Infinite lists and generators

[Terry Reedy]

"Steven D. Majewski" <sdm7g at Virginia.EDU> wrote in message
news:mailman.1005757000.20449.python-list at python.org...
>
>
> On Wed, 14 Nov 2001, Terry Reedy wrote:
> > The problem with filtering an infinite stream is that if there are
> > only N items that pass, the filter will never halt looking for the
> > N+1st.
>
> You just do 'lazy' filtering -- in fact you try to avoid forcing
> evaluation of the whole stream until the very end, and then you
> stop when you get a sufficient number, or reach the end of the
> stream.

The topic was infinite streams that have no end!

> ( And if possible, you may not have to force full evaluation
> at all if you can do your output one item at a time. You should
> avoid list comprehensions though, which force evaluation -- kind
> of like Icon's 'every' )
>
> Filter is just:
>
> def filter( f, stream ):
>   for item in stream:
>     if f(item): yield item

This is what I had in mind when I wrote my comment.  To illustrate,
try

def n_true(n):
   if n < 0: raise StopIteration
   while n:
      yield n
      n -= 1
   while 1:
      yield 0

for item in filter(lambda x: x, n_true(10)): print item

Terry J. Reedy