# CSE 481E
# Python Warmup Excercise Questions 2-5

from numpy import *

# 2. Higher-Order Functions in Python


# scale takes a number n, and a list of numbers s, and returns 
# a new list with each element in s multiplied by n.
def scale(n, s):
    return map(lambda x: n*x, s)

# evens takes a list of numbers as an argument, and returns a list of just
# the even numbers
def evens(s):
  return filter(lambda n: n%2==0, s)


# make_table function
def make_table(n):
    ns = arange(n)
    
    return vstack(
        (
            ns,
            sqrt(ns),
            ns*ns,
            log(ns),
            exp(ns)
        )
    )


# Random tester
def rand_tester(n):
    # Take the sum of an array consisting of 1s for every random element
    # greater than 0.5, and 0 for every other random element:
    return sum (where (random.rand(n) > 0.5, 1, 0))


# Desirability computations
def desirable_apartment_count(incomes, rents):
    # Desirability is the outer product of the rents and 1/incomes
    cost_to_income = outer(rents, 1.0 / incomes)
    
    # Desirable counts are the number of elements less than 1/3 in each column:
    return sum( where (cost_to_income < 1/3., 1, 0), axis = 0)