gini-1.py

from __future__ import division


def gini(population):
    """
    https://en.wikipedia.org/wiki/Gini_coefficient#Calculation
    see "This may be simplified to:"
    """
    pop = sorted(population)
    n = len(pop)

    a = 0
    for idx, item in enumerate(pop, 1):
        a += (idx * item)
    a *= 2

    b = 0
    for item in pop:
        b += n * item

    return (a / b) - (n+1)/n


def gini2(population):
    """
    https://en.wikipedia.org/wiki/Gini_coefficient#Calculation
    see "This may be simplified to:"
    (as above, but using reduce (with "initial=0" as accumulator)
    """
    pop = sorted(population)
    n = len(pop)

    a = 2 * reduce(
        lambda prev, (idx, item): prev + (idx * item),
        enumerate(pop, 1),
        0
    )

    b = reduce(
        lambda prev, item: prev + (n * item),
        pop,
        0
    )

    return (a / b) - (n+1)/n


def gini3(population):
    """
    https://en.wikipedia.org/wiki/Gini_coefficient#Calculation
    see "This may be simplified to:"
    (as above, but basically doing map then reduce
    """
    pop = sorted(population)
    n = len(pop)

    a = 2 * sum(map(
        lambda (idx, item): idx * item,
        enumerate(pop, 1)
    ))

    b = sum(map(
        lambda item: n * item,
        pop
    ))

    return (a / b) - (n+1)/n