Lab 4a — Strings

Preparation

1 minute

Download the zip file for this lab, located above. This zip file has code that you will use for this assignment. Extract the files and put them in your cs110 directory in a folder called lab4a.

Tick Tick Boom

10 minutes

In the file tick_boom.py, you will find a set of short puzzles. The main() function calls a set of functions that process strings. For example:

f1('a!_pA35S222y')

Every function, f1()…f4() is a string bomb. It should return a string that contains one 'tick' for every character in the string, but then end with 'BOOM' when a character is reached that triggers the bomb.

Unfortunately, all of the bombs are duds right now, because all functions return one 'tick' for every character and never explode. Your job is to add the logic that activates each string bomb.

For example, you can see that f1('a!_pA35S222y') should return 'tickticktickticktickBOOM':

    text = 'a!_pA35S222y'
    value = f1(text)
    if value != 'tickticktickticktickBOOM':
        print(f"Error on f1 with {text}")
        return

However, this same function, when called with f1('gh 9af380Pb') should return 'tickticktickBOOM':

 text = 'gh 9af380Pb'
    value = f1(text)
    if value != 'tickticktickBOOM':
        print(f"Error on f1 with {text}")
        return

What logic can you add to trigger this bomb correctly? Work one bomb at a time.

Discussion with TA:

• What is the trigger for each string bomb?
• How did you write code to trigger the bombs?

Make a string

10 minutes

Write a fuction called make_a_string() that takes a list of strings and puts all of these strings into one string. It does this by capitalizing all the strings in the list and concatenating them with a dash in between. For example when called like this:

    the_list = ['yer','a','wizard','harry']
    print(make_a_string(the_list))

then make_a_string() should return 'YER-A-WIZARD-HARRY'.

You can find starter code in make_string.py.

Discussion with TA:

• How did you make sure the dashes get put in the right place?

Hint, see the collect words() example in the guide on strings.

Password checker

20 minutes

Write a function that checks whether a password is a “good” password. A password is good only if it:

• has at least 8 characters
• has at least one lowercase character
• has at least one uppercase character
• has at least one number

For example this

print(check_password('password'))

prints False and this

print(check_password('a87fRPesT'))

prints True.

There is starter code in password_checker.py and a set of examples that should alternate False and True.

To check whether a string has at least one lowercase letter, you can:

• set a variable, has_lowercase to False
• loop through the string checking every character
  • if you find a character that is lowercase, set has_lowercase to True

Discussion with TA:

• What is the algorithm to solve this problem?
• Share a solution and walk through it line-by-line

Note, you will see that this password checker doesn’t particularly ensure that passwords are strong, meaning difficult for an attacker to break. The latest advice from security experts is for websites to:

• require only a minimum length (e.g. at least 8 characters)
• check whether your password is in a list of known breaches from sites that have been hacked (and then have you change it if it is found)
• check whether they can easily guess your password (and have you pick a stronger one if you have a weak password)

Grading

To finish this lab and receive a grade, take the canvas quiz.

Solution