Lab 5: Sequences and Generators

Due at 11:59pm on 10/30/2016.

Starter Files

Download lab05.zip. Inside the archive, you will find starter files for the questions in this lab, along with a copy of the OK autograder.

Submission

By the end of this lab, you should have submitted the lab using ok. You may submit more than once before the deadline; only the final submission will be graded.

Generators

Question 1: Character generator

Write a generator that outputs each character of a string.

def char_gen(s):
    """
    >>> for char in char_gen("hello"):
    ...     print(char)
    ...
    h
    e
    l
    l
    o
    """
    "*** YOUR CODE HERE ***"
    for char in s:
        yield char

Use OK to test your code:

python ok -q char_gen --local

Question 2: Countdown

Write a generator that counts down to 0.

def countdown(n):
    """
    >>> from types import GeneratorType
    >>> type(countdown(0)) is GeneratorType # countdown is a generator
    True
    >>> for number in countdown(5):
    ...     print(number)
    ...
    5
    4
    3
    2
    1
    0
    """
    "*** YOUR CODE HERE ***"
    while n >= 0:
        yield n
        n = n - 1

Use OK to test your code:

python ok -q countdown --local

Question 3: Primes

Write a generator that generates prime numbers. Fill out the is_prime helper function and use that to create your generator.

def is_prime(n):
    """
    Return True if n is prime, false otherwise.

    >>> is_prime(1)
    False
    >>> is_prime(2)
    True
    >>> is_prime(19)
    True
    """
    "*** YOUR CODE HERE ***"
    if n < 2:
        return False
    counter = 2
    while counter <= sqrt(n):
        if n % counter == 0:
            return False
        counter += 1
    return True

def primes():
    """
    An infinite generator that outputs primes. 

    >>> p = primes()
    >>> for i in range(3):
    ...     print(next(p))
    ...
    2
    3
    5
    """
    "*** YOUR CODE HERE ***"
    num = 0
    while True:
        if is_prime(num):
            yield num
        num += 1

Use OK to test your code:

python ok -q is_prime --local

Use OK to test your code:

python ok -q primes --local

Question 4: Scale

Implement the generator function scale(s, k), which yields elements of the given iterable s, scaled by k.

def scale(s, k):
    """Yield elements of the iterable s scaled by a number k.

    >>> s = scale([1, 5, 2], 5)
    >>> type(s)
    <class 'generator'>
    >>> list(s)
    [5, 25, 10]

    >>> m = scale(naturals(), 2)
    >>> [next(m) for _ in range(5)]
    [2, 4, 6, 8, 10]
    """
    "*** YOUR CODE HERE ***"
    for elem in s:
        yield elem * k

Use OK to test your code:

python ok -q scale --local

Sequences

Question 5: Count palindromes

def count_palindromes(L):
    """The number of palindromic words in the sequence of strings
    L (ignoring case).

    >>> count_palindromes(("Acme", "Madam", "Pivot", "Pip"))
    2
    """
    "*** YOUR CODE HERE ***"

Hint: The easiest way to get the reversed version of a string s is to use the Python slicing notation trick s[::-1]. Also, the function lower, when called on strings, converts all of the characters in the string to lowercase. For instance, if the variable s contains the string "PyThoN", the expression s.lower() evaluates to "python".

Use OK to test your code:

python ok -q count_palindromes --local