[Tutor] exercise is recursion

[D-Man]

On Tue, 21 Nov 2000 09:22:25 Moshe Zadka wrote:
 | On Tue, 21 Nov 2000, Remco Gerlich wrote:
 | 
 | > I would write it like this:
 | > 
 | > import math
 | > 
 | > def factors(n, highestsofar=2):
 | >    for i in range(highestsofar, math.sqrt(n)+1):
 | >       if n % i == 0:
 | >          return [i] + factors(n/i, i)
 | >    return [n]
 | 
 | def factors(n, highestsofar=2, factorssofar=None):
 | 	if factorssofar is None:
 | 		factorssofar = []
 | 	for i in range(highestsofar, math.sqrt(n)+1):
 | 		if n % i == 0:
 | 			factorssofar.append(i)
 | 			return factors(n/i, i, factorssofar)
 | 	factorssofar.append(n)
 | 	return factorssofar
 | 
 | Is more efficient, and almost as clear.
 | 

Why not 

def factors( n , highestsofar=2 , factorssofar=[] ) :
	...

Then when the client calls it
	result = factors( number , factorssofar=[] )
The list will be empty.  One less arg to pass in the recursion and also removes
the need for the "if ( factorssofar == None )"  check.  (I know the default arg
is only evaluated once, but that can be useful in this case)

 | --
 | Moshe Zadka <moshez@math.huji.ac.il> -- 95855124
 | http://advogato.org/person/moshez
 | 
 
Does python do tail-recursion yet?  If so, then Moshe's is definitely more
efficient (read better).  If not, I think the previous solution is easier to
read, but both are quite clear.

Just my $0.02.

-D