reference a hash whose elements are defined with the => operator. #hash %month = ( "01"=>"Jan", "02"=>"Feb", "03"=>"Mar", "04"=>"Apr", "05"=>"May", "06"=>"Jun", "07"=>"Jul", "08"=>"Aug", "09"=>"Sep", "10"=>"Oct", "11"=>"Nov", "12"=>"Dec" ); $p = \%month; # = = values(%$p); for ($i=0; $i<@k; $i++) print "$k[$i] : $v[$i]", br; Another way to de-reference a reference is: %$p. The following is an example. %x = ("CA", "California", "PA", "Pennsylvania", 258

8 "WA", "Washington"); #hash $p = \%x; # reference print %$p, " "; %x = (TX=>"Texas", MI=>"Michigan", OH=>"Ohio"); # hash $p = \%x; # reference print %$p, " "; Reference for subroutines A Perl subroutine or function is a group of statements that together perform a task. Perl allows programmers to create a reference of subroutines. The syntax to reference a subroutine is: $reference = \&subroutinename; The following example contains a subroutine named print_it. The instructor creates a reference, $p, to represent the print_it subroutine. The instructor uses &$p() to call the subroutine by the reference. sub print_it print "I am a subroutine\n" ; $p = \&print_it; &$p(); The following is another example that illustrates how to call a named subroutine with its reference. The syntax to call a named subroutine by its reference is: $reference->(). sub hello print "Hello world!"; $p = \&hello; # reference $p->(); # Call the subroutine via its reference In the following example, the anonymous subroutine can calculate the sum of n, where n is passed by the reference $p->(n). By the way, the value of n is given by the user through the HTML form. 259

9 print "<form action='$envscript_name' method='post'>"; print "Enter an integer: <input type='text' name='n'>"; print "<input type='submit' value='submit'>"; print "</form>"; if (param) $n = param('n'); $p = sub $sum = 0; for ($i=0; $i <= $_[0]; $i++) $sum += $i; return $sum; ; print $p->($n); The following is another example that compares the &$p() and $p->() formats. sub findmax if ($_[0] >= $_[1]) return $_[0]; else return $_[i]; ; $p = \&findmax; print &$p(9, 3). " ". $p->(6, 4); Perl programmers can also create a reference to an anonymous subroutine. 260

10 $p = sub print "I am a subroutine\n"; ; &$p(); The following uses the -> operator to call a anonymous subroutine by its reference with a parameter, $p. $p = sub print shift; ; $p->("hello!"); The following is an example that passes an array to a subroutine. The subroutine then returns the sum of the elements in the = (1, 2, 3, 4, 5); print getsum(@x); sub getsum $sum = 0; foreach (@_) $sum += $_; return $sum; The following illustrates how the instructor (a) changes the getsum subroutine to an anonymous subroutine, (b) creates a reference $p to represent the anonymous subroutine, and (c) then uses &$p() to represent the = (1, 2, 3, 4, 5); $p = sub 261

11 $sum = 0; foreach (@_) $sum += $_; return $sum; ; print &$p(@x). " "; References are particularly handy for passing in arguments to subroutines, or returning values from them. sub add_number $sum = 0; foreach (@_) $sum += $_; return = (4, 17, 14, 23); $p = add_number(@n); The following is an example which produces a reference to a function by preceding that function name with \& and to dereference that reference you simply need to prefix reference variable using ampersand &. # Function definition sub PrintHash foreach (@_) print "Item : $_ "; %x = ('fn' => 'Jennifer', 'ln' => 'Lopez', 'age' => 19); 262

12 # Create a reference to above function. $p = \&PrintHash; # Function call using reference. &$p(%x); In the following example, the shift keyword retrieves the first element of the array and returns it. sub add_number return = (4, 17, 14, 23); $p = add_number(@n); Circular References A circular reference occurs when two references contain a reference to each other. You have to be careful while creating references otherwise a circular reference can lead to memory leaks. Following is an example: $x = 100; $x = \$x; print "Value of \"x\" is : ", $$x, "\n"; When above program is executed, it produces the following result. Note that the data type is REF. Value of "x" is : REF(0x1a7256c) Similarly, the following is the string version. $x = 'apple'; $x = \$x; print "Value of \"x\" is : ", $$x, "\n"; 263

