Chapter 5 List Variables and Loops

Chapter 5 List Variables and Loops

Objectives • Describe how list variables work • Describe four types of Perl looping statements • Explain how logical conditional operators are used

Lists Advantages • Using list 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 list 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.

Creating List Variables • Suppose wanted to create a list variable to hold 4 student names: • Creates list variable @students with values ‘Johnson’, ‘Jones’, ‘Jackson’, and ‘Jefferson’

Creating List Variables Of Scalars • Suppose wanted to create a list variable to hold 4 student grades (numerical values): @grades = ( 66, 75, 85, 80 ); • Creates list variable @grades with values 66, 75, 85, 80.

Referencing List Items • Items within a list variable are referenced by a set of related scalar variables • For example, $students[0],$students[1],$students[2], and $students[3] • Reference in a variable name/subscript pair:

Referencing List Items - II • Subscripts can be whole numbers, another variable, or even expressions enclosed within the square brackets. • Consider the following example: $i=3; @preferences = (“ketchup ”, “mustard ”, “pickles ”, “lettuce ” ); print “$preferences[$i] $preferences[$i-1] $preferences[$i-2] $preferences[0]”; • Outputs the list in reverse order: lettuce pickles mustard ketchup

Creating List Variables Of Scalars • Suppose wanted to create a list variable to hold 4 student grades (numerical values): @grades = ( 66, 75, 85, 80 ); • Creates list variable @grades with values 66, 75, 85, 80.

Changing Items In A List Variable • Change values in a list variable and use them in expressions like other scalar variables. For example: • @scores = ( 75, 65, 85, 90); • $scores[3] = 95; • $average = ( $scores[0] + $scores[1] + • $scores[2] + $scores[3] ) / 4; • The third line sets $average equal to (75 + 65 + 85 + 95 ) / 4—that is, to 80.

A Complete List Example Program 1. #!/usr/bin/perl 2. use CGI ':standard'; 3. print header, start_html('Menu Choice'); 4. @menu = ('Meat Loaf', 'Meat Pie', 'Minced Meat', 'Meat Surprise' ); 5. $item = int(rand(4)); 6. print '<FORM ACTION="http://perl-pgm.com/cgi-bin/C5/choice.cgi" METHOD="POST">'; 7. print "<FONT SIZE=4 > What do you want to eat for dinner? "; 8. print br, "Our special is <FONT COLOR=BLUE> $menu[$item] </FONT>"; 9. print br, '<INPUT TYPE="RADIO" NAME="eat" VALUE="0">', "$menu[0]"; 10. print br, '<INPUT TYPE="RADIO" NAME="eat" VALUE="1" checked>', "$menu[1]"; 11. print br, '<INPUT TYPE="RADIO" NAME="eat" VALUE="2">', "$menu[2]"; 12. print br, '<INPUT TYPE="RADIO" NAME="eat" VALUE="3">', "$menu[3]"; 13. print br, '<INPUT TYPE=SUBMIT VALUE="Submit">'; 14.print '<INPUT TYPE=RESET></FORM>', end_html

Would Output The Following ...

Outputting the Entire List Variable • Output all of the elements of a list variable by using the list variable with print • For example, @workWeek = ('Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday' ); print "My work week is @workWeek"; • Would output the following: • My work week is Monday Tuesday Wednesday Thursday Friday

Getting the Number in a List Variable • Use Range operator to find last element of list • For example: @grades = ( 66, 75, 85, 80 ); $last_one = $grades[$#grades];

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). @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.

Using Range Operator for list length 1. #!/usr/bin/perl 2. use CGI ':standard'; 3. print header, start_html("Got Your Preference"); 4. @menu = ("Meat Loaf", "Meat Pie", "Minced Meat", "Meat Surprise" ); 5. $response = param('eat'); 6. print "<FONT size=4> You Selected @menu[$response]"; 7. $menuLen = $#menu + 1; 8. print br, "We had $menuLen items available"; 9. print br, "These items were @menu", end_html;

A Better Way to Get List Length • You can also find the length of a list variable by assigning the list variable name to a scalar variable. • For example, the following code assigns to $size the number of elements in the list variable @grades: • $size=@grades;

Adding and Removing List Items • shift()andunshift(): add/remove elements from the beginning of a list. • shift() removes an item from the beginning of a list. For example, @workWeek = (‘Monday’, ‘Tuesday’, ‘Wednesday’,‘Thursday’, ‘Friday’ ); $dayOff = shift(@workWeek); print "dayOff= $dayOff workWeek=@workWeek"; • Would output the following: dayOff= Monday workWeek=Tuesday Wednesday Thursday Friday

