Favorite non-python language trick?
spam.csubich+block at block.subich.spam.com
Sun Jul 3 04:28:53 CEST 2005
Steven D'Aprano wrote:
> On Fri, 01 Jul 2005 12:24:44 -0700, Devan L wrote:
>>With the exception of reduce(lambda x,y:x*y, sequence), reduce can be
>>replaced with sum, and Guido wants to add a product function.
> How do you replace:
> reduce(lambda x,y: x*y-1/y, sequence)
> with sum?
You don't, but an almost equally short replacement works just as well,
and doesn't even need the lambda:
>>>_res = 0.0
>>>for x in sequence: _res = _res*x + 1/x
>>><blank line if in interactive mode>
Sure, this isn't a sum, but I'd argue that the for loop solution is
1) For single expressions, the guts of the operation is still a single line
2) This completely avoids lambda -- while I myself am ambivalent about
the idea of lambda going away, lambda syntax can get hairy for
complicated expressions -- the comma changes meaning halfway through the
expression, from 'parameter delimiter in lambda' to 'next parameter in
3) This trivially extends to a block of code, which a lambda doesn't
4) Behavior for zero- and one-length lists is explicit and obvious.
There are, of course, a few disadvantages, but I think they're more
corner corner cases.
1) This solution obviously isn't itself an expression (although the
result is a single variable), so it can't be used in totality as a
component to a larger call.
[Rebuttal: When exactly would this be a good thing, anyway? Reduce
statements are at least 11 characters long, 13 with a one-character
default value. Using this as a parameter to just about anything else,
even a function call, seems a bit unreadable to me.]
2) An explicit intermediate/result value is needed. This seems to be
more of a 'cleanliness' argument than anything.
Besides, rewriting this as a for loop actually improves performance:
>>> sequence = range(1,100)
>>> def f1():
j = 0.0
for x in sequence: j = j*x+1/x
>>> def f2():
return reduce(lambda x,y: x*y - 1/y, sequence)
>>> def runtime(f,n):
starttime = time.time()
for i in xrange(n):
Making the series bigger results in even worse relative performance (no
>>> sequence = range(1,1000)
So really, 'reduce' is already useless for large anonymous blocks of
code (which can't be defined in lambdas), and it seems slower than 'for
.. in' for even simple expressions.
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