Prelab 3 Notes

Here we discuss an interesting problem that has parallels with the work you are doing this week for Prelab 3 and Lab 3. There is nothing to turn in for this write-up – it is just for extra practice.

Building up strings

Many of our functions will use a similar pattern in which we build up a new string piece-by-piece by appending characters to a result string (initialized as the empty string or ""). We saw some examples of this pattern in class and will tackle another example here. Specifically let’s write a function named password_gen that takes a single parameter length and generates a random password (as a string) of length characters.

How could we implement our password generator? There are many ways, but a simple one is to use randint to index into a string of allowed characters.

Let’s solve this problem in several steps:

1. Define a constant CHARS with the allowed characters

    CHARS = "abcdefghijklmnopqrstuvwxyz0123456789_!@#$%^&*"
   

2. Create a version of password_gen to create a string of the specified length with a fixed character

    CHARS = "abcdefghijklmnopqrstuvwxyz0123456789_!@#$%^&*"
       
    def password_gen(length):
        result = ""
        for i in range(length):
            result = result + CHARS[0]
        return result
   

3. Enhance password_gen to create a string with random characters

    from random import randint
       
    CHARS = "abcdefghijklmnopqrstuvwxyz0123456789_!@#$%^&*"
       
    def password_gen(length):
        result = ""
        for i in range(length):
            result = result + CHARS[randint(0, len(CHARS)-1)]
        return result
   

   Recall that randint has inclusive end, and so to not exceed the length of CHARS we need to use len(CHARS)-1 as the end argument.

4. And finally add the finishing touches, e.g., docstrings.

    from random import randint
       
    CHARS = "abcdefghijklmnopqrstuvwxyz0123456789_!@#$%^&*"
       
    def password_gen(length):
        """
        Generate a random password
           
        Args:
            length: number of characters in the password
               
        Returns:
            Password string
        """
        result = ""
        for i in range(length):
            result = result + CHARS[randint(0, len(CHARS)-1)]
        return result