# Example of incorrect attempt to write recursive function
# that counts the number of 0s in a list
#
# This demonstrate a common (among new-to-recursion students) but
# incorrect approach to writing recursive functions.
# Usually, people who write code like this believe that the recursive
# calls (marked with ***) will update the value of the total
# variable at the calling level.  But they DO NOT.  Each time
# count0s is called, a new set 'total' variable is created and only
# that new variable is modified.  You can "watch" what happens very clearly
# if you run this code in pythontutor.com.
#
# count0sB and counts0sC demonstrate correct ways to solve this problem.
#
def count0s (inList):
    total = 0
    if inList != []:
        if inList[0] == 0:
            total = total + 1
            count0s(inList[1:])  # *** recursive call
        else:
            count0s(inList[1:])  # *** recursive call
    return total

def count0sB (inList):
    total = 0
    if inList != []:
        restTotal = count0sB(inList[1:])
        
        if inList[0] == 0:
            total = 1 + restTotal
        else:
            total = restTotal
    return total

def count0sC (inList):
    if inList == []:
        return 0
    elif inList[0] == 0:
        return 1 + count0sC(inList[1:])
    else:
        return count0sC(inList[1:])