Discussion 4: Recursion🖨️

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Q1: Warm Up

What is the value of result after executing result = (lambda x: 2 * (lambda x: 3)(4) * x)(5)? Talk about it with your whole group and make sure you all agree before anybody checks the answer.

Recursion

Many students find this discussion challenging. Everything gets easier with practice. Please help each other learn. It's also long. If you don't finish, don't worry.

VERY IMPORTANT: In this discussion, don't check your answers until your whole group is sure that the answer is right. Figure things out and check your work by thinking about what your code will do. Your goal should be to have all answers right the first time! If you need help, ask.

Q2: Swipe

Implement swipe, which prints the digits of argument n, one per line, first backward then forward. The left-most digit is printed only once. Do not use while or for or str. (Use recursion, of course!)

def swipe(n):
    """Print the digits of n, one per line, first backward then forward.

    >>> swipe(2837)
    7
    3
    8
    2
    8
    3
    7
    """
    if n < 10:
        print(n)
    else:
        "*** YOUR CODE HERE ***"

Hint

First print the first line of the output, then make a recursive call, then print the last line of the output.

Q3: Is Prime

Implement is_prime that takes an integer n greater than 1. It returns True if n is a prime number and False otherwise. Try following the approach below, but implement it recursively without using a while (or for) statement.

You will need to define another "helper" function (a function that exists just to help implement this one). Does it matter whether you define it within is_prime or as a separate function in the global frame? Try to define it to take as few arguments as possible.

def is_prime(n):
    """Returns True if n is a prime number and False otherwise.
    >>> is_prime(2)
    True
    >>> is_prime(16)
    False
    >>> is_prime(521)
    True
    """
    "*** YOUR CODE HERE ***"

Hint

Define an inner function that checks whether some integer between i and n evenly divides n. Then you can call it starting with i=2:

def is_prime(n):
    def f(i):
        if i == n:
            return ____
        elif ____:
            return ____
        else:
            return f(____)
    return f(2)

Recommended, but more challenging (if there's time)

Q4: Sevens

The Game of Sevens: Players in a circle count up from 1 in the clockwise direction. (The starting player says 1, the player to their left says 2, etc.) If a number is divisible by 7 or contains a 7 (or both), switch directions. Numbers must be said on the beat at 60 beats per minute. If someone says a number when it's not their turn or someone misses the beat on their turn, the game ends.

For example, 5 people would count to 20 like this:

Player 1 says 1
Player 2 says 2
Player 3 says 3
Player 4 says 4
Player 5 says 5
Player 1 says 6  # All the way around the circle
Player 2 says 7  # Switch to counterclockwise
Player 1 says 8
Player 5 says 9  # Back around the circle counterclockwise
Player 4 says 10
Player 3 says 11
Player 2 says 12
Player 1 says 13
Player 5 says 14 # Switch back to clockwise
Player 1 says 15
Player 2 says 16
Player 3 says 17 # Switch back to counterclockwise
Player 2 says 18
Player 1 says 19
Player 5 says 20

Play a few games with your group.

Then, implement sevens which takes a positive integer n and a number of players k. It returns which of the k players says n. You may call has_seven.

An effective approach to this problem is to simulate the game, stopping on turn n. The implementation must keep track of the final number n, the current number i, the player who will say i, and the current direction that determines the next player (either increasing or decreasing). It works well to use integers to represent all of these, with direction switching between 1 (increase) and -1 (decreasing).

def sevens(n, k):
    """Return the (clockwise) position of who says n among k players.

    >>> sevens(2, 5)
    2
    >>> sevens(6, 5)
    1
    >>> sevens(7, 5)
    2
    >>> sevens(8, 5)
    1
    >>> sevens(9, 5)
    5
    >>> sevens(18, 5)
    2
    """
    def f(i, who, direction):
        if i == n:
            return who
        "*** YOUR CODE HERE ***"
    return f(1, 1, 1)

def has_seven(n):
    if n == 0:
        return False
    elif n % 10 == 7:
        return True
    else:
        return has_seven(n // 10)

Hint

First check if i is a multiple of 7 or contains a 7, and if so, switch directions. Then, add the direction to who and ensure that who has not become smaller than 1 or greater than k.