Bengt Richter bokr at oz.net
Tue Aug 6 17:54:42 CEST 2002

```On 6 Aug 2002 01:21:18 -0700, tebeka at cs.bgu.ac.il (Miki Tebeka) wrote:

>Hello Brit,
>
>> So I know what lambda functions are, they're syntax and how they're used.
>> However I'm not sure *why* one would use a lambda function. What's the
>> advantage that they offer over a regular function?
>
>Warning: I've bee a Scheme believer before I've converted to Python
>:-)
>
>Sometime lambda functions yeild more (IMO) elegant solution that is
>One example is success/fail continuations.
>
Couldn't resist ;-) Borrowing and modifying your code, you might find
this solution interesting (I modified your tree display to make it
--< treex.py >------------------------------------
class Tree:
'''Simple binary tree'''
def __init__(self, val, lson=None, rson=None):
self.val = val
self.lson = lson
self.rson = rson

def display(self, indent = 0, label=''):
print '%s%s%s' % ('  ' * indent, label, self.val)
if self.lson:
self.lson.display(indent + 1, 'L:')
if self.rson:
self.rson.display(indent + 1, 'R:')

class Success(Exception):
def __init__(self, crumbs): self.crumbs= crumbs

def path2(t, v):
'''Find path to v in t'''
try:
sfpath2(t, v, '')
return "Couldn't find path to %s" % v
except Success, s:
return 'Found %s by walking %s from tree root' % (v, s.crumbs)

def sfpath2(t, v, crumbs):
'''raise Success solution'''
if not t: return  # Can't find
if t.val == v: # Found right here
raise Success(crumbs or 'nowhere')
# Recursive case, explore subtrees
sfpath2(t.lson, v, crumbs+'L')
sfpath2(t.rson, v, crumbs+'R')

t = Tree(1,
Tree(2,
Tree(3)),
Tree(4,
Tree(5)))

def test(t,x):
print path2(t, x)
--------------------------------------------------
>>> import treex
>>> treex.t.display()
1
L:2
L:3
R:4
L:5
>>> treex.test(treex.t, 5)
Found 5 by walking RL from tree root
>>> treex.test(treex.t, 3)
Found 3 by walking LL from tree root
>>> treex.test(treex.t, 4)
Found 4 by walking R from tree root
>>> treex.test(treex.t, 9)
Couldn't find path to 9
>>> treex.test(treex.t, 1)
Found 1 by walking nowhere from tree root

Regards,
Bengt Richter

```