Adding and Removing List Items • unshift() adds an element to the beginning of the list For example, @workWeek = (‘Monday’, ’Tuesday’, ’Wednesday’, ’Thursday’, ’Friday’ ); unshift(@workWeek, “Sunday”); print “workWeek is now =@workWeek”; • would output the following: workWeek is now =Sunday Monday Tuesday Wednesday Thursday Friday

Adding and Removing List Items • pop()and push(): add/remove elements from the end of a list. • shift() removes an item from the end of a list. For example, @workWeek = (“Monday”, “Tuesday”, “Wednesday”, “Thursday”, “Friday” ); $dayOff = pop(@workWeek); • Would output the following: dayOff= Friday workWeek=Monday Tuesday Wednesday Thursday

Adding and Removing List Items • push() adds an element to the end of the list For example, @workWeek = (‘Monday’, ‘Tuesday’, ‘Wednesday’, ‘Thursday’, ‘Friday’ ); push(@workWeek, ‘Saturday’); print “workWeek is now =@workWeek”; • would output the following: workWeek is now =Monday Tuesday Wednesday Thursday Friday Saturday

Extracting Multiple List Values • So for we have accessed one item at a time from a list variable. For example, @myList = ( 'hot dogs’, 'ketchup', 'lettuce', 'celery'); $essentials = @myList[ 2 ]; print "essentials=$essentials"; • would output: essentials=lettuce

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, @myList = ( 'hot dogs’, 'ketchup', 'lettuce', 'celery'); @essentials = @myList[ 2, 3 ]; print "essentials=@essentials"; • The output of this code is essentials=lettuce celery

Extracting Multiple List Values • In a similar fashion, you can use a list variableas an index into another list variable. • Consider the following code @myList = ( 'hot dogs’, 'ketchup', 'lettuce', 'celery'); @keyones = ( 0, 3 ); @essentials = @myList[ @keyones ]; print "essentials=@essentials"; • The output of this code is essentials=hot dogs celery • Useful with certain HTML form elements.

Example Multiple Arguments 1. #!/usr/bin/perl 2. use CGI ':standard'; 3. print header; 4. @menu = ('USA', 'China', 'Canada', 'Mexico' ); 5. print '<FORM ACTION="http://perl-pgm.com/cgi- bin/C5/multichoice.cgi” METHOD="POST">'; 6. print '<FONT SIZE=4 > What countries have you visited?'; 7. print '<INPUT TYPE="checkbox" NAME="places" VALUE="0">', “$menu[0]"; 8. print '<INPUT TYPE="checkbox" NAME="places" VALUE="1" checked>', "$menu[1]"; 9. print '<INPUT TYPE="checkbox" NAME="places" VALUE="2">', "$menu[2]"; 10. print '<INPUT TYPE="checkbox" NAME="places" VALUE="3">', "$menu[3]"; 11. print br, '<INPUT TYPE=SUBMIT VALUE="Submit">'; 12. print '<INPUT TYPE=RESET></FORM>', end_html;

Lists of Lists (or multidimensional lists) • Some data are best represented by a list of lists

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, $Inventory[0][0] points to AC1000, $Inventory[1][0] points to AC1001, and $Inventory[2][0] points to AC1002

A Partial Example ... @Inventory = ( [ 'AC1000', 'Hammer', 122, 12.50 ], [ 'AC1001', 'Wrench', 344, 5.50 ], [ 'AC1002', 'Hand Saw', 150, 10.00] ); $numHammers = $Inventory[0][2]; $firstPartNo = $Inventory[0][0]; $Inventory[0][3] = 15; print “$numHammers, $firstPartNo, $Inventory[0][3]”; • This would output 122, AC1000, 15

A Complete Example ... • Suppose the following HTML code creates radio buttons and sets CGI variable item (0-3) • The output is sent tohttp://perl-pgm.com/cgi-bin/C5/GetInvent.cgi. <BR><INPUT TYPE=radio NAME="item" VALUE=0 >Hammers <BR><INPUT TYPE=radio NAME="item" VALUE=1 >Wrenches <BR><INPUT TYPE=radio NAME="item" VALUE=2 >Hand Saws <BR><INPUT TYPE=radio NAME="item" VALUE=3 >Screw Drivers

Receiving CGI/Perl Program 1. #!/usr/bin/perl 2. use CGI ':standard'; 3. print header, start_html('Inventory Answer'); 4. # Inventory: PartNO, Item, Num In Stock, Price 5. $PRTNO=0; $ITEM=1; $NUM=2; $PRICE=3; 6. @Inventory = ( [ 'AC1000', 'Hammers', 122, 12 ], [ 'AC1001', 'Wrenches', 344, 5 ], [ 'AC1002', 'Hand Saws', 150, 10], [ 'AC1003', 'Screw Drivers', 250, 2] ); 7. $pick=param('item'); 8. if ( $pick >= 0 && $pick <= 3 ) { 9. print "Yes we have $Inventory[$pick][$ITEM]."; 10. print br, "In fact we have $Inventory[$pick][$NUM] of them."; 11. print br, "They are $Inventory[$pick][$PRICE] dollars."; 12. } else { "print sorry we do not have that item" } 13. print end_html;

Looping Statements • Advantages of using loops: • Your programs can be much more concise. When similar sections of statements need to be repeated in your program, you can often put them into a loop and reduce the total number of lines of code required. • You can write more flexible programs. Loops allow you to repeat sections of your program until you reach the end of a data structure such as a list or a file (covered later).

