Recursion defined

Recursion, see Recursion.  :)

Something defined in terms itself.  Or sometimes CS scientists or programmers making point through
GNU - "GNU's Not Unix"
YAML - "YAML Ain't Markup Language"
Or beautiful Sierpinski Traingles

When a function call's itself some interesting things happen behind the scene like holding onto the variables which later used when computer execution unwinds the stack.  In a typical example of recursion in solving a factorial, one may write
#!/usr/bin/env perl

use strict;
sub factorial {
    my $v = shift;
    return 1 if $v == 1;
    return $v * factorial($v - 1);
}
factorial(5);
When a first call is made to factorial(5), the execution jumps to factorial function (subroutine) and gets to last line, where while evaluating encounters another function call to factorial ($v -1) which then again makes a call to function or subroutine. This pushes a new call frame on to stack (with args).  If a function returns it is pop-ed out of the stack and done (lost).

Few things are working together with call stack, heap, garbage collector (which removes any memory of any variable or func obj that doesn't have reference count 1 or more), and execution system.

Now to see more on recursion you can try the following


  1 #!/usr/bin/env  perl
  2 $! = 1;
  3 use strict;
  4 use IO::Handle;
  5 use Carp qw(cluck);
  6
  7 STDOUT->autoflush(1);      # Flush output immediately
  8 STDERR->autoflush(1);
  9
 10 sub factorial {
 11     my $v = shift;
 12  
 13     dummy_func();             # Sub that returns immediately printing call stack
 14     return 1 if $v == 1;
 15     print "Variable v value: $v and it's address:", \$v,
                     "\nCurrent sub factorial addr:", \&factorial, "\n","-"x40;
 16     return $v * factorial($v - 1);    # Builds on call for each func call
 17 }
 18  
 19 sub dummy_func {
 20     cluck;
 21 }
 22
 23 factorial(5);
Resulting output:
  1     main::dummy_func() called at ./t_recursion.pl line 13
  2     main::factorial(5) called at ./t_recursion.pl line 23
  3 Variable v value: 5 and its address:SCALAR(0x7ff6240546a0)
  4 Current sub factorial addr:CODE(0x7ff62402f2c0)
  5 ----------------------------------------
  6     main::dummy_func() called at ./t_recursion.pl line 13
  7     main::factorial(4) called at ./t_recursion.pl line 16
  8     main::factorial(5) called at ./t_recursion.pl line 23
  9 Variable v value: 4 and its address:SCALAR(0x7ff6240610e8)
 10 Current sub factorial addr:CODE(0x7ff62402f2c0)
 11 ----------------------------------------
 12     main::dummy_func() called at ./t_recursion.pl line 13
 13     main::factorial(3) called at ./t_recursion.pl line 16
 14     main::factorial(4) called at ./t_recursion.pl line 16
 15     main::factorial(5) called at ./t_recursion.pl line 23
 16 Variable v value: 3 and its address:SCALAR(0x7ff6240612f8)
 17 Current sub factorial addr:CODE(0x7ff62402f2c0)
 18 ----------------------------------------
 19     main::dummy_func() called at ./t_recursion.pl line 13
 20     main::factorial(2) called at ./t_recursion.pl line 16
 21     main::factorial(3) called at ./t_recursion.pl line 16
 22     main::factorial(4) called at ./t_recursion.pl line 16
 23     main::factorial(5) called at ./t_recursion.pl line 23
 24 Variable v value: 2 and its address:SCALAR(0x7ff624061538)
 25 Current sub factorial addr:CODE(0x7ff62402f2c0)
 26 ----------------------------------------
 27     main::dummy_func() called at ./t_recursion.pl line 13
 28     main::factorial(1) called at ./t_recursion.pl line 16
 29     main::factorial(2) called at ./t_recursion.pl line 16
 30     main::factorial(3) called at ./t_recursion.pl line 16
 31     main::factorial(4) called at ./t_recursion.pl line 16
 32     main::factorial(5) called at ./t_recursion.pl line 23

When recursion script is kicked-off, it pushes factorial(5) first frame on to the call stack (line 2 above) which calls another dummy_func which then goes on to the stack (line 1).   Hence when cluck is called in dummy_func there are two calls on the stack along with any arguments passed.

Then dummy_call returns and is pop-ed from the stack.  Program moves to line 15 (script above) and evaluates to false.  Then prints lines 3&4 output ($v and its location, factorial sub location).

Script line 16 calls factorial which pushes the new function call on to stack and at the point the value of $v is 5.  The function and this variable are in same scope and on stack.  So later when this function returns is multiplied with $v (value 5).

When factorial is called 2nd time (but first time at line 16 and pushed onto call stack) $v is reduced by 1 ($v -1) which is then copied and execution starts at top of this subroutine again.  Remember copy of definition of function always the same at some location (CODE(0x7ff62402f2c0)) in program memory.

This execution then calls dummy_func which spits out the call stack and as you expected now you have dummy_func at top, 2nd factorial in middle and 1st factorial call at bottom.  Stack is FILO (First In Last Out or LIFO - Last In First Out).  Then execution moves to lines 14 & 15.   Output looks like:

  6     main::dummy_func() called at ./t_recursion.pl line 13
  7     main::factorial(4) called at ./t_recursion.pl line 16
  8     main::factorial(5) called at ./t_recursion.pl line 23
  9  Variable v value: 4 and its address:SCALAR(0x7ff6240610e8)
 10 Current sub factorial addr:CODE(0x7ff62402f2c0)

At script line 16 the recursion continues and you get output lines 12 to 32.  At the last function the base or terminal condition of recursion is met ( return 1 if $v == 1; ) and it returns 1.

factorial of 1 => 1! = 1;

Now the stack rewinding begins, the return value of 1 (when factorial (1) returned) is multiplied with the variable $v (value 2) and results in 2 which is returned by  return $v * factorial($v - 1);  statement.

Finally,  5! = 120.

All this happen behind the scene and it might be just better to know and recognize the common pattern when this happen :).  I wouldn't worry about how the implementation is done when I run query like

SELECT column_N FROM table_X;
It is so darn simple but so much goes behind that SQL statement from mapping table to file and exact location in file to extract correct values.  It is all hidden from the application program.

For more details take a look at "Call Stack" or "Activation Record".

But if you like to dig deeper through debugging, try


> perl -d t_recursion.pl
Loading DB routines from perl5db.pl version 1.33
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(t_recursion.pl:2):	$! = 1;
  DB<1> n
main::(t_recursion.pl:7):	STDOUT->autoflush(1);
  DB<1> n
main::(t_recursion.pl:8):	STDERR->autoflush(1);
  DB<1> n
main::(t_recursion.pl:23):	factorial(5);
  DB<1> s
main::factorial(t_recursion.pl:11):
11:	    my $v = shift;
  DB<1> s
main::factorial(t_recursion.pl:13):
13:	    dummy_func();
  DB<1> s
main::dummy_func(t_recursion.pl:20):
20:	    cluck;
  DB<1> T
. = main::dummy_func() called from file `t_recursion.pl' line 13
. = main::factorial(5) called from file `t_recursion.pl' line 23
  DB<1> 


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