Vanity Plates -Problem Set 2 (CS50's Introduction to Programming with Python)

[@SomeStuff9]

this was very helpful. i could not think thru the logic to [index: ] to make sure everything after the first digit was also a a digit. I was slicing and dicing and failing all over the place. very nice.

[@NickRoeder]

Freaking brilliant. I had 45 lines of code that passed check50 but not technically the requirements of the pset. I had to see how it was properly done. This is great. Thank you.

[@deljimaetalk7766]

Mine is 59 lines 🤣 Got 8 /10. Brilliant is an understatement

[@Pr0feBboy]

I spent a whole day trying to find a way to check if there are middle numbers and finally decided to check a tutorial that was actually wrong.... Is it just me or this for a beginner with 1 month of experience in python is really difficult ? It's kind of discouraging to fail all the time

[@6hbaufhehdj]

Dont be discouraged. Be happy you are learning, this was a hard problem for me as I am a computer science student, but it comes down to learning the string methods.

[@Pr0feBboy]

@@6hbaufhehdj Thanks. Im still going but where I'll get 5 months from now no idea

[@miquelr2353]

Exactly the same. Im trying now for about hour and a half, I wrote that last loop to check like 8 or 9 different ways. Each time it decided a different input to mess up This is very hard

[@noisi_tc3068]

It is a difficult and steep learning curve and this problem is definitief not easy

[@abdullahadel1141]

Thanks Omar for the amazing explanation, It really helped me a lot

[@TheCodingCreator]

Glad it helped!

[@studywithshin2205]

if we set int(char) != 0 wouldn't 'CS50' return False? Why does it return True?? Can someone please explain ?

[@MohammedArab-f6i]

cus only the first number in the phrase will enter the if statments. If the first number is 0 then return false vai the else statment.

[@getstart98]

wow, amazing!! my solutions passed the test but one was about 40 lines long and the other was almost 50😶

[@minibus1351]

This worked for me def main(): plate = input("Plate: ") if is_valid (plate): print("Valid") else: print("Invalid") def is_valid(s): if 6 >= len(s) and s[0:2].isalpha() and s.isalnum(): for char in s: if char.isdigit(): index = s.index(char) if s[index:].isdigit() and int(char) != 0: return True else: return False return True if __name__ == "__main__": main()

[@tchavofrund1908]

Hi thanks a lot for the explaination it is very concise ! However, I don't understand the role of the very last 'return True'. From what I understand, if all the 'if' statments are True, it goes all the way down to the last one and if the last one is True then it works. If not, there is the 'else' that returns False and then it is invalid. So why after that 'else' do we still need the 'return True' ? Wouldn't it mean that even if the 'if' above return False, at the end, it overwrites it and it is still True ? Thanks

[@ravishankar-ix7qj]

We used the last "Return True " if the string contains only letters. if there are no number it will not enter if condition....

[@aigerimabseit816]

def main(): plate = input("Plate: ") if not plate.isalnum() or (len(plate) < 2 or len(plate) > 6): print("Invalid") return False if is_valid(plate): print("Valid") else: print("Invalid") def is_valid(s): if s.isalpha() or s[:2].isalpha() and s[-2:].isnumeric() and s[-2:][0] != "0": return True else: return False main() try this way too

[@TheCodingCreator]

great work. but also try putting your code inside the is_valid() function, and leave the main function for calling the function and printing a message to the user.

[@aristhera]

doesn't work. Let s = 'aaa888' That's valid, right? Now let it be 'aaa088' That's invalid because of the leading 0 but s[-2:][0] matches only 88 (and then 8) and is therefor True

[@aristhera]

so you will definitely need a for-loop like in the video, check for isdigit(), create an index (see vid) and then s[index:][0] instead of [-2:] and check if the string before index is alphabetical with s[index - 1].isalpha()

[@jessicaly8893]

can you explain why there is a return True on line 18?

[@TheCodingCreator]

because the for loop checks if a string containing digits is valid or not and that's it, so we also have to consider a string with no digits, and that's also a valid plate and the function should return true.

