Code Golf: Shortest code to find a weighted median?

Question

My try at code golfing.

The problem of finding the minimum value of ∑W_i*|X-X_i| reduces to finding the weighted median of a list of x[i] with weights <

Answer 1

short, and does what you'd expect. Not particularly space-efficient.

def f(l,i):
   x,y=[],sum(i)
   map(x.extend,([m]*n for m,n in zip(l,i)))
   return (x[y/2]+x[(y-1)/2])/2.

here's the constant-space version using itertools. it still has to iterate sum(i)/2 times so it won't beat the index-calculating algorithms.

from itertools import *
def f(l,i):
   y=sum(i)-1
   return sum(islice(
       chain(*([m]*n for m,n in zip(l,i))),
       y/2,
       (y+1)/2+1
   ))/(y%2+1.)