Subroutines.Pdf

Subroutines • aka: user-defined functions, methods, Subroutines procdures, sub-procedures, etc etc etc • We’ll just say Subroutines. – “Functions” generally means built-in functions • We’ll attempt to start out most basic, and work our way up to complicated. The Basics Parameters sub myfunc{ • (aka Arguments, inputs, etc) print “Hey, I’m in a function!\n”; • Can call any subroutine with any number of parameters. } • Get passed in via local @_ variable. … sub myfunc{ myfunc( ); foreach $word (@_){ • Because function already declared, ( ) are optional print “$word ”; myfunc; (ie, can just say ) } • Can declare without defining: $foobar = 82 – sub myfunc; myfunc “hello”, “world”, $foobar; – Make sure you define it eventually…. • prints “hello world 82” • official name of subroutine is &myfunc – ampersand not normally necessary to call it Passing current parameters Squashing array parameters • Can call a function with the current value of • If arrays or hashes passed into a subroutine, @_ as the parameter list by using &. they get ‘squashed’ into one flat array: @_ • &myfunc; @a = (1, 2, 3); @b=(8, 9, 10); – myfunc’s @_ is alias to current @_ myfunc (@a, @b); • same as saying myfunc(@_); • inside myfunc, @_ = (1, 2, 3, 8, 9, 10); – it’s faster internally… • Maybe this is what you want. – if not, need to use references… 1 References in Parameters Return values • In Perl, subroutines return last expression evaluated. • To pass arrays (or hashes), and not squash them: sub count { sub myfunc{ $sum = $_[0] + $_[1]; } ($ref1, $ref2) = @_; $total = count(4, 5); @x = @$ref1; @y = @$ref2; #$total = 9 … • Standard practice is to use return keyword } sub myfunc{ … @a = (1, 2, 3); @b = (8, 9, 10); return $retval; myfunc (\@a, \@b); } Return issues Scalar vs List Returns • Can return values in list or scalar context. • wantarray function sub toupper{ – Built-in function in Perl. – If subroutine called in list context, return true (1) @params = @_; – If subroutine called in scalar context, return false (“”) foreach (@params) {tr/a-z/A-Z/;} – If subroutine called in void context, return undef. return @params; • Perhaps we want to return entire list, or first element if } called in scalar context: @uppers = toupper ($word1, $word2); sub fctn{ $upper = toupper($word1, $word2); … #$upper gets size of @params return wantarray ? @params : $params[0]; } Anonymous functions Scoping • Can declare a function without giving it a name. • Up to now, we’ve used global variables • call it by storing it’s return value in definition exclusively. – $subref = sub { print “Hello\n”; }; • Perl has two ways of creating local • to call, de-reference the return value: variables – &$subref; – local and my • works with parameters too.. • what you may think of as local (from – &$subref($param1, $param2); C/C++) is really achieved via my. 2 my lexical variables • my creates a new variable lexically scoped to inner • Variables declared with my are called “lexical most block variables” or “lexicals” – block may be subroutine, loop, or bare { } • Not only are they not visible outside block, • variables created with my are not accessible (or even mask globals with same name: visible) to anything outside scope. $foo = 10; sub fctn{ { my $x = shift(@_); my $foo = 3; … print $foo; #prints 3 } } print $x; #ERROR!!! print $foo; #prints 10 local Where’s the scope • subroutines declared within a lexical’s scope have • local does not create new variable access to that lexical • instead, assigns temporary value to existing (global) variable – this is one way of implementing static variables in Perl • has dynamic scope, rather than lexical { • functions called from within scope of local variable get the my $num = 20; temporary value sub add_to_num { $num++ } sub fctn { print “a = $a, b = $b\n”; }; $a = 10; $b = 20; sub print_num { print “num = num\n”; } { local $a = 1; } my $b = 2; add_to_num; fctn(); print_num; } print $num; #ERROR! #prints a = 1, b = 20 What to know about scope Prototypes • my is statically (lexically) scoped • Perl’s way of letting you limit how you’ll – look at code. whatever block encloses my is the allow your subroutine to be called. scope of the variable • when defining the function, give it the ‘type’ • local is dynamically scoped of variable you want it to take: – scope is enclosing block, plus subroutines called • sub f1 ($$) {…} from within that block – f1 must take two scalars • Almost always want my instead of local • sub f2($@) {…} – notable exception: cannot create lexical variables – f2 takes a scalar, followed by a list such as $_. Only ‘normal’, alpha-numeric variables • sub f3(\@$) {…} – for built-in variables, localize them. – f3 takes an actual array, followed by a scalar 3 Prototype generalities Prototype conversions if prototype char is: Perl expects: \$ actual scalar variable • sub fctn($$) { … } \@ actual array variable • fctn(@foo, $bar) \% actual hash variable • Perl converts @foo to scalar (ie, takes its $ scalar size), and passes that into the function @ array – ‘eats’ rest of params and • sub fctn2(\@$) {…} force list context % hash – ‘eats’ rest of params and • fctn2(@foo, $bar) forces hash context • Perl automatically creates reference to * file handle @foo to pass as first member of @_ & subroutine (name or definition) Getting around parameters • If you want to ignore parameters, call subroutine with & character in front • sub myfunc (\$\$){ … } • myfunc (@array); #ERROR! • &myfunc (@array); #No error here 4.

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