13 The ref() function The ref() function returns a string that indicates the type of reference. In other words, programmers can write a subroutine that takes a different action based on the type of reference it received. sub mysub # subroutine print "This is a subroutine."; ; $p1 = \&mysub; #variable $x = "This is a variable."; $p2 = = ("apple", "orange", "grape"); #array $p3 = \@y; %z = (CA=>"California", PA=>"Pennsylvania"); #hash $p4 =\%z; print ref($p1). " ". ref($p2). " ". ref($p3). " ". ref($p4); A sample output looks: CODE SCALAR ARRAY HASH The following table compares the principles of variables, arrays, and hashes as well as how conference works with them in Perl. Item \ Data Type Variable Array Hash Example $x = = ("apple", "orange"); %x = (a=>"apple", o=>"orange"); Reference $p = \$x; $p = \@x; $p = \%x; De-reference $$p or %$p or %$p Accessing element $$p $$p[0] or $p->[0] $p'a' or $p->'a' The following example reviews how to reference to handle variable, array, and hash in Perl. $x = "apple"; 264

14 $p1 = \$x; print $$p1. = ("apple", "orange"); $p2 = \@y; print $$p2[0]. " ". $p2->[1]. " "; %z = (a=>"apple", o=>"orange"); $p3 = \%z; print $$p3'a'. " ". $p3->'o'. " "; Review Question 1. Given the following code, which is a possible output? $x = ['apple', 'banana']; print $x; A. ARRAY(0x1a81d64) B. applebanana C. 'apple' 'banana' D. apple banana 2. Given the following code, which is a possible output? $x = PA => "Pennsylvania", CA => "California", ; print $x; A. HASH(0x1ac1d54) B. PA => "Pennsylvania", CA => "California", C. PAPennsylvaniaCACalifornia D. $x 3. Given the following code, which is a possible output? $x = 15; $p = \$x; print $p; A. ARRAY(0x1a81d64) B. HASH(0x1a81d64) C. SCALAR(0x1a81d64) D. CODE(0x1a81d64) 4. Given the following code, which is a possible output? A. $x B. 15 C. $p D. $$p $x = 15; $p = \$x; print $$p; 5. Given the following code, which is a possible = (4, 7, 3, 5, 8, 9); $p = \@x; $size 265

15 for ($i = 0; $i < $size; $i++) print $x[$i]. " "; A. (4, 7, 3, 5, 8, 9) B. $p C D. $x[$i] 6. Given the following code, which is a possible = ("apple", "orange", "banana"); $p = \@x; foreach (@$p) print $_. " "; A. ("apple", "orange", "banana") B. "appleorangebanana" C. "apple orange banana" D. apple orange banana 7. Given the following code, which is a possible = qw(apple orange banana); $p = \@x; print $$p[0]; A. apple B. apple orange banana C. $apple D. $$[0] 8. Given the following code, which is a possible output? %x = ('01'=>"one", '02'=>"two"); $p=\%x; print %$p; A. onetwo B. 01one02two C D. '01'=>"one", '02'=>"two" 9. Given the following code, which is a possible output? $p = sub print "Perl Programmming"; ; &$p(); A. sub print "Perl Programmming"; ; B. &$p() C. Perl Programmming D. $p 10. Given the following code, which is a possible output? 266

16 sub add_number return = (4, 17, 14, 23); $p = add_number(@n); print $p; A. 4 B C. (4, 17, 14, 23) D. $p 267

$Close all the applications that may cause conflicts with XAMPP such as Skype IIS, Wampserver, VMware, etc. 3. Insert the USB that contains XAMPP. Determine the drive name (such as F:\ ). 4.$

