countdown.py

# look, it's art. okay? it's art about how functional programming in Python is sometimes ugly
#
# it just so happens this art has a terrible worst-case complexity.
#

# usage: echo "1 2 3" | python countdown.py

from itertools import chain, izip_longest, product, permutations
import operator
import sys

TOLERANCE = 0.6

operators = (operator.add, operator.sub, operator.mul, operator.truediv)

READABLE_OPERATORS = {
        operator.add: "+",
        operator.sub: "-",
        operator.mul: "*",
        operator.truediv: "/"
}

class NoSolutionException(Exception): pass # let's be civilized here

def solve(operands, solvand):
    for opchoice in product(operators, repeat=(len(operands) - 1)):
        # wasn't sure if order of operands is meant to matter, so I'll increase
        # the worst-case complexity a bit.
        #
        # I mean, it wasn't a fast solution to start with ...
        for permutation in permutations(operands):
            _opchoice = list(opchoice)
            if abs(reduce(lambda x, y: _opchoice.pop(0)(x, y), permutation) - solvand) <= TOLERANCE: # :)
                return (permutation, opchoice)
    raise NoSolutionException()

if __name__ == '__main__':
    operands = [float(token) for token in sys.stdin.read().split(' ')]
    solvand = operands.pop(-1)
    try:
        permutation, opchoice = solve(operands, solvand)
        # this next bit is my favorite <3
        print "solution: {} = {} (more or less)".format(" ".join(chain(*izip_longest(map(str, permutation),
                                                map(READABLE_OPERATORS.get, opchoice), fillvalue=''))),
                                         solvand)
    except NoSolutionException:
        print "Invalid"