5 queens

[Steven D'Aprano]

On Mon, 24 Dec 2007 00:18:29 -0800, cf29 wrote:

> On Dec 23, 2:04 pm, Steven D'Aprano <st... at REMOVE-THIS-
> cybersource.com.au> wrote:
>> def combinations(seq, n):
>>     if n == 0:
>>         yield []
>>     else:
>>         for i in xrange(len(seq)):
>>             for cc in combinations(seq[i+1:], n-1):
>>                 yield [seq[i]]+cc
>>
>> >>> for c in combinations(range(4), 3):
>> ...     print c
>> Steven
> 
> Thank you Steven, it works. I am so new to Python that I don't
> understand fully how this function works to be able to adapt it to my
> original problem. I wish I had a teacher :-)

(1) Read up on generators. Google is your friend.

(2) Experiment in an interactive Python session. For example, look at 
this pair of functions:


>>> def function(x, y, z):
...     L = []
...     for i in (x, y, z):
...             L.append(i)
...     return L
...
>>> def generator(x, y, z):
...     for i in (x, y, z):
...             yield i
...
>>> print function(1, 2, 3)
[1, 2, 3]
>>> print generator(1, 2, 3)
<generator object at 0xb7f7ee8c>
>>> 
>>> 
>>> for i in function(1, 2, 3):
...     print i
...
1
2
3
>>> for i in generator(1, 2, 3):
...     print i
...
1
2
3


Now imagine, instead of returning three items, suppose you had a function 
that needed to return millions of items. A function using return would 
need to produce all of those items before it could make them available to 
you. A generator using yield can feed them to you one at a time, as soon 
as they're ready.


(3) Read up on recursion, when a function calls itself.



Hope that helps.


-- 
Steven