Solution Files

Attendance

If you are in a regular lab in CS 61A, get an arrival number from your TA and then fill out this lab attendance form. (If you are in the mega lab, you don't need to fill out the form.) You need to submit the lab problems in addition to the attendance form to get credit for lab.

Required Questions

Review

Important: If the python3 command doesn't work, please try using python or py.

python3 ok -q FUNCTION

print("DEBUG:", x)

>>> 1 / 5
0.2

>>> 25 / 4
6.25

>>> 4 / 2
2.0

>>> 5 / 0
ZeroDivisionError

>>> 1 // 5 # truncate result of true division
0

>>> 25 // 4
6

>>> 4 // 2
2

>>> 5 // 0
ZeroDivisionError

>>> 1 % 5
1

>>> 25 % 4
1

>>> 4 % 2
0

>>> 5 % 0
ZeroDivisionError

x % y == 0

def what_prints():
    print('Hello World!')
    return 'Exiting this function.'
    print('This course is awesome!')

>>> what_prints()
Hello World!
'Exiting this function.'

What Would Python Display? (WWPD)

Q1: Return and Print

Use Ok to test your knowledge with the following "What Would Python Display?" questions:

python3 ok -q return-and-print -u
Copy
✂️

>>> def welcome():
...     print('Go')
...     return 'hello'
...
>>> def cal():
...     print('Bears')
...     return 'world'
...
>>> welcome()
______
Go
'hello'
>>> print(welcome(), cal())
______
Go
Bears
hello world

Write Code

Q2: Debugging Quiz

The following is a quick quiz on different debugging techniques that will be helpful for you to use in this class. You can refer to the debugging article to answer the questions.

Use Ok to test your understanding:

python3 ok -q debugging-quiz -u
Copy
✂️

Q3: Pick a Digit

Implement digit, which takes positive integers n and k and has only a single return statement as its body. It returns the digit of n that is k positions to the left of the rightmost digit (the one's digit). If k is 0, return the rightmost digit. If there is no digit of n that is k positions to the left of the rightmost digit, return 0.

Hint: Use // and % and the built-in pow function to isolate a particular digit of n.

def digit(n, k):
    """Return the k-th digit from the right of n for positive integers n and k.

    >>> digit(3579, 2)
    5
    >>> digit(3579, 0)
    9
    >>> digit(3579, 10)
    0
    """
    return n // pow(10, k) % 10

Use Ok to test your code:

python3 ok -q digit
Copy
✂️

Q4: Middle Number

Implement middle by writing a single return expression that evaluates to the value that is neither the largest or smallest among three different integers a, b, and c.

Hint: Try combining all the numbers and then taking away the ones you don't want to return by using the built-in min and max functions.

def middle(a, b, c):
    """Return the number among a, b, and c that is not the smallest or largest.
    Assume a, b, and c are all different numbers.

    >>> middle(3, 5, 4)
    4
    >>> middle(30, 5, 4)
    5
    >>> middle(3, 5, 40)
    5
    >>> middle(3, 5, 40)
    5
    >>> middle(30, 5, 40)
    30
    """
    return a + b + c - min(a, b, c) - max(a, b, c)

Use Ok to test your code:

python3 ok -q middle
Copy
✂️

Syllabus Quiz

Q5: Syllabus Quiz

Please fill out the Syllabus Quiz, which confirms your understanding of the policies on the syllabus page (linked in the toolbar above).

Check Your Score Locally

You can locally check your score on each question of this assignment by running

python3 ok --score

This does NOT submit the assignment! When you are satisfied with your score, submit the assignment to Gradescope to receive credit for it.

Submit Assignment

If you are in a regular lab in CS 61A, let your TA know you're done so that you can get a departure number, then fill out this lab attendance form before you leave. (If you are in the mega lab, you don't need to fill out the form.)

Then, submit this assignment by uploading any files you've edited to the appropriate Gradescope assignment. Lab 00 has detailed instructions.

Optional Questions

These questions are optional. If you don't complete them, you will still receive credit for this assignment. They are great practice, so do them anyway!

