tricky nested list unpacking problem
Scott David Daniels
Scott.Daniels at Acm.Org
Mon Dec 15 17:28:43 EST 2008
Kirk Strauser wrote:
> At 2008-12-15T19:06:16Z, Reckoner <reckoner at gmail.com> writes:
>
>> The problem is that I don't know ahead of time how many lists there are or
>> how deep they go. In other words, you could have:
>
> Recursion is your friend.
>
> Write a function to unpack one "sublist" and call itself again with the new
> list. For instance, something like:
>
> def unpack(pattern):
> # Find the first subpattern to replace
> # [...]
> results = []
> for number in subpattern:
> results.append(pattern.replace(subpattern, number))
> return results
>
> Calling unpack([1,2,3,[5,6],[7,8,9]]) would look cause it to call
> unpack([1,2,3,5,[7,8,9]]) and unpack([1,2,3,6,[7,8,9]]), compile the
> results, and return them.
Along these lines generators are the bees knees.
def expands(source):
'''Nested lists to list of flat lists'''
for n, val in enumerate(source):
if isinstance(val, list):
assert val, 'empty list @%s in %s undefined.' % (
n, len(source))
head = source[: n]
tail = source[n + 1 :]
for element in val:
for row in expands(head + [element] + tail):
yield row
break
else:
if source: # Just to make expands([]) return an empty list)
yield source
def answer(source):
'''Do the requested printing'''
for row in expands(source):
print '-'.join(str(x) for x in source)
--Scott David Daniels
Scott.Daniels at Acm.Org
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