# count21.py
#  A function to count all the 21's in a list of numbers
#  P. Conrad for CS5nm 11/17/08


        
# check_expect checks whether a function produces the expected result
# consumes:
#     test: a string describing the test
#    check: the value we are testing (typically result of a function call)
#   expect: the value we are expecting (typically a literal value)
# produces:
#    nothing (there is no return value)
# side effect:
#    prints a message indicating whether the test passed or failed

def check_expect(test,check,expect):
    if (check == expect):
        print "Test " + test + " passed."
    else:
        print "Test " + test + " failed: expected " + str(expect)  + \
              " but I got " + str(check)




# consume: a list of anything
# produce: boolean, True if the list is empty, otherwise False

def isEmpty(myList):
    return len(myList)==0


def first(myList):
    return myList[0]

def rest(myList):
    return myList[1:]

# consumes: a list of numbers
# produce: an int, how many 21s are in the list

def count21(myList):
    if (isEmpty(myList)):
        return 0
    else:
        if (first(myList) == 21):
            return 1 + count21(rest(myList))
        else:
            return 0 + count21(rest(myList))


check_expect("count21([21, 3, 4, 5, 21]",
             count21([21, 3, 4, 5, 21]),
             2)

check_expect("count21([21, 3, 4])",
             count21([21, 3, 4]),
             1)


check_expect("count21([])",
             count21([]),
             0)