Perl Basics Learning Objectives: 1. To understand the commands available for control flow in Perl 2. To learn some useful operators in Perl

COMP111 Lecture 10 / Slide 2 Perl Basics Table of Content  Control flow  “if” statement  “if … else “ statement  “if … elsif … else “ statement  Relational Operator  Truth in Perl  And, Or, Not  “while” statement  Looping using for  “last” command  String Operator  Variable Interpolation  Exponentiation  Operator Precedence

COMP111 Lecture 10 / Slide 3 Control Flow  Perl has several control flow statements:  if  while  for  unless  until  do while  do until  foreach

COMP111 Lecture 10 / Slide 4 “ if “ statement  The Perl if statement works almost the same as in C++: #!/usr/local/bin/perl5 -w $user = `whoami`; chomp($user); if($user eq "clinton"){ print "Hi Bill!\n"; }  The eq operator compares two strings, and returns true if they are equal (use == for numeric comparisons).  The curly braces { } are always required in Perl (even if only one statement inside, unlike C++). This avoids the “ dangling else ” problem.

COMP111 Lecture 10 / Slide 5 “ if … else “ statement  The if else statement is similar: #!/usr/local/bin/perl5 -w $user = `whoami`; chomp($user); if ($user eq "clinton") { print "Hi Bill!\n"; } else { print "Hi $user!\n"; }

COMP111 Lecture 10 / Slide 6 “ if … elsif … else “ statement  You can also handle a list of cases: #!/usr/local/bin/perl5 -w $users = `who | wc -l`; chomp($users); if ($users > 4){ print "Heavy load!\n"; } elsif ($users > 1){ print "Medium load\n"; } else { print "Just me!\n"; }

COMP111 Lecture 10 / Slide 7 Relational Operators  Perl ’ s numeric and string comparison operators: ComparisonNumericString Equal ==eq Not equal !=ne Less than <lt Greater than >gt Less than or equal to <=le Greater than or equal to >=ge

COMP111 Lecture 10 / Slide 8 Truth in Perl  Truth is flexible in Perl:  Expressions that evaluate to false 0# traditional false value ""# the null string "0"# only non-zero length false string  Some examples of truth: 1 # traditional true value 684 # non-zero numerical values are true " " # whitespace is true "hello" # strings are true "00" # a string

COMP111 Lecture 10 / Slide 9 And, Or, Not  1 represents true, and 0 false (as in C++).  You can also combine and negate expressions with logical and ( && ), logical or ( || ), and not ( ! ) just like in C++: #!/usr/local/bin/perl5 -w chomp($user = `whoami`); chomp($nme = `who | grep $user | wc -l`); chomp($nusers = `who | wc -l`); if($nusers - $nme && $user ne "clinton"){ print "Someone else is logged in!\n"; } else{ print "All is well!\n"; }

COMP111 Lecture 10 / Slide 10 “ while “ statement  The while statement loops indefinitely, while the condition is true, such as a user-controlled condition: #!/usr/local/bin/perl5 -w $resp = "no"; while($resp ne "yes"){ print "Wakeup [yes/no]? "; chomp($resp = ); } $ test11 Wakeup [yes/no]? no Wakeup [yes/no]? y Wakeup [yes/no]? yes $

COMP111 Lecture 10 / Slide 11 Looping using for (1)  for can be used as in C++ to do incrementing loops: $ cat fac #!/usr/local/bin/perl5 -w print "Enter number: "; $n = ; $fac = 1; for($i=1; $i<=$n; $i++){ $fac *= $i; } print "The factorial of $n is $fac\n"; $ fac Enter number: 5 The factorial of 5 is 120 $ Don’t forget to chomp $n

COMP111 Lecture 10 / Slide 12  With chomp(): $ cat fac #!/usr/local/bin/perl5 -w print "Enter number: "; chomp($n = ); $fac = 1; for($i=1; $i<=$n; $i++){ $fac *= $i; } print "The factorial of $n is $fac\n"; $ fac Enter number: 5 The factorial of 5 is 120 $ Looping using for (2)

COMP111 Lecture 10 / Slide 13 “last “ command  The last command works like the C++ break command, breaking out of the innermost loop : $ cat test12 #!/usr/local/bin/perl5 -w while(1){ print "Wakeup [yes/no]? "; chomp($resp = ); if($resp eq "yes"){ last; } $ test12 Wakeup [yes/no]? no Wakeup [yes/no]? y Wakeup [yes/no]? yes $

COMP111 Lecture 10 / Slide 14 String Operators (1)  Concatenate strings with the “. ” operator (a period). $ cat string #!/usr/local/bin/perl5 -w $name = "Bill". "Clinton"; print "$name\n"; print "Bill"."Gates"."\n"; $ string BillClinton BillGates $

COMP111 Lecture 10 / Slide 15 String Operators (2)  The string repetition operator x allows you to repeat a string several times: $ cat string1 #!/usr/local/bin/perl5 -w $name = "Bill"x3; print "$name\n"; $n = 4; print "Bill" x 2. "Gates" x $n. "\n"; print 5; print "\n"; $test = ($n+1) x 4; print "$test\n"; $ string2 BillBillBill BillBillGatesGatesGatesGates $

COMP111 Lecture 10 / Slide 16 Variable Interpolation (1)  Putting variables inside double quotes is called variable interpolation. We have seen many examples of this.  The variable name will be the longest possible variable name that makes sense at that part of the string.  Enclose the variable in a pair of curly braces if needed to override this.

COMP111 Lecture 10 / Slide 17 Variable Interpolation (2) $ cat bill1 #!/usr/local/bin/perl5 -w $bill = "trouble"; $billgates = "cheap"; print "Bill is $bill\n"; print "Bill is $billgates\n"; print "Bill is ${bill}gates\n"; print "Bill is "."$bill\n"; print "Bill is "."$bill"."\n"; $ bill1 Bill is trouble Bill is cheap Bill is troublegates Bill is trouble $

COMP111 Lecture 10 / Slide 18 Exponentiation  Perl has an exponentiation operator ** unlike C++: $ cat exp #!/usr/local/bin/perl5 -w $n = 2; $m = 3; $result = $n ** $m; print "$n raised to the $m power is $result\n"; $ exp 2 raised to the 3 power is 8 $

COMP111 Lecture 10 / Slide 19 Operator Precedence  Operator precedence is basically the same as in C++.  As in C++, you can use parentheses to override precedence, and to clarify the grouping. $ cat prec #!/usr/local/bin/perl5 -w $n = 2; $m = 3; $result = $n + 1 * $m; print "$n plus one times $m is $result\n"; $result = ($n + 1) * $m; print "$n plus one times $m is $result\n"; $ prec 2 plus one times 3 is 5 $ prec 2 plus one times 3 is 9