list comprehension question

[Hans Mulder]

On 17/10/12 09:13:57, rusi wrote:
> On Oct 17, 10:22 am, Terry Reedy <tjre... at udel.edu> wrote:
>> On 10/16/2012 9:54 PM, Kevin Anthony wrote:
>>
>>> I've been teaching myself list comprehension, and i've run across
>>> something i'm not able to convert.
>>
>> list comprehensions specifically abbreviate the code that they are
>> (essentially) equivalent to.
>>
>> res = []
>> for item in source:
>>    res.append(f(item))
>> res
>>
>> <==>
>>
>> [f(item) for item in source]
>>
>> Matrix multiplication does not fit the pattern above. The reduction is
>> number addition rather than list appending.
> 
> Dunno why you say that. Heres matrix multiply using list
> comprehensions:
> 
> from operator import add
> def dot(p,q): return reduce(add, (x*y for x,y in zip(p,q)))
> 
> def transpose(m): return zip(*m)
> 
> def mm(a,b): return mmt(a, transpose(b))
> 
> def mmt(a,b): return [[dot(ra, rb) for rb in b] for ra in a]
> 
> which can then be 'reduced' to a one-liner if that takes your fancy

I can golf it down to two lines without losing readability:

def dot(p,q): return sum(x*y for x,y in zip(p,q))

def mm(a,b): return [[dot(ra, rb) for rb in zip(*b)] for ra in a]


Hope this helps,

-- HansM