1. More Perl Control Flow Learning Objectives: 1. To learn more commands for control flow 2. To apply the new commands learnt for writing tidier program 3. To understand the limitation of using some control flow statements

2. COMP111 Lecture 12 / Slide 2 Perl Arrays and Lists Table of Content  Control Flow  Unless  Until  do … while  do … until  foreach  foreach $_  “last” again  Next  Redo  Next & redo  Redo & last/next  Labeled Blocks  Labeled Blocks Examples  Backward if  Backward unless, while, until  && if  || unless

3. COMP111 Lecture 12 / Slide 3 Control Flow  We have already seen several Perl control flow statements:  if  while  for  last  Other control flow statements:  unless  until  do while  do until  foreach  next  redo

4. COMP111 Lecture 12 / Slide 4 “ unless “ statement (1)  The Perl unless statement is like an if statement with the condition negated: if($temperature <= 20){ print "too cold!\n"; } unless($temperature > 20){# same thing print "too cold!\n"; } ------------------------------------------- if(!$hot){ print "too cold!\n"; } unless($hot){# same thing print "too cold!\n"; }

5. COMP111 Lecture 12 / Slide 5  unless can have else, just like if : #!/usr/local/bin/perl5 -w print "Enter temperature: "; chomp($temperature = ); unless($temperature > 20){ print "too cold!\n"; }else{ print "too hot!\n"; } “ unless “ statement (2)

6. COMP111 Lecture 12 / Slide 6  unless can also have elsif, just like if : #!/usr/local/bin/perl5 -w print "Enter temperature: "; chomp($temperature = ); unless($temperature >= 20){ print "too cold!\n"; }elsif($temperature == 20){ print "ok!\n"; }else{ print "too hot!\n"; } “ unless “ statement (3)

7. COMP111 Lecture 12 / Slide 7  The Perl until statement is like an while statement with the condition negated.  Sometimes is is easier to say “ until something is true ” rather than “ while not this is true ” : while(!$endoffile){... } until($endoffile){# same thing... } “ until “ statement (1)

8. COMP111 Lecture 12 / Slide 8  The until statement loops indefinitely, until the condition is true, such as a user-controlled condition: #!/usr/local/bin/perl5 -w $resp = "no"; until($resp eq "yes"){ print "Wakeup [yes/no]? "; chomp($resp = ); } $ test11 Wakeup [yes/no]? no Wakeup [yes/no]? y Wakeup [yes/no]? yes $ “ until “ statement (2)

9. COMP111 Lecture 12 / Slide 9  The do while statement is like the C++ do while statement.  It loops indefinitely, while the condition is true, such as a user- controlled condition:  do while always executes the body of the loop at least once. #!/usr/local/bin/perl5 -w do{ print "Wakeup [yes/no]? "; chomp($resp = ); }while($resp ne "yes"); $ test11 Wakeup [yes/no]? no Wakeup [yes/no]? yes $ “ do … while “ statement (1)

10. COMP111 Lecture 12 / Slide 10  The do until statement loops indefinitely, until the condition is true, such as a user-controlled condition.  do until always executes the body of the loop at least once. #!/usr/local/bin/perl5 -w do{ print "Wakeup [yes/no]? "; chomp($resp = ); }until($resp eq "yes"); $ test11 Wakeup [yes/no]? no Wakeup [yes/no]? y Wakeup [yes/no]? yes $ “ do … until “ statement (1)

11. COMP111 Lecture 12 / Slide 11  foreach takes a list of values and assigns them one by one to a scalar variable.  The body of the loops is executed once for each successive assignment.  foreach is similar to the shell programming ’ s for statement. foreach $i (@some_list){... } “ foreach “ statement (1)

12. COMP111 Lecture 12 / Slide 12  The following example sums the contents of an array: $ cat sum #!/usr/local/bin/perl5 -w @a = (21,32,3,44,75,16,19); $sum = 0; foreach $b (@a){ $sum += $b; } print "The array sum is: $sum\n"; $ sum The array sum is: 210 $ “ foreach “ statement (2)