[@mohammadamintorabi8415]

Thanks. small improvement though, instead of " for char in s: if s.isdigit(): index = s.index(char) ..... you can say: for i in range(len(s)): if s[i:].isdigit() and s[i]!= '0': ..... "i" is your index

[@BaraBarabere.]

not working

[@MohammedArab-f6i]

yea but then you would have to change every instance of char to s[i], which honestly is more annoying then just assigning the index to a var 🦐

[@KurtGibson-p4q]

Can someone explain to me the syntax of line 14 i dont get why the : in [index:] ?

[@seifyounes5306]

+ 1

[@studywithshin2205]

please watch tutorials on index function. here in this code index is a variable assigned to index = s.index(char) if char is a digit and [index:] means starting from that index in the iterated string of variable char, it will be returned True. if there is an alphabet after that index, it will return False since numbers cannot be in the middle.

[@btobmelody3217]

can i do the for loop outside the if statement in line 10? like after that conditional, return true else false. then do the for loop?

[@KawsarAhmed-xd4jg]

Hey! So a bug may arise because the if statement at first handles most of the logic except it does not encounter the '0' or zero that comes just after the leading alphabets. In that case, it may return True which isn't the correct output. For this reason, it's necessary to implement the for loop inside the if statement to check whether the zero is positioned just after the alphabets or not. Hope you will find it helpful.

[@chikamatthew959]

I don’t understand the s[0:2].isalpha() function in the program

[@HollowSunny69]

Check the hints in the problem set. It explains it well.

[@TheCodingCreator]

it checks if the first two charachters in our string is alphabetic letters, because as the problem stated “All vanity plates must start with at least two letters.”

[@jermovankatwijk2290]

Makes sure that the first spots are always metters

[@MrKamoliddin]

the code should fail with inputs with , such as CS50P2.

[@MrKamoliddin]

but this works: def is_valid(s): if 2

[@miquelr2353]

I like how i did check50 and instantly got 4 red ansers with this code

[@mariakeramida1032]

Hi all. I m trying to solve the problem but the elif statement is not working. I checked everything but the CS50P has a result Valid. Can you please help me with my mistake? Here is my code: def main(): plate = input("Plate: ") if is_valid(plate): print("Valid") else: print("Invalid") def is_valid(s): if len(s) >= 2 and len(s)

[@hongcao309]

appreciate this video

[@jdubiez]

Why do you is .isdigit instead of .isnumeric ? Or does it not matter?

[@hernansaa]

both work well in this case.

[@lucascamarasa2081]

Do you mind explaining line 13 and line 14 of your final code again? I don't quite get the syntax, it looks cryptic to me

[@abdullashafi580]

yup this line was confusing! but there is a built in function in python called index() "The Python index() method helps you find the index position of an element or an item in a string of characters or a list of items. It spits out the lowest possible index of the specified element in the list. In case the specified item does not exist in the list, a ValueError is returned " __google

[@floki0407]

I did same for first three.. But instead i used nested loop for each and ended up in problem.. For next 2 conditions... But writing them in only one line and nesting only tei conditions make it better......

[@nted]

Loops are annoying to me because I break them very easily

[@TheCodingCreator]

sometimes it be annoying, but you have to get used to it because it is very useful and most of the time you will need loops in your program.

[@9ionet]

For those who don't quite get it, hopefully below code is less cryptic. def main(): plate = input("Plate: ") if is_valid(plate): print("Valid") else: print("Invalid") def is_valid(s): if not 2

[@julialett]

hii can you please explain to me the "stack = []" part of the code pls?

[@seifyounes5306]

i still can't get it or understand it at all

[@seifyounes5306]

i feel like im the only one who can't do it

[@cruseder2]

def main(): plate = input("Plate: ") if is_valid(plate): print("Valid") else: print("Invalid") def is_valid(s): list = [] length = len(s) - 1 if len(s) < 2 or len(s) > 6: return False for i in range(length): if s[i].isnumeric() and s[i+1].isalpha(): return False for i in s: if i.isnumeric(): list.append(i) if i[0] == 0: return False if s.isalnum() and s[0:2].isalpha(): return True if __name__ == "__main__": main()

[@tonybuss8242]

Your code needs an improvement, the way you typed it above will not work.... after your if 6 >= len(s) >= 2 and s[0:2].isalpha() and s.isalnum(): you need to add if s.isalpha(): return True if you do not, a standar all alfa AAAAAA will not work. // TAB

[@TheCodingCreator]

i tried and it is working, why do you think it won't work?