Advantages of Using Loops

The Perl Looping Constructs • Perl supports four types of looping constructs: • The for loop • The foreach loop • The while loop • The until loop • Each type of loop is described below.

The for loop • You use the for loop to repeat a section of codea specified number of times. • (typically used when you know how many times to repeat)

3 Parts to the for Loop • The initialization expression defines the initial value of a variable used to control the loop. ($iis used above with value of 0). • The loop-ending condition defines the condition for termination of the loop. It is evaluated during each loop iteration. When false, the loop ends. (The loop above will repeat as long as $i is less than $max). • The iteration expression is evaluated at end of each loop iteration. (In above loop the expression $i++ means to add 1 to the value of $iduring each iteration of the loop. )

Consider the following program 1. #!/usr/bin/perl 2. use CGI ':standard'; 3. print header, start_html(‘For Loop’); 4. $start = param ('start'); 5. $end = param ( 'end'); 6. print '<TABLE BORDER=1><TH>Numb</TH><TH>Sqr</TH><TH>Cubed</TH>'; 7. for ( $i = $start; $i < $end; $i++ ) { 8. $i_sq=$i**2; 9. $i_cubed=$i**3; 10. print "<TR><TD> $i </TD> <TD> $i_sq </TD> <TD> $i_cubed </TR>"; 11. } 12. print "</TABLE>That is the end and i=$i"; 13. print end_html;

The foreach Loop • The foreach loop is typically used to repeat a set of statements for every element in a list. • If @items_array = (“A”, “B”, “C”); • Then $item would “A” then “B” then “C”.

A For Each Example Program 1. #!/usr/bin/perl 2. use CGI ':standard'; 3. print header, start_html(' A foreach Example'); 4. @secretNums = ( 3, 6, 9 ); 5. $uinput = param( 'guess' ); 6. $ctr=0; $found=0; 7. foreach $item ( @secretNums ) { 8. $ctr=$ctr+1; 9. if ( $item == $uinput ) { 10. print "Number $item. Item found was number $ctr<BR>"; 11. $found=1; 12.last; 13. } 14. } 15. if ( $found ) { 16. print "<BR> I checked $ctr item(s)."; 17. } else { 18. print "<BR>Your guess, $uinput was NOT FOUND! I checked $ctr item(s)."; 19. }

The while Loop • You use awhile loop to repeat a section of code as long as a test condition remains true.

Consider The Following ... 1. #!/usr/bin/perl 2. use CGI ':standard'; 3. print header, start_html(' A While Example'); 4. $upick=param('upick'); 5. $ctr = 1; 6. print ( "<FONT Size=4> Numb of times to find $upick " ); 7. print ( '<TABLE BORDER=1> <TH> Numb <TH> Rand ' ); 8. $rnum= int(rand(10)); 9. while ( $rnum != $upick ) { 10. print ("<TR> <TD> $ctr <TD> $rnum </TR>"); 11. $ctr = $ctr + 1; 12. $rnum= int(rand(10)); 13. } 14. print ("<TR> <TD> $ctr <TD> $rnum </TR>"); 15. print ('</TABLE>', br, "FOUND in $ctr times random=$rnum" );

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

Example Program 1. #!/usr/bin/perl 2. use CGI ':standard'; 3. print header, start_html(' A While Example'); 4. $upick=param('upick'); 5. $ctr = 1; 6. print ( "<FONT Size=4> Numb of times to find $upick " ); 7. print ( '<TABLE BORDER=1> <TH> Numb <TH> Rand ' ); 8. do { 9. $rnum= int(rand(10)); 10. print ("<TR> <TD> $ctr <TD> $rnum </TR>"); 11. $ctr = $ctr + 1; 12. } until ( $rnum == $upick ); 13. print ('</TABLE>', br, "FOUND in $ctr times random=$rnum" );

Creating Compound Conditionals • 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 > $max && $found ne ‘TRUE’ )

Some basic Conditional Operators • &&—the AND operator - Both tests must be true: while ( $ctr < $max && $flag == 0 ) { • ||—the OR operator. Either test is true. if ( $name eq “SAM” || $name eq “MITCH” ) { • !—the NOT operator. Condition is false. if ( !$FLAG == 0 )

Chapter 5 List Variables and Loops