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Perl Control Flow

Perl Control Flow. Learning Objectives: To understand the commands available for control flow in Perl To learn some useful operators in Perl. “ if “ statement. The Perl if statement works almost the same as in C++: #!/usr/local/bin/perl5 -w $user = `whoami`; chomp($user);

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Perl Control Flow

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

  2. “ 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"; } … if (-e “datafile”) {…} # if file exists… • 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.

  3. “ 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"; }

  4. Relational Operators • Perl’s numeric and string comparison operators: Comparison Numeric String Equal == eq Not equal != ne Less than < lt Greater than > gt Less than or equal to <= le Greater than or equal to >= ge

  5. 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

  6. And, Or, Not • 1represents 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"; }

  7. “ while“ statement • The whilestatement 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 = <STDIN>); } $ test11 Wakeup [yes/no]? no Wakeup [yes/no]? y Wakeup [yes/no]? yes $

  8. 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 = <STDIN>; $fac = 1; for($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

  9. Looping using for (2) • Withchomp(): $ cat fac #!/usr/local/bin/perl5 -w print "Enter number: "; chomp($n = <STDIN>); $fac = 1; for($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 $

  10. “last “ command • The lastcommand 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($resp = <STDIN>); if($resp eq "yes"){ last; } } $ test12 Wakeup [yes/no]? no Wakeup [yes/no]? y Wakeup [yes/no]? yes $

  11. 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 $

  12. 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 5 5555 $

  13. 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.

  14. 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 Bill is trouble $

  15. 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 $

  16. 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

  17. Write a Program to compute the Sum #!/usr/local/bin/perl5 –w $sum=0; While (...) { ...

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