17 Lab #9 Perl Reference Preparation #1: Running the server and create default database 1. Insert the USB flash drive. Record the drive name:. 2. Close all the applications that may cause conflicts with XAMPP such as Skype IIS, Wampserver, VMware, etc. 3. Insert the USB that contains XAMPP. Determine the drive name (such as F:\ ). 4. Change to the X:\xampp\ directory (where X must be the correct drive name) to find the setup_xampp.bat and then execute it. You should see: ###################################################################### # ApacheFriends XAMPP setup win32 Version # # # # Copyright (C) Apachefriends # # # # Authors: Kay Vogelgesang (kvo@apachefriends.org) # # Carsten Wiedmann (webmaster@wiedmann-online.de) # ###################################################################### 5. Read the option below the above lines. You should see one of the following: Sorry, but... nothing to do! Press any key to continue... Do you want to refresh the XAMPP installation? 1) Refresh now! x) Exit 6. If you got the Sorry, but... option, skip to the next step to launch Apache with XAMPP Control Panel; otherwise, press 1 and then [Enter]. If re-configuration succeeds, you should see the following message. XAMPP is refreshing now... Refreshing all paths in config files... Configure XAMPP with awk for Windows_NT ###### Have fun with ApacheFriends XAMPP! ###### Press any key to continue In the X:\xampp\ directory, click the xampp-control.exe file to launch XAMPP control panel. 8. Click the Start button next to Apache to start the Apache server. Learning Activity #1: 268

$1. In the X:\xampp\htdocs\myperl directory, use Notepad to create a new script file named lab9_1.pl with the following lines. Be sure to replace X with the correct drive name.$

18 1. In the X:\xampp\htdocs\myperl directory, use Notepad to create a new script file named lab9_1.pl with the following lines. Be sure to replace X with the correct drive name. $x = 15; # variable $p = \$x; # reference print $x. " ". $p; $name = "Jennifer Lopez"; # variable $p = \$name; # reference print $name. " ". $p; $x = 15; # variable $p = \$x; # reference print $x. " ". $$p; #deference $name = "Jennifer Lopez"; # variable $p = \$name; # reference print $name. " ". $$p; $x = "apple"; $p = \$x; print $$p; 2. Use Web browser to visit (where localhost is your computer), you should see the following window. Notice that you need to use if you modified the port number to resolve the port conflicts (as specified in Lab #1). The output looks: 269

19 3. Download the assignment template, and rename it to lab9.doc if necessary. Capture a screen shot similar to the above figure and paste it to a Word document named lab9.doc (or lab9.docx). Learning Activity #2: 1. In the X:\xampp\htdocs\myperl directory, use Notepad to create a new script file named lab9_2.pl with the following = (4, 7, 3, 5, 8, 9); # array $p = \@x; # reference $size for ($i = 0; $i < $size; $i++) print $x[$i]. = (4, 7, 3, 5, 8, 9); # array $p = \@x; # reference $size for ($i = 0; $i < $size; $i++) print $$p[$i]. = ("apple", "orange", "banana"); # array $p = \@x; # reference foreach (@$p) print $_. = qw(apple orange banana); print " "; $p = \@x; print "\$p[0]: ", $$p[0], " "; print "\$p[1]: ", $$p[1], " "; print "\$p[2]: ", $$p[2], " "; 270

$@x = qw(apple orange banana); print @x; print " "; $p = \@x; print $p->[0], " "; print $p->[1], " "; print $p->[2], " "; 2. Test the program. A sample output looks: 3.$

20 @x = qw(apple orange banana); print " "; $p = \@x; print $p->[0], " "; print $p->[1], " "; print $p->[2], " "; 2. Test the program. A sample output looks: 3. Capture a screen shot similar to the above figure and paste it to a Word document named lab9.doc (or lab9.docx). Learning Activity #3: 1. In the X:\xampp\htdocs\myperl directory, use Notepad to create a new script file named lab9_3.pl with the following = (4, 7, 3, 5, 8, 9); # array $p = \@x; # reference = ("apple", "orange", "banana"); # array $p = \@x; # reference = ( [1, 2, 3, 4], ["one", "two", "three", "four"], ['ichi', 'nii', 'san', 'shi'], ); $s 271

21 for ($i = 0; $i < $s; $i++) $s1 for ($j = 0; $j < $s1; $j++) print $x[$i][$j]. = ( [1, 2, 3, 4], ["one", "two", "three", "four"], ['ichi', 'nii', 'san', 'shi'], ); $p = \@x; $s for ($i = 0; $i < $s; $i++) $s1 for ($j = 0; $j < $s1; $j++) print $$p[$i][$j]. " "; 2. Test the program. A sample output looks: 3. Capture a screen shot similar to the above figure and paste it to a Word document named lab9.doc (or lab9.docx). Learning Activity #4: 1. In the X:\xampp\htdocs\myperl directory, use Notepad to create a new script file named lab9_4.pl with the following lines: 272