13. COMP111 Lecture 12 / Slide 13  foreach allows us to easily print an array in our own customized way.  The following example prints an array with each element separated by 2 spaces: $ cat print1 #!/usr/local/bin/perl5 -w @a = (1,2,3,4,5); foreach $i (@a){ print "$i "; } print "\n"; $ print1 1 2 3 4 5 $ “ foreach “ statement (3)

14. COMP111 Lecture 12 / Slide 14  The following example prints the numbers in reverse order without changing the array: $ cat print2 #!/usr/local/bin/perl5 -w @a = (1,2,3,4,5); foreach $i (reverse @a){ print "$i "; } print "\n"; $ print2 5 4 3 2 1 $  reverse @a is the same as writing reverse(@a). Parenthesis are always optional on Perl functions. “ foreach “ statement (4)

15. COMP111 Lecture 12 / Slide 15  If you omit the scalar variable in foreach, Perl will use $_ automatically: $ cat print3 #!/usr/local/bin/perl5 -w @a = (1,2,3,4,5); foreach (reverse @a){ print; } print "\n"; $ print3 54321 $  print (and other Perl functions) use $_ as the default if nothing is specified. “ foreach “ statement (5)

16. COMP111 Lecture 12 / Slide 16  Of course, if you want double spaces, you will have to use $_ explicitly: $ cat print3a #!/usr/local/bin/perl5 -w @a = (1,2,3,4,5); foreach (reverse @a){ print "$_ "; } print "\n"; $ print3a 5 4 3 2 1 $ “ foreach “ statement (6)

17. COMP111 Lecture 12 / Slide 17  The scalar variable in foreach is an alias for each variable in the list, not a copy. (Tricky!)  If you modify the scalar variable in foreach, the aliased element in the list is also changed: $ cat double #!/usr/local/bin/perl5 -w @a = (1,2,3,4); foreach $b (@a){ $b *= 2; } print "@a\n"; $ double 2 4 6 8 $ “ foreach “ statement (7)

18. COMP111 Lecture 12 / Slide 18 Backward “ if ” (1)  A simple way to write “ if this, then that ” is: chomp($user = `whoami`); print("Hi Bill!\n") if($user eq "gates"); is the same as: chomp($user = `whoami`); if($user eq "gates"){ print "Hi Bill!\n"; }  Backward if avoids having to write the curly braces { }.  There can only be one statement inside the block.

19. COMP111 Lecture 12 / Slide 19  Backward if is a natural and tidy way to exit from a loop: $ cat backif #!/usr/local/bin/perl5 -w while(1){ print "Wakeup [yes/no]? "; chomp($resp = ); last if $resp eq "yes"; }; $ backif Wakeup [yes/no]? no Wakeup [yes/no]? y Wakeup [yes/no]? yes $ Backward “ if ” (2)

20. COMP111 Lecture 12 / Slide 20  Another simple way to write “ if this, then that ” is: chomp($user = `whoami`); $user eq "gates" && print("Hi Bill!\n"); is the same as: chomp($user = `whoami`); if($user eq "gates"){ print("Hi Bill!\n"); } && “ if ” (1)

21. COMP111 Lecture 12 / Slide 21 this && that;  Why does this work?  Isn ’ t && the logical-and operator?  Consider what happens when this and that take on values of true and false:  If this is true, then the value of the entire expression is still not known, because it depends on the value of that. So that has to be evaluated.  If this is false, there is no need to look at that, because the value of the whole expression must be false. Since there is no need to evaluate that, Perl skips it. && “ if ” (2)

22. COMP111 Lecture 12 / Slide 22  && if is also a tidy way to exit from a loop: $ cat backif1 #!/usr/local/bin/perl5 -w while(1){ print "Wakeup [yes/no]? "; chomp($resp = ); $resp eq "yes" && last; }; $ backif1 Wakeup [yes/no]? no Wakeup [yes/no]? y Wakeup [yes/no]? yes $ && “ if ” (3)