[@jermovankatwijk2290]

Jea indeed, without this addition NVROUS would be Invalid

[@NickRoeder]

AAAAAA.isalnum is True so it will return valid. It does indeed return valid too.

[@janardhan2jordan]

thanks a lot bud

[@simeonrf7047]

def main(): plate = input("Plate: ") if is_valid(plate): print("Valid") else: print("Invalid") def is_valid(input_string): while True: length = len(input_string) if ( input_string[0:2].isalpha() and 2

[@ramireddyvarun26]

feel so foolish! I tried enumerate to get the index etc but never explored index directly from string methods! lesson learnt ... read the bloody documentation! :) Thanks though.

[@amirprx3]

❤❤

[@curiousLeafy]

``` # import re def main(): plate = input("Plate: ") if is_valid(plate): print("Valid") else: print("Invalid") def is_valid(s): if(len(s) < 2): return False s = s.lower() length = len(s) is_min_two_letters = s[0: 2].isalpha() valid_length = length >=2 and length

[@adarshgurung9432]

Please atleast try of explain what you are doing.

[@TheCodingCreator]

which part do u need explanation for?

[@adarshgurung9432]

@@TheCodingCreator I dont understand the use of "char" , "index" and line 14 .

[@TheCodingCreator]

@@adarshgurung9432 my goal was to look for the first digit that appears in a string, so that is why I made the for loop. after knowing that the character is a digit, I want to check if that character doesn't equal 0, and each subsequent character is also a digit (line 14). char, and index are just variable names, you can call them whatever u like. for further understanding, let's assume that our string is "CS50" and we are on line 11. first loop, char will store "C", then we check if char is a digit on line 12, since it is not a digit, our loop ends. second loop, char will store "S", then again we check if char is a digit, since it is not, our loop ends. third loop, char will store "5", then we check if char is a digit, since it is a digit, the variable index will store the index of this character which will be 2 (index count start from 0), then on line 14, s[index:] slices our string starting at index till the last character, in our case: s[2:], which returns "50", then we check if that string contains only digits (that is true). in line 14 we also check if char != 0, in our case 5 != 0, which return true. i hope that makes sense.

[@adarshgurung9432]

@@TheCodingCreator Thanks, I understand most of it except, Which line of code ensures that if first numeric value is not zero. I guess it is int(char) != 0? But I don't understand how.

[@adarshgurung9432]

I mean in line 14 ( int(char) != 0), won't this line disturb the "0" in "CS50" as input, even if 0 is at the end of the word CS50?

[@ayounis5158]

what do you think of my solution: def is_valid(inp): if 2

[@koenokatachiii9218]

trash

[@ayounis5158]

@@koenokatachiii9218 fatherless behaviour

[@koenokatachiii9218]

​@@ayounis5158 youre editing anime girls, get a life

[@ayounis5158]

@@koenokatachiii9218 get a family, oh you can't, srry :(

[@koenokatachiii9218]

@@ayounis5158 what an insult!!!!!!!!!!!!!

[@TheFailMistress]

i know an egyptian when i hear one 😅

[@TheCodingCreator]

was it that obvious 😂

[@christopherreina4901]

without using index: def main(): plate = input("Plate: ") if is_valid(plate): print("Valid") else: print("Invalid") def is_valid(s): if s[0:2].isalpha() and 2

[@oltionr]

That's not fair to distribute the solutions. I hope CS50 will not consider this type of solution. Anyone should try to learn it on their own.

[@TheCodingCreator]

that is not the solution, that is just a solution. the point of these videos is not to make you copy and paste the solution and submit it, the point is to explain the language and learn how to solve problems and make programs with it. but yeah everyone should try it on their own, I should probably mention that in my videos.

[@jorgeav527-xyz]

if re.search("0.", frase): return False