Lab 1: Functions
Due by 11:59pm on Wednesday, September 4.
Starter Files
Download lab01.zip.
Required Questions
Review
Using no commandline options will run the code in the file you provide and return you to the command line. If your file just contains function definitions, you'll see no output unless there is a syntax error.
python3 lab00.py
i
: Thei
option runs the code in the file you provide, then opens an interactive session (with a>>>
prompt). You can then evaluate expressions, for example calling functions you defined. To exit, typeexit()
. You can also use the keyboard shortcutCtrlD
on Linux/Mac machines orCtrlZ Enter
on Windows.If you edit the Python file while running it interactively, you will need to exit and restart the interpreter in order for those changes to take effect.
Here's how we can run
lab00.py
interactively:python3 i lab00.py
m doctest
: Runs the doctests in a file, which are the examples in the docstrings of functions.Each test in the file consists of
>>>
followed by some Python code and the expected output.Here's how we can run the doctests in
lab00.py
:python3 m doctest lab00.py
When our code passes all of the doctests, no output is displayed. Otherwise, information about the tests that failed will be displayed.
To use Ok to test a function, run the following command (replacing FUNCTION
with the name of the function):
python3 ok q FUNCTION
If your function contains a call to print
that starts with "DEBUG:"
, then this line will be ignored by OK. (Otherwise, including extra print
calls can cause tests to fail because of the additional output displayed.)
print("DEBUG:", x)
There are more features described on the Using OK page. You can quickly generate most ok commands at okhelp.
True Division: / (decimal division) 
Floor Division: // (integer division) 
Modulo: % (remainder) 




A ZeroDivisionError
occurs when dividing by 0.
One useful technique involving the %
operator is to check
whether a number x
is divisible by another number y
:
x % y == 0
For example, in order to check if x
is an even number: x % 2 == 0
return
statement that provides
the value of the call expression used to call the function.
When Python executes a return
statement, the function call terminates
immediately. If Python reaches the end of the function body without executing
a return
statement, the function returns None
.
In contrast, the print
function is used to display values.
Unlike a return
statement, when Python evaluates a call to print
, the
function does not terminate immediately.
def what_prints():
print('Hello World!')
return 'Exiting this function.'
print('61A is awesome!')
>>> what_prints()
Hello World!
'Exiting this function.'
Notice also that
return
will preserve the quotes.
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 returnandprint u
>>> 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 debuggingquiz u
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 builtin pow
function to isolate a
particular digit of n
.
def digit(n, k):
"""Return the digit that is k from the right of n for positive integers n and k.
>>> digit(3579, 2)
5
>>> digit(3579, 0)
9
>>> digit(3579, 10)
0
"""
return ____
Use Ok to test your code:
python3 ok q digit
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.
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 ____
Use Ok to test your code:
python3 ok q middle
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 section of CS 61A, fill out this lab attendance and feedback form. (If you are in the mega section, 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!
After you've watched the lecture videos on Control (lecture 3), come back and try these practice problems! You're welcome to ask questions about them in this lab, a future lab, or office hours.
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
"""
"*** YOUR CODE HERE ***"
Use Ok to test your code:
python3 ok q falling
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 divisible by 7
>>> c
0
"""
"*** YOUR CODE HERE ***"
Use Ok to test your code:
python3 ok q divisible_by_k
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
"""
"*** YOUR CODE HERE ***"
Use Ok to test your code:
python3 ok q sum_digits
Q9: WWPD: What If?
Use Ok to test your knowledge with the following "What Would Python Display?" questions:
python3 ok q ifstatements u
Hint:
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
"""
"*** YOUR CODE HERE ***"
Use Ok to test your code:
python3 ok q double_eights