You really should work on these today! They are marked as optional only because some students just joined the course and are getting caught up, but students who have already attended the first three lectures should be well prepared to solve these, and Homework 1 will be much smoother with the experience of solving these.

Q6: Falling Factorial

Let's write a function falling, which is a "falling" factorial that takes two arguments, n and k, and returns the product of k consecutive numbers, starting from n and working downwards. When k is 0, the function should return 1.

def falling(n, k):
    """Compute the falling factorial of n to depth k.

    >>> falling(6, 3)  # 6 * 5 * 4
    120
    >>> falling(4, 3)  # 4 * 3 * 2
    24
    >>> falling(4, 1)  # 4
    4
    >>> falling(4, 0)
    1
    """
    total, stop = 1, n-k
    while n > stop:
        total, n = total*n, n-1
    return total

Use Ok to test your code:

python3 ok -q falling
Copy
✂️

Q7: Divisible By k

Write a function divisible_by_k that takes positive integers n and k. It prints all positive integers less than or equal to n that are divisible by k from smallest to largest. Then, it returns how many numbers were printed.

def divisible_by_k(n, k):
    """
    >>> a = divisible_by_k(10, 2)  # 2, 4, 6, 8, and 10 are divisible by 2
    2
    4
    6
    8
    10
    >>> a
    5
    >>> b = divisible_by_k(3, 1)  # 1, 2, and 3 are divisible by 1
    1
    2
    3
    >>> b
    3
    >>> c = divisible_by_k(6, 7)  # There are no integers up to 6 that are divisible by 7
    >>> c
    0
    """
    count = 0
    i = 1
    while i <= n:
        if i % k == 0:
            print(i)
            count += 1
        i += 1
    return count

Use Ok to test your code:

python3 ok -q divisible_by_k
Copy
✂️

Q8: Sum Digits

Write a function that takes in a nonnegative integer and sums its digits. (Using floor division and modulo might be helpful here!)

def sum_digits(y):
    """Sum all the digits of y.

    >>> sum_digits(10) # 1 + 0 = 1
    1
    >>> sum_digits(4224) # 4 + 2 + 2 + 4 = 12
    12
    >>> sum_digits(1234567890)
    45
    >>> a = sum_digits(123) # make sure that you are using return rather than print
    >>> a
    6
    """
    total = 0
    while y > 0:
        total, y = total + y % 10, y // 10
    return total

Use Ok to test your code:

python3 ok -q sum_digits
Copy
✂️

Q9: WWPD: What If?

Use Ok to test your knowledge with the following "What Would Python Display?" questions:

python3 ok -q if-statements -u
Copy
✂️

Hint: print (unlike return) does not cause the function to exit.

>>> def ab(c, d):
...     if c > 5:
...         print(c)
...     elif c > 7:
...         print(d)
...     print('foo')
>>> ab(10, 20)
______
10
foo

>>> def bake(cake, make):
...     if cake == 0:
...         cake = cake + 1
...         print(cake)
...     if cake == 1:
...         print(make)
...     else:
...         return cake
...     return make
>>> bake(0, 29)
______
1
29
29
>>> bake(1, "mashed potatoes")
______
mashed potatoes
'mashed potatoes'

Q10: Double Eights

Write a function that takes in a number and determines if the digits contain two adjacent 8s.

def double_eights(n):
    """Return true if n has two eights in a row.
    >>> double_eights(8)
    False
    >>> double_eights(88)
    True
    >>> double_eights(2882)
    True
    >>> double_eights(880088)
    True
    >>> double_eights(12345)
    False
    >>> double_eights(80808080)
    False
    """
    prev_eight = False
    while n > 0:
        last_digit = n % 10
        if last_digit == 8 and prev_eight:
            return True
        elif last_digit == 8:
            prev_eight = True
        else:
            prev_eight = False
        n = n // 10
    return False

# Alternate solution
def double_eights_alt(n):
    while n:
        if n % 10 == 8 and n // 10 % 10 == 8:
            return True
        n //= 10
    return False

Use Ok to test your code:

python3 ok -q double_eights
Copy
✂️