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Python Tutorial Was: Guido's regrets: filter and map

Bengt Richter bokr at oz.net
Tue Nov 26 11:21:57 EST 2002 
On Mon, 25 Nov 2002 23:19:46 -0800, Terry Hancock <hancock at anansispaceworks.com> wrote:
[...]
>
>map(), filter(), and reduce() do not represent all of the uses for filter,
>and thus, they can't all be replaced by list comps (BTW, I never used
>reduce, never understood what it was good for, and don't immediately
>see how it can be done using list-comps -- would be genuinely interested
>in seeing a couple of example if anyone's willing). 
>
I guess I'll bite:

 >>> def lcreduce(fn, seq, init=None):
 ...     return [first
 ...         for first, rest in [ init is None and
 ...             (list(seq[:1]), list(seq[1:])+[id]) or
 ...             ([init], list(seq)+[id])]
 ...         for y in rest if y is id or first.append(fn(first.pop(), y))][0][0]
 ...
 >>> seq = range(10)
 >>> lcreduce(int.__add__,seq)
 45
 >>> lcreduce(int.__add__,seq,5)
 50
 >>> seq = (1,2)
 >>> lcreduce(int.__add__,seq)
 3
 >>> lcreduce(int.__add__,seq,5)
 8
 >>> seq = [3]
 >>> lcreduce(int.__add__,seq)
 3
 >>> lcreduce(int.__add__,seq,5)
 8
 >>> seq = ()
 >>> lcreduce(int.__add__,seq,5)
 5
 >>> lcreduce(int.__add__,seq)
 Traceback (most recent call last):
   File "<stdin>", line 1, in ?
   File "<stdin>", line 3, in lcreduce
 IndexError: list index out of range

But that last is ok:
 >>> reduce(int.__add__,seq)
 Traceback (most recent call last):
   File "<stdin>", line 1, in ?
 TypeError: reduce() of empty sequence with no initial value

I suspect reduce is a little faster than lcreduce ;-)

Regards,
Bengt Richter
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