[PEP] matching and mismatching

[Luke Kenneth Casson Leighton]

On Thu, May 02, 2002 at 11:10:07AM -0500, Mark McEahern wrote:

> [Luke Kenneth Casson Leighton]
> > the idea: a function/operator similar to map and reduce _or_ a keyword
> > similar to "in", such that:
> > 
> > for x in ([1,3,2,5,9] matching lambda x:x > 2):
> > 	print x
> > will output:
> > 3
> > 5
> > 9
> 
> What's wrong with using list comprehensions?  E.g.,
 
 great!  thanks!

> >>> nums = range(1, 11)
> >>> odds = [x for x in nums if x % 2]
> >>> evens = [x for x in nums if not (x % 2)]
> >>> nums
> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
> >>> odds
> [1, 3, 5, 7, 9]
> >>> evens
> [2, 4, 6, 8, 10]

interesting that you raise something that aspex's chip can optimise.

the x % 2 and the not (x % 2) operations, apart from being able
to perform the test in parallel, can do separate offloading of all
numbers that "match" or "mismatch" the test.

i.e. the match or mismatch is stored as a flag in every processing
element, such that, well, it's best if i try to express it in
pseudo-python:


(odds, evens) = [x for x in nums (if x % 2, if not x % 2)]

so, you performed the test on every element in the list.
you offload into "odds".

whilst the data is still loaded into the parallel array, you
perform a second test (which happens to be the opposite of
the first test, so you don't _actually_ need to issue a second
instruction - that's the optimisation), and do a second offload
(into "evens").

hm.  you could also even extend this idea further:

(odds, evens, triples) = [x for x in nums (if x % 2, if not x % 2, if x % 3)]

what do think?

worth a PEP?

l.

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