February 13th, 2014, 11:02 PM

Hanoi Towers
Code:
def hanoi_move ( start , via , target ,n ,k ):
""" finds the kth move in an Hanoi Towers instance
with n discs """
if n <=0:
return "zero or fewer disks"
elif k <=0 or k >=2** n or type(k )!= int :
return "number of moves is illegal"
elif k ==2**( n 1):
return str . format ("disk {} from {} to {}",n , start , target )
elif k <2**( n 1):
return hanoi_move ( start , target , via ,n 1 , k)
else:
return hanoi_move ( via , start , target ,n 1 ,k 2**( n 1))
may you explain the last row, why it is not (via, target, start ,n 1 ,k 2**( n 1)? why k=2^(n1) when disk n is moved from start to target?
February 16th, 2014, 07:05 PM

I think I do understand the quick code to determine which move is the kth move. Clear explanationanother story. I wrote a lot of confusion which I deleted before posting. Then wrote a program. See if you can figure it out from this code which plays ToH. In this output the towers are in constant position, original source, original extra, and original sink.
Code:
'''
$ python p.py
0: (dcba) ( ) ( )
1: (dcb ) (a ) ( )
2: (dc ) (a ) (b )
3: (dc ) ( ) (ba )
4: (d ) (c ) (ba )
5: (da ) (c ) (b )
6: (da ) (cb ) ( )
7: (d ) (cba ) ( )
8: ( ) (cba ) (d )
9: ( ) (cb ) (da )
10: (b ) (c ) (da )
11: (ba ) (c ) (d )
12: (ba ) ( ) (dc )
13: (b ) (a ) (dc )
14: ( ) (a ) (dcb )
15: ( ) ( ) (dcba)
'''
class Hanoi:
def __init__(self, n, verbosity=0):
self.n = n
self.verbosity = verbosity
self.fmt = '{:4d}:'+(' ({:%ds})'%self.n)*3
self.reset()
def verbose(self):
if self.verbosity:
print(self)
def reset(self):
self.towers = [list(reversed('abcdefghijklmnop'[:self.n])), [], []]
self.count = 0
def __str__(self):
return self.fmt.format(self.count, *(''.join(t) for t in self.towers))
def __call__(self):
self.verbose()
self.hanoi(0, 1, 2, self.n)
def hanoi(self,a,b,c,n):
if n:
self.hanoi(a,c,b,n1) # move all but 1 onto the extra pile b
t = self.towers
self.count += 1
t[c].append(t[a].pop()) # move the 1 onto the destination
self.verbose()
self.hanoi(b,a,c,n1) # move those of b to destination
h = Hanoi(4, True)
h()
notes: I think there's a way to have string formatting use a format method. I didn't look it up. I think there are ways to incorporate field widths and various braces into string formats. I didn't study that either.
Last edited by b49P23TIvg; February 17th, 2014 at 10:35 AM.
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