def years_to_double(money, rate):
    '''Return how many years it takes for an investment to double at a
    given interest rate.'''

    years = 0
    increased_value = money
    # Each year until the investment has doubled, add 
    # interest for the year.
    while increased_value < (money * 2):
        years += 1
        increased_value += rate * increased_value
    
    return years

def num_dogs(L):
    ''' Given a pet list L, return the number of dogs in the list. '''
    n = 0 
    for l in L:
        if l[1] == 'dog':
            n += 1
    return n

def ages(L):
    '''Given a pet list L, return the sum of the ages of the
       animals in the list.'''
    n = 0 
    for l in L:
       n += l[2]
    return n

def nested_lengths(L):
    ''' Given a nested list L, returns a list of the lengths
        of the sublists. More formally: for each element e in L,
        the returned list contains a corresponding
        element, c, that represents the number of elements in e.'''

    lens = []
    for l in L:
        lens.append(len(l))
    return lens            


if __name__ == "__main__":
    
    # How many years will it take to turn 10 dollars into 20 dollars
    # if 10% interest is added every year?
    interest = 0.1
    investment = 10
    years = years_to_double(investment, interest)
    
    print "At an interest rate of " + str(interest),
    print "your money will double in " + str(years) + " years."

    pets = [["Shoji", "cat", 18], ["Hanako", "dog", 15], \
            ["Sir Toby", "cat", 10], ["Sachiko", "cat", 7],\
            ["Sasha", "dog", 3], ["Lopez", "dog", 13]] 

    # Your testing should include more cases, to make sure you
    # are correct
    print num_dogs(pets), "sb", 3
    print num_dogs([]), "sb", 0    
    print  ages(pets), "sb", 66
    print nested_lengths(pets), "sb", [3,3,3,3,3,3]
    print nested_lengths([[0,2],['a'],[2,3,5,4,5]]), "sb", [2,1,5]