On Fri, 2011-03-25 at 09:49 +0100, Andrea Crotti wrote: > Terry Reedy <tjre...@udel.edu> writes: > > > > For the reason Stefan explained and hinted above. Try the following instead: > > > > def fib_iter(n, _cache = [1,1]): > > k = len(_cache) > > if n >= k: > > for i in range(k, n+1): > > _cache.append(_cache[i-2] + _cache[i-1]) > > return _cache[n] > > > I don't get what's the difference between that and: > > def fib_iter(n): > ls = [0, 1] > for i in range(2, n+1): > ls.append(ls[i-2] + ls[i-1]) > > return ls[n]
> How can passing a default "_cache" argument can make such a difference? Default values are initialised at definition time. Since lists are mutable, the _cache variable really is a cache - the following time you call the function it will store all the previous calculations. e.g. >>> def default_value(_cache=[]): ... _cache.append(len(_cache)) ... print _cache ... >>> default_value() [0] >>> default_value() [0, 1] >>> default_value() [0, 1, 2] >>> default_value() [0, 1, 2, 3] -- http://mail.python.org/mailman/listinfo/python-list
