#Function - grouping of code, called by name, do some work, return some value
#User-defined functions => subroutines in Perl

countdown();  #call function below
print "Blastoff!\n";

&countdown();  #call function below, different syntax
print "Blastoff!\n";

sub countdown{
	for ($i=10; $i>=0; $i--) {
		print "$i -";
	}
}

######################################################
#perl subroutine can be called anywhere in the program
sub world{
	print "World!\n";
}

sub hello{
	print "Hello, ";
	world(); #call subroutine above
}

hello();

######################################################
#returning values from subroutine
sub two_by_four{
	return 2*4;
	# 2*4; #strange, both works
}
print 8*two_by_four()."\n";

######################################################
#returning values from subroutine

sub x_greaterthan100 {
	#relies on the value of $x being set elsewhere
	if ($x>100) {
		return 1;	#value for true 
	}
	else {
		return 0;
	}
}
$x = 70;
if (x_greaterthan100()) {
	print "$x is greater than 100\n";
} else {
	print "$x is NOT greater than 100\n";
}

######################################################
#returning arrays/hashes from subroutine

sub shift_to_uppercase {
	@words = qw (cia fbi un nato unicef);
	foreach (@words) {
		$_ = uc($_);
	}
	return @words;
}
@acronyms = shift_to_uppercase();
print join(', ', @acronyms)."\n";  
#output a string, connect all element of array, with ","

######################################################
#arguments, access through special variable @_, an array

sub printargs {
	print join (", ", @_);
}
printargs('market', 'home' , 10, 20);
print "\n";

sub print_3rd_argument {
	print $_[2];  #third element of array
}
print_3rd_argument('market', 'home' , 10, 20);
print "\n";

sub display_box_score {
	($num_hits, $num_at_bats) = @_;
	print "For $num_at_bats trips to the plate, ";
	print "he's hitting ", $num_hits/$num_at_bats,"\n";
}
display_box_score(50, 210);

#passing hashes and arrays
sub sort_numerically {
	print "Sorting ...";
	return (sort {$a<=>$b} @_);  
	# {$a<=>$b} sort numerically ascending
	# refer to documentation for sort syntax
}
@items = (10, 12, 5, 15, 14);
@sorted_items = sort_numerically(@items);
print join(",", @sorted_items);
print "\n";
#note: passing 2 or more hashes or arrays does not work
#2 arrays are join to one array, not an array of arrays

#pass in scalar and array
sub updated_sort_numerically {
	($message, @myarray) = @_;
	print "$message ...";
	return (sort {$a<=>$b} @myarray);  
	# {$a<=>$b} sort numerically ascending
	# refer to documentation for sort syntax
}
@sorted_items = updated_sort_numerically("Welcome! ",@items);
print join(",", @sorted_items);
print "\n";

######################################################
#use "my" operator to delcare variables private to a subroutine
sub myfunc{
	my $x;
	$x=20;
	print "$x\n";  #this is a private $x
}
$x=10; 	#this is a global $x
print "$x\n";
myfunc();
print "$x\n";