Closures

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Revision as of 07:50, 13 June 2007 (view source) Rina (Talk | contribs) (New page: ==Closure And its Usage== my sub xx { my $n; $n = 5 } # $n inaccessible from outside function #here $n always disappears # Or does it always ??? ==Anonymous Function== sub { ...) ← Older edit		Revision as of 14:27, 14 June 2007 (view source) Amire80 (Talk | contribs) (→Closure - basic: pre, good bullets, link to HOP) Newer edit →
Line 24:		Line 24:
	Lets combine my + anonymous function		Lets combine my + anonymous function
-	sub xx {	+	<pre>
		+	sub xx {
	my $n;		my $n;
	return  sub {		return  sub {
	$n++;		$n++;
	}		}
-	}	+	}
		+	</pre>
-	• $n not  accessible outside xx	+	* $n not  accessible outside xx
-	• $n does not disappear, since anonymous function needs it.	+	* $n does not disappear, since anonymous function needs it.
-	• For good explanation see M.J. Dominus ‘Higher Order Perl’ pp. 71-79	+	* For good explanation see [http://hop.perl.plover.com/ M.J. Dominus Higher Order Perl] pp. 71-79
	==Example - counter==		==Example - counter==

---

Revision as of 14:27, 14 June 2007

Contents 1 Closure And its Usage 2 Anonymous Function 3 Closure - basic 4 Example - counter 5 Counter - use 6 Counter – more interesting 7 Interval 8 Interval - use 9 Regexes - Task 10 Regex with Closure

Closure And its Usage

my
sub xx {
  my $n;
  $n = 5
}

1. $n inaccessible from outside function
2. here $n always disappears
3. Or does it always ???

Anonymous Function

sub {
…
…
…
}

What is it good for ?

Closure - basic

Lets combine my + anonymous function

sub xx {
   my $n;
   return  sub {
   $n++;
   }
}

• $n not accessible outside xx
• $n does not disappear, since anonymous function needs it.
• For good explanation see M.J. Dominus Higher Order Perl pp. 71-79

Example - counter

sub create_cnt {
  my $start = 1;
  return sub  {
  $start++;
  }
}

Counter - use

$c1 = create_cnt();  #cnt1
$c2 = create_cnt(); #cnt2
$c1->();  
$c1->();
$c1->();
$v1 = $c1->();

$c2->();
$v2 = $c2->();
print "v1 = $v1, v2 = $v2\n";

sub create_cnt {
  my $start = 1;
  return sub  {
  $start++;
  }
}

Counter – more interesting

$c1 = create_cnt(1, 1);
$c2 = create_cnt(7, 3);
$c1->();
$c1->();
$c1->();
$v1 = $c1->();
$c2->();
$c2->();
$v2 = $c2->();
print "v1 = $v1, v2 = $v2\n";

different start and delta

sub create_cnt {
  my ($start, $inc) = @_;
  my $current;
  return sub  {
       $current =  $current ? $current += $inc: $start;
  }
}

Interval

sub time_int () {
  my $start = time();
  return sub (){
     my $current = time();
     my $diff = $current - $start;
     $start = $current;
     return $diff;
    }
}

Interval - use

my $t1 = time_int();
my $t2 = time_int();
for (1..4) {
sleep(3);
my $d1 = $t1->();
print "$d1\n";
}
my $d2 = $t2->();
print "$d2\n";

sub time_int () {
  my $start = time();
  return sub (){
     my $current = time();
     my $diff = $current - $start;
     $start = $current;
     return $diff;
   }
}

Regexes - Task

• For ‘12345678’ • For any substring, e.g.34 • Print char before, then substring, then char after, e.g 1234 • Program to implement:

' ' 'Regexes – implementation 1' ' '

my @x = @ARGV;
$str = '12345678';
foreach $cur  (@x) {
   my ($res) = $str =~ /(.?$cur.?)/;
   print "cur = $cur, res = $res\n"
}

Probem • Regex is compiled for each substring • We cannot anticipate all substrings • Closure to the rescue!

Regex with Closure

my @x = @ARGV;
$str = '12345678';
foreach $cur  (@x) {
   unless (defined $regex{$cur}) {
      $regex{$cur} = sdd_regex($cur); # key substr, val closure
      print "create for $cur\n";
   }
   else {
        print "already exists for $cur\n";
    }
    my $res = $regex{$cur}->(\$str); # use closure
    print "cur = $cur, res = $res\n";
}

sub sdd_regex () {
   my $s = shift;
   my $reg = qr/(.?$s.?)/; ## compile once for substr
   sub () {
      my $str = shift;
      #my ($r) =  $$str =~ /(.?$s.?)/;
       my ($r) =  $$str =~ /$reg/;
       return $r;
    }
}