Comments on: Tail recursion in Python http://paulbutler.org/archives/tail-recursion-in-python/ Thu, 09 Feb 2012 16:01:42 +0000 hourly 1 http://wordpress.org/?v=3.0 By: Akhmed http://paulbutler.org/archives/tail-recursion-in-python/#comment-53748 Akhmed Thu, 12 Jan 2012 20:05:55 +0000 http://www.paulbutler.org/archives/tail-recursion-in-python/#comment-53748 Paul, very interesting post and great solution! I see one problem with your code though: your tail_rec() function has it hardcoded that recursive functions must have just 1 argument. I rewrote your code as follows. Seems to be a more general solution. <code> # tail_rec receives fun(.) and returns digger(fun(.)) def tail_rec(fun): # digger(f) keeps digging recursively into f # until it encounters some value def digger(f): retval = f; # keep digging into the recursion # while returned value is a function while (callable( retval )): retval = retval(); return retval; # return the digger(fun(.)) return (digger(fun)); # Recursive functions def even(x): if x == 0: return True; else: return (lambda: odd(x-1)); def odd(x): if x == 0: return False; else: return (lambda: even(x-1)); is_even = tail_rec( even(2000001) ); is_odd = tail_rec( odd(2000001) ); print is_even; print is_odd; </code> Paul, very interesting post and great solution!

I see one problem with your code though: your tail_rec() function has it hardcoded that recursive functions must have just 1 argument. I rewrote your code as follows. Seems to be a more general solution.

# tail_rec receives fun(.) and returns digger(fun(.))
def tail_rec(fun):

# digger(f) keeps digging recursively into f
# until it encounters some value
def digger(f):
retval = f;

# keep digging into the recursion
# while returned value is a function
while (callable( retval )):
retval = retval();

return retval;

# return the digger(fun(.))
return (digger(fun));

# Recursive functions
def even(x):
if x == 0:
return True;
else:
return (lambda: odd(x-1));

def odd(x):
if x == 0:
return False;
else:
return (lambda: even(x-1));

is_even = tail_rec( even(2000001) );
is_odd = tail_rec( odd(2000001) );

print is_even;
print is_odd;

]]> By: Rich Katz http://paulbutler.org/archives/tail-recursion-in-python/#comment-192 Rich Katz Tue, 31 Mar 2009 18:32:49 +0000 http://www.paulbutler.org/archives/tail-recursion-in-python/#comment-192 See Python Tail Recursion Decorators. For instance: http://code.activestate.com/recipes/496691/ Regards, Rich See Python Tail Recursion Decorators.

For instance:

http://code.activestate.com/recipes/496691/

Regards,

Rich

]]> By: Paul Butler http://paulbutler.org/archives/tail-recursion-in-python/#comment-189 Paul Butler Mon, 30 Mar 2009 21:45:00 +0000 http://www.paulbutler.org/archives/tail-recursion-in-python/#comment-189 ythefoe, that works, but only if you have the memory and you know in advance an upper bound on the size of the stack. Neither of these are always the case. ythefoe, that works, but only if you have the memory and you know in advance an upper bound on the size of the stack. Neither of these are always the case.

]]> By: Paul Butler http://paulbutler.org/archives/tail-recursion-in-python/#comment-188 Paul Butler Mon, 30 Mar 2009 21:42:00 +0000 http://www.paulbutler.org/archives/tail-recursion-in-python/#comment-188 Brian, thanks. Stackless looks pretty cool. Brian, thanks. Stackless looks pretty cool.

]]> By: Greg Stoll http://paulbutler.org/archives/tail-recursion-in-python/#comment-187 Greg Stoll Mon, 30 Mar 2009 21:31:30 +0000 http://www.paulbutler.org/archives/tail-recursion-in-python/#comment-187 Interesting technique - thanks! Interesting technique – thanks!

]]> By: ythefoe http://paulbutler.org/archives/tail-recursion-in-python/#comment-186 ythefoe Mon, 30 Mar 2009 20:54:04 +0000 http://www.paulbutler.org/archives/tail-recursion-in-python/#comment-186 Why not setting your recursion limit higher? From the python doc: sys.setrecursionlimit(limit) Set the maximum depth of the Python interpreter stack to limit. This limit prevents infinite recursion from causing an overflow of the C stack and crashing Python. The highest possible limit is platform-dependent. A user may need to set the limit higher when she has a program that requires deep recursion and a platform that supports a higher limit. This should be done with care, because a too-high limit can lead to a crash. Why not setting your recursion limit higher?
From the python doc:

sys.setrecursionlimit(limit)

Set the maximum depth of the Python interpreter stack to limit. This limit prevents infinite recursion from causing an overflow of the C stack and crashing Python.

The highest possible limit is platform-dependent. A user may need to set the limit higher when she has a program that requires deep recursion and a platform that supports a higher limit. This should be done with care, because a too-high limit can lead to a crash.

]]> By: Pádraig Brady http://paulbutler.org/archives/tail-recursion-in-python/#comment-185 Pádraig Brady Mon, 30 Mar 2009 09:46:56 +0000 http://www.paulbutler.org/archives/tail-recursion-in-python/#comment-185 So you explicitly store state rather than implicitly on the stack. This is better as I noted here: http://www.pixelbeat.org/programming/functional_python_state.html So you explicitly store state rather than implicitly on the stack. This is better as I noted here:
http://www.pixelbeat.org/programming/functional_python_state.html

]]> By: popurls.com // popular today http://paulbutler.org/archives/tail-recursion-in-python/#comment-184 popurls.com // popular today Mon, 30 Mar 2009 09:40:05 +0000 http://www.paulbutler.org/archives/tail-recursion-in-python/#comment-184 <strong>popurls.com // popular today...</strong> story has entered the popular today section on popurls.com... popurls.com // popular today…

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]]> By: username223 http://paulbutler.org/archives/tail-recursion-in-python/#comment-183 username223 Mon, 30 Mar 2009 09:09:58 +0000 http://www.paulbutler.org/archives/tail-recursion-in-python/#comment-183 "a = a()"? Objectivist? ;-) “a = a()”? Objectivist? ;-)

]]> By: praxis' me2DAY http://paulbutler.org/archives/tail-recursion-in-python/#comment-182 praxis' me2DAY Mon, 30 Mar 2009 08:43:21 +0000 http://www.paulbutler.org/archives/tail-recursion-in-python/#comment-182 <strong>praxis? ??...</strong> tail recursion in python, ??? ?? ???.... praxis? ??…

tail recursion in python, ??? ?? ???….

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