22 #hash %month = ( "01", "Jan", "02", "Feb", "03", "Mar", "04", "Apr", "05", "May", "06", "Jun", "07", "Jul", "08", "Aug", "09", "Sep", "10", "Oct", "11", "Nov", "12", "Dec"); $p = \%month; # reference foreach (keys %$p) print $_. " : ". $$p$_. " "; %x = ("CA", "California", "PA", "Pennsylvania", "WA", "Washington"); #hash $p = \%x; # reference print %$p, " "; %x = (TX=>"Texas", MI=>"Michigan", OH=>"Ohio"); # hash $p = \%x; # reference print %$p, " "; sub findmax if ($_[0] >= $_[1]) return $_[0]; else return $_[i]; ; $p = \&findmax; print &$p(9, 3). " ". $p->(6, 4); sub add_number $sum = 0; foreach (@_) $sum += $_; 273

$doc (or lab9.docx). Learning Activity #5: 1. In the X:\xampp\htdocs\myperl directory, use Notepad to create a new script file named lab9_5.$

23 return = (4, 17, 14, 23); $p = add_number(@n); 2. Test the program. A sample output looks: 3. Capture a screen shot similar to the above figure and paste it to a Word document named lab9.doc (or lab9.docx). Learning Activity #5: 1. In the X:\xampp\htdocs\myperl directory, use Notepad to create a new script file named lab9_5.pl with the following lines: sub mysub # subroutine print "This is a subroutine."; ; $p1 = \&mysub; #variable $x = "This is a variable."; $p2 = = ("apple", "orange", "grape"); #array $p3 = \@y; %z = (CA=>"California", PA=>"Pennsylvania"); #hash 274

$$p4 =\%z; print ref($p1). " ". ref($p2). " ". ref($p3). " ". ref($p4); 2. Test the program. A sample output looks: 3.$ zip file named lab9.zip containing ONLY the following script files. lab9_1.pl lab9_2.pl lab9_3.pl lab9_4.pl lab9_5.pl lab9.doc (or.

zip file named lab9.zip containing ONLY the following script files. lab9_1.pl lab9_2.pl lab9_3.pl lab9_4.pl lab9_5.pl lab9.doc (or.

24 $p4 =\%z; print ref($p1). " ". ref($p2). " ". ref($p3). " ". ref($p4); 2. Test the program. A sample output looks: 3. Capture a screen shot similar to the above figure and paste it to a Word document named lab9.doc (or lab9.docx). Submittal 1. Complete all the 5 learning activities in this lab. 2. Create a.zip file named lab9.zip containing ONLY the following script files. lab9_1.pl lab9_2.pl lab9_3.pl lab9_4.pl lab9_5.pl lab9.doc (or.docx) [you may be given zero point if this word document is missing] 3. Upload the zipped file to Question 11 of Assignment 9 as the response. Programming Exercise #09 1. Use Notepad to create a new file named ex09.pl with the following lines in it (be sure to replace YourFullNameHere with the correct one): ## File name: ex09.pl ## Student: YourFullNameHere 2. Next to the above two lines, create an anonymous subroutine that can calculate the sum of n 2, where n is passed by a reference, $p. Additionally, create an HTML form that will let the user enter an integer. The integer is then processed by param() function as n to be passed to $p. A sample output looks: 3. Download the programming exercise template, and rename it to ex09.doc. Capture a screen shot similar to the above figure and paste it to a Word document named ex09.doc (or.docx). 4. Create a.zip file named ex09.zip with the following two files. Upload the.zip file for grading. ex09.pl ex09.doc (or.docx) [You may be given zero point if this Word document is missing] and 275

25 Grading Criteria 1. You code must fully comply with the requirement to earn full credits. No partial credit is given. 276

#!"X:\xampp\perl\bin\perl.exe" use CGI qw(:standard); print header, start_html;

$#!X:\xampp\perl\bin\perl.exe use CGI qw(:standard); print header, start_html;$ Lecture #8 Introduction Declaring a subroutine Subroutines In terms of programming, a subroutine is a sequence of program instructions that perform a specific task, packaged as a unit. This unit can then