Perl Intro 7 Subroutines
Download as PPT, PDF0 likes391 views
This document summarizes a presentation on Perl subroutines and closures. The presentation covers subroutine basics like defining and calling subs, passing arguments, and return values. It also discusses closures, including using static variables with anonymous subs to cache repeated function calls and enclosing lexical variables. Examples are provided for recursion, checking call context, and creating anonymous subs.
Perl Intro 7 Subroutines
- 1. Perl Brown Bag Subroutines Closures Shaun Griffith April 24, 2006 05/13/12 ATI Confidential 1
- 2. Agenda •Subs •Basics •Arguments •Return values •Closures •Static variables •Anonymous subs 05/13/12 2
- 3. Subs Basics sub my_sub { code goes here } # no semi-colon Calling my_sub( arg1, arg2, …); $catch = my_sub(); @stuff = my_sub(@args); Ambiguous my_sub @args; # must be defined earlier &my_sub; # current @_ available inside 05/13/12 3
- 4. Arguments Passing Args my_sub( “one”, $two, @three, %four ); Catching Args sub my_sub { my $x1 = shift @_; my $x2 = shift; # @_ is default my $x3 = shift; # reference my %x4 = @_; # all the rest … } With a long list, that’s a lot of work! 05/13/12 4
- 5. Try It And See™ Do this on your own machine: Save the file sub1.pl on you PC go to Start, Run, cmd (to get to DOS) cd to the directory with sub1.pl perl –d sub1.pl > s Now use <enter> to step through the program, and x $var to examine variables at key places. 05/13/12 5
- 6. Try It And See™… Now answer these questions: •Did you get “Odd number of elements…”? What does that mean? How did it happen? •What’s odd about %four in the second call to simple? •Why doesn’t print %four do what you expect? •Why isn’t smart all that good? •Is smarter better? •What happens if smarter gets a parameter it doesn’t know? What is it missing? 05/13/12 6
- 7. Recursion Factorial sub fac { my $x = shift; # exit conditions return if ( $x < 0 ); # bad input return 1 if ( ($x == 0) or ($x == 1)); my $y = $x * fac($x-1); return $y; } return $y could be written as just $y, since the last expression evaluated will be returned in the absence of an explicit return. 05/13/12 7
- 8. Return How many values should the sub return? 0: return; 1: return $x; list: return( $x, $y); # parens are better ref: return @array; Here’s a gotcha: Sometimes you want to signal an error or an empty result, so you might try returning undef: return undef; However, in list context, undef is a one-element list: if ( scalar( @x = ( undef ) ) )… This is always true!!! 05/13/12 8
- 9. Return with Context wantarray (should have been named “wantlist”): sub check_context { if (not defined wantarray) { print “void” } elsif ( wantarray ) { print “list” } else { print “scalar” } print “ contextn”; } (For a more complete mechanism, see the Want module.) 05/13/12 9
- 10. Closures Closures “enclose” lexical (my) variables. Recall sub fac – once fac(317)is computed, there’s no reason to compute it again, is there? One use of closures is for simple cache variables: { # closure for fac my %cache; # only visible to fac, but “static” sub fac { my $x = shift; return if ( $x < 0 ); # bad input return 1 if ( ($x == 0) or ($x == 1)); if ( not exists( $cache{$x} ) ) { $cache{$x} = $x * fac($x-1); } return $cache{$x}; } # end sub fac } # end closure for fac Closures must be defined above the first call! 05/13/12 10
- 11. Anonymous Subs Create a sub with no name (but save it as a coderef): my $two = 2; my $times_2 = sub { $two * shift }; $z = $times_2->(17); Note that $two is “captured” in the anonymous sub – as long as $times_2 exists, so will $two. The uglier syntax for this is: $z = &$times_2(17); # less clear & is the sub sigil, like $ is for scalars. 05/13/12 11
- 12. Next Time? Filehandles? •Open •Close •EOF •Pipes Command Line Arguments? •Catching •Checking •Using 05/13/12 12