Lab 1 Solutions
Quick Logistics Review
Using Python
When running a Python file, you can use flags on the command line to inspect your code further. Here are a few that will come in handy. If you want to learn more about other Python flags, take a look at the documentation.
Using no flags will run the code in the file you provide and return you to the command line.
python3 lab01.py
i
: Thei
option runs your Python script, then opens an interactive session. To exit, typeexit()
into the interpreter prompt. 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.
python3 i lab01.py
m doctest
: Runs doctests in a particular file. Doctests are marked by triple quotes ("""
) and are usually located within functions.python3 m doctest lab01.py
Using OK
In CS 88, we use a program called Ok for autograding labs, homeworks, and projects. You should have Ok in the starter files downloaded at the start of this lab. For more information on using Ok commands, learn more here. To use Ok to run doctests for a specified function, run the following command:
python3 ok q <specified function>
By default, only tests that did not pass will show up. You can use the v
option to show all tests, including tests you have passed:
python3 ok v
You can also use the debug printing feature in OK by writing
print("DEBUG:", x)
Finally, when you have finished all the questions in
lab01.py, you must submit the assignment using the
submit
option:
python3 ok submit
For more Ok commands, visit here.
Topics
Consult this section if you need a refresher on the material for this lab. It's okay to skip directly to the questions and refer back here should you get stuck.
Division
Let's compare the different divisionrelated operators in Python:
True Division (decimal division) The / Operator 
Floor Division (integer division) The // Operator 
Modulo (similar to a remainder) The % Operator 




Note that floor division and modulo both return an integer while true division always returns a floating point number.
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
Functions
If we want to execute a series of statements over and over, we can abstract them away into a function to avoid repeating code.
For example, let's say we want to know the results of multiplying the numbers 13 by 3 and then adding 2 to it. Here's one way to do it:
>>> 1 * 3 + 2
5
>>> 2 * 3