cnb wrote:
this recursive definition of sum thrumped me, is this some sort of
gotcha or am I just braindead today?
and yes i know this is easy a a for x in xs acc += x or just using the
builtin.
def suma(xs, acc=0):
if len(xs) == 0:
acc
else:
suma(xs[1:], acc+xs[0])
it returns none.
Without return statement, the only recursive solution is a lambda expr :
>>> suma = lambda xs : xs[0]+suma(xs[1:]) if xs else 0
>>> suma(range(101))
5050
Note that suma(xs[1:]) implies copying the remainder of xs, what in turn makes
the time grow quadratically with the length of xs. So instead of passing a
superfluous acc second variable, you could pass an index into the list, eg
def suma(xs,pos=0) :
if pos>=len(xs) :
return 0
else :
return xs[pos]+suma(xs,pos+1)
Cheers, BB
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