More Perl Control Flow

1. Software Tools More Perl Control Flow

2. 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

3. unless • 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"; }

4. unless • unless can have else, just like if: #!/usr/local/bin/perl5 -w print "Enter temperature: "; chomp($temperature = <STDIN>); unless($temperature > 20){ print "too cold!\n"; }else{ print "too hot!\n"; }

5. unless • unless can also have elsif, just like if: #!/usr/local/bin/perl5 -w print "Enter temperature: "; chomp($temperature = <STDIN>); unless($temperature >= 20){ print "too cold!\n"; }elsif($temperature == 20){ print "ok!\n"; }else{ print "too hot!\n"; }

6. until • The Perl until statement is like a 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 ... }

7. until • The untilstatement 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 = <STDIN>); } $ test11 Wakeup [yes/no]? no Wakeup [yes/no]? y Wakeup [yes/no]? yes $

8. do while • The do whilestatement 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($resp = <STDIN>); }while($resp ne "yes"); $ test11 Wakeup [yes/no]? no Wakeup [yes/no]? yes $

9. do until • The do untilstatement 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($resp = <STDIN>); }until($resp eq "yes"); $ test11 Wakeup [yes/no]? no Wakeup [yes/no]? y Wakeup [yes/no]? yes $

10. foreach • foreach takes a list of values and assigns them one by one to a scalar variable. • The body of the loop is executed once for each successive assignment. • foreach is similar to the shell programming’s for statement. foreach $i (@some_list){ ... }

11. foreach • 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 $

12. foreach • 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 $

13. foreach • 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.

14. foreach $_ • 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.

15. foreach $_ • 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 $

16. foreach • The scalar variable in foreach is an alias for each variable in the list, not just 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 $

17. lastAgain • The lastcommand only breaks out of while, until, for, and foreach loops. • For mysterious reasons, do while and do until do not count as loops in Perl. $ cat last1 #!/usr/local/bin/perl5 -w do{ print "Wakeup [yes/no]? "; chomp($resp = <STDIN>); if($resp eq "yes"){ last; } }while(1); $ last1 Wakeup [yes/no]? yes Can't "last" outside a block at last1 line 6, <STDIN> chunk 1. $

18. lastAgain • In the example below, the last statement causes the outer while loop to be broken: $ cat last2 #!/usr/local/bin/perl5 -w while(1){ do{ print "Wakeup [yes/no]? "; chomp($resp = <STDIN>); if($resp eq "yes"){ last; } }while(1); } $ last2 Wakeup [yes/no]? yes $

19. next • The nextcommand causes a jump to the end of the loop, but without terminating the loop. $ cat next1 #!/usr/local/bin/perl5 -w $resp = "no"; until($resp eq "quit" || $resp eq "yes"){ print "Wakeup [yes/no/quit]? "; chomp($resp = <STDIN>); if($resp eq "yes"){ next; } print "sleeping...\n"; } $ next1 Wakeup [yes/no/quit]? no sleeping... Wakeup [yes/no/quit]? yes $ next1 Wakeup [yes/no/quit]? quit sleeping... $

20. redo • The redocommand causes a jump back to the beginning of the loop, but without re-testing the condition (next re-tests the condition). $ cat redo #!/usr/local/bin/perl5 -w $resp = "no"; until($resp eq "quit" || $resp eq "yes"){ print "Wakeup [yes/no/quit]? "; chomp($resp = <STDIN>); if($resp eq "yes"){ redo; } print "sleeping...\n"; } $ redo Wakeup [yes/no/quit]? no sleeping... Wakeup [yes/no/quit]? yes Wakeup [yes/no/quit]? quit sleeping... $

21. nextandredo • Like last, nextand redoonly break out of while, until, for, and foreach loops (not do while and do until loops). $ cat redo1 #!/usr/local/bin/perl5 -w $resp = "no"; do{ print "Wakeup [yes/no/quit]? "; chomp($resp = <STDIN>); if($resp eq "yes"){ redo; } print "sleeping...\n"; }until($resp eq "quit" || $resp eq "yes"); $ redo1 Wakeup [yes/no/quit]? no sleeping... Wakeup [yes/no/quit]? yes Can't "redo" outside a block at redo1 line 7, <STDIN> chunk 2. $

22. redoandlast/next • You can make a whileloop using redo and last/next in a “naked” block (a “naked” block is a block { } that is not part of a loop or if statement): $ cat redo2 #!/usr/local/bin/perl5 -w { print "Wakeup [yes/no]? "; chomp($resp = <STDIN>); if($resp eq "yes"){ last; # could also use "next;" } redo; } $ redo2 Wakeup [yes/no]? no Wakeup [yes/no]? y Wakeup [yes/no]? yes $

23. Labeled Blocks • What if you want to jump out of two nested loops? • In C++, you’d be in trouble. • In Perl, you can use labeled loops to break out of an outer loop using last, next, and redo. • For clarity (and other reasons), it is best to choose label names that are all upper case letters and digits. • Labels always end in a colon “:”. OUTERLOOP: for($i=1; $i<3; $i++){ INNERLOOP: for($j=1; $j<5; $j++){ ... • Add the label (without the colon) as a parameter to last, for example, if you want to exit OUTERLOOP. last OUTERLOOP;

24. Labeled Blocks Example $ cat label #!/usr/local/bin/perl5 -w $sum = 0; LOOP1: for($i1=1; $i1<=2; $i1++){ LOOP2: for($i2=1; $i2<=2; $i2++){ LOOP3: for($i3=1; $i3<=2; $i3++){ $sum += 1; print "sum so far: $sum\n"; if($sum > 2){ last LOOP2; } } $sum += 2; } $sum += 3; } print "sum at end: $sum\n"; $ label sum so far: 1 sum so far: 2 sum so far: 5 sum so far: 9 sum at end: 12 $

25. Backwardif • 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.

26. Backwardif • 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 = <STDIN>); last if $resp eq "yes"; }; $ backif Wakeup [yes/no]? no Wakeup [yes/no]? y Wakeup [yes/no]? yes $

27. Backwardunless, while, until • You can also use backward unless, while, and until (if there is only one statement in the block): $ cat sum #!/usr/local/bin/perl5 -w print "Enter numbers to sum (0 to quit): \n"; $sum = 0; $n = 1; $sum += $n = <STDIN> while($n != 0); print "The sum is: $sum: \n"; $ sum Enter numbers to sum (0 to quit): 1 1 0 The sum is: 2 $

28. && if • 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"); }

29. && if 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.

30. && if • &&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 = <STDIN>); $resp eq "yes" && last; }; $ backif1 Wakeup [yes/no]? no Wakeup [yes/no]? y Wakeup [yes/no]? yes $

31. || unless • Similarly, another simple way to write an unless statement is: $temperature > 20 || print "too cold!\n"; is the same as: unless($temperature > 20){ # same thing print "too cold!\n"; }