possibly trivial newbie list/array question

[Duncan Booth]

Paul Rubin <phr-n2001 at nightsong.com> wrote in 
news:7xy9obovj8.fsf at ruckus.brouhaha.com:

> Can you tell me the actual purpose of list comprehension?  
> 
>         [f(x) for x in a]
> 
> just seems like confusing syntactic hair that's otherwise equivalent
> to, though as we've seen sometimes slower than,
> 
>         map(lambda x: f(x), a).
Which would be much better written, and faster should it matter, as:
     map(f, a)

> 
> Am I missing something?  Unless there's more to it than I see (which
> is quite possible), I don't understand why this feature made it into
> the language.
> 
Once you get used to it it feels much cleaner and easier to read than the 
lambda equivalent and less error prone as well. Would you care to rewrite 
the following using map, filter and lambda?

    [ f(x, y) for x in a for y in b if g(x,y) ]

As far as speed is concerned, you will find it very difficult in general to 
predict whether the map or the list builder notation will be faster, and by 
the time you have worked it out you will almost certainly have wasted more 
time than you will ever save. Dont try to do micro-optimisations. Wait 
until your code really is too slow then optimise the bits that matter.

-- 
Duncan Booth                                             duncan at rcp.co.uk
int month(char *p){return(124864/((p[0]+p[1]-p[2]&0x1f)+1)%12)["\5\x8\3"
"\6\7\xb\1\x9\xa\2\0\4"];} // Who said my code was obscure?