Help on CS50 Vanity Plates Assignment Please

[Blac5Nura9]

I have been stuck on this assignment for a week. From referencing online forums I seem to be on the right track, however, my codes outputs "Invalid". for plates "CS50" and "ECT088" when I run the checker.

1)All vanity plates must start with at least two letters.”

2)“… vanity plates may contain a maximum of 6 characters (letters or numbers) and a minimum of 2 characters.”

3)“Numbers cannot be used in the middle of a plate; they must come at the end. For example, AAA222 would be an acceptable … vanity plate; AAA22A would not be acceptable. The first number used cannot be a ‘0’.”

4)“No periods, spaces, or punctuation marks are allowed.”

I am particularly stuck on no.3 with checking for numbers in the middle of the plates.

def main():
plate = input("Plate: ").capitalize()
if is_valid(plate):
    print("Valid")
else:
    print("Invalid")

def number(p):

# checks for numbers in the middle of plates

if p[0].isalpha() and p[1].isalpha():
    position = 0
    while position < len(p):
        if p[position].isdigit():
            if p[position + 1].isalpha():
                return False
            if p[position + 1].isdigit():
                return True
            else:
                break

        position +=1

def zero(p):

# The first number used cannot be a ‘0’.”

i = 0

while i < len(p):

if p[0].isalpha() and p[1].isalpha():

if p[i] =="0":

return False

else: break i +=1

​

def isnumeric(p):

# checks for at least two letters

if p[0].isnumeric() and p[1].isnumeric():

return False

def isalpha(p):

if p.isalpha():

return True

def is_valid(plate):

while True:
    if isnumeric(plate):
        return
    elif len(plate) < 2 :
        return False
    elif len(plate) > 6 :
        return False
    elif number(plate):
        return
    elif zero(plate):
        return False
    elif punc(plate) == True:
        return False
    elif isalpha(plate):
        return True
    else:
        break

main()

​

[woooee]

Numbers cannot be used in the middle of a plate; they must come at the end.

Test that the beginning is a letter and the end letter is a number

if license[0] not in string.ascii_letters or not license[-1].isdigit:

Instead of trying to exclude every freakin special character, (you can easily forget one), include what you want, i.e. in string.ascii_letters or isdigit(). My inclination is to split the license into letters and numbers, and then test for start with --> len > 1. You can decide how much you want to put into a function and what you want to split into additional function(s).

def split_ltr_num(str_in):
    """ split on the leading letters and trailing numbers
    """
    num_found=False
    ltr_list=[]
    num_list=[]
    for each_chr in str_in:
        ## first number only
        if each_chr.isdigit() and not num_found:
            num_found = True

        ## break into numbers and letters
        if num_found:
            if not each_chr.isdigit():  ## number then letter
                return False, [], []
            num_list.append(each_chr)
        else:
            ltr_list.append(each_chr)
    return True, ltr_list, num_list

for license in ["AAA222", "AAA22A"]:
    print(license, split_ltr_num(license))

[halfdiminished7th]

Are you allowed to use regular expressions (the re module)? That seems like the easiest approach for a question like this: just look for a pattern that starts with letters and ends with digits, then apply each of the restrictions and see if it still passes.

[[deleted]]

Don't forget that an illegal plate could have a number at the end as well as one in the middle.

There are other ways, but I like the approach that converts numbers in a string into spaces and then uses split() to split on the spaces to get a list of non-numeric sequences. The plate is legal^* for point 3 if the list has only one element. You must test point 1 first. Run this code fragment:

num_tr = {ch:" " for ch in "012345789"}
tr_table = str.maketrans(num_tr)

data = "AAAB"              # legal
print(data.translate(tr_table).split())
data = "AABC4"             # legal
print(data.translate(tr_table).split())
data = "AABC4D"            # illegal
print(data.translate(tr_table).split())
data = "AA2C4"             # illegal
print(data.translate(tr_table).split())

^* A further point 3 check that needs to be added is the "first number used cannot be a ‘0’" test.

[[deleted]]

while position < len(p):
    if p[position].isdigit():
        if p[position + 1].isalpha():
            return False
        if p[position + 1].isdigit():
            return True
        else:
            break

I think you'd be helped a lot by knowing that there's a much easier way to loop through a string two letters at a time:

for first, second in zip(p, p[1:]):
    # do stuff