functional programming with map()

[Bengt Richter]

On 24 Feb 2002 20:18:53 -0800, Paul Rubin <phr-n2002a at nightsong.com> wrote:

>David Eppstein <eppstein at ics.uci.edu> writes:
>> > : But what is the functional equvalent of:
>> > 
>> > : for x in items:
>> > :     x.f()
>> > 
>> > 
>> > 
>> > Here's one way to do it:
>> > 
>> > ###
>> > map(lambda x: x.f(), items)
>> > ###
>> 
>> I'd prefer
>> [x.f() for x in items]
>> 
>> It's not functional syntax, but so what?
>
>Both of those build up a new list of the results, instead of
>discarding the values.  If the f function takes an integer and
>computes a 20-megabyte structure, you've got a problem.  This really
>calls for a generator comprehension as discussed a few weeks back.
>
>   reduce(lambda x,y:(y.f(),0)[1], items, 0)
>
>almost works, but invokes f an extra time at the end.
>
>Someone see how to fix it?

 >>> class F:
 ...     def __init__(self,msg):
 ...         self.msg = msg
 ...     def f(self):
 ...         print self.msg
 ...
 >>> a=F('this is a')
 >>> b=F('this is b')
 >>> c=F('this is c')
 >>> items = [a,b,c]
 >>> reduce(lambda x,y:y.f() and 0, items, 0)
 this is a
 this is b
 this is c

but I didn't get an extra call trying your version:

 >>> reduce(lambda x,y:(y.f(),0)[1], items, 0)
 this is a
 this is b
 this is c
 0

you can use the 'and 0' ploy to get a zero length
list comprehension also:

 >>> [ 0 for x in items if x.f() and 0]
 this is a
 this is b
 this is c
 []

Regards,
Bengt Richter