Mailing List Archive

David Berthelot writes:
>I would like to sum the numeric values by using map.
>I've tried this:
>>>> map(lamda x,y:x+y,map(L.getitem,(1,)*len(L)))
>But of course it doesn't work...

The reduce() built-in does this sort of thing. Try:
>>> reduce(lambda total,(name,amt): total + amt, L, 0)
75

--
A.M. Kuchling http://starship.python.net/crew/amk/
I'm an excuse for medical experiments and art theory. You must get me out of
here and out of the hospital.
-- Peter Greenaway, _A Zed and Two Noughts_ (1986)

David Berthelot <berthelo@lirmm.fr> writes:

| Hi,
|
| i like to program in functional style, for instance, to get the sum of
| square values for a range i do :
| >>> reduce(lambda x,y:x+y,map(lambda x:x*x,range(10)))
| 285
| >>>
|
| Ok, but i've a problem, how to access a list element in this fashion,
| for instance:
| >>> L = [ ("John",40), ("Monthy",35)]
|
| I would like to sum the numeric values by using map.
| I've tried this:
| >>> map(lamda x,y:x+y,map(L.getitem,(1,)*len(L)))
|
| But of course it doesn't work...

I think you meant 'reduce' instead of 'map' at the beginning, as in
your first example.

Also, of course, 'lamda' should be 'lambda'.

To solve your problem, instead of trying to use __getitem__ somehow,
you can just write another lambda:

>>> L = [ ("John",40), ("Monthy",35)]
>>> reduce (lambda x,y:x+y, map(lambda l:l[1], L))
75

In this case the story has a happy ending. But be aware that Python's
support for functional programming is not very complete, and often it
is harder to do things functionally than to do them with iteration.

--
Dan Schmidt -> dfan@harmonixmusic.com, dfan@alum.mit.edu
Honest Bob & the http://www2.thecia.net/users/dfan/
Factory-to-Dealer Incentives -> http://www2.thecia.net/users/dfan/hbob/
Gamelan Galak Tika -> http://web.mit.edu/galak-tika/www/

You might want to look into the operator module. Instead of

>>> map(lamda x,y:x+y,map(L.getitem,(1,)*len(L)))

the commands

>>> L = [ ("John",40), ("Monthy",35)]
>>> import operator
>>> reduce (operator.add, \
map (operator.getitem, L, len(L) * (1, )), \
0)
75

perhaps does what you expected. Gets messy very fast, though...

Alex.

You might want to look into the operator module. Instead of

>>> map(lamda x,y:x+y,map(L.getitem,(1,)*len(L)))

the commands

>>> L = [ ("John",40), ("Monthy",35)]
>>> import operator
>>> reduce (operator.add, \
map (operator.getitem, L, len(L) * (1, )), \
0)
75

perhaps do what you expected. Gets messy very fast, though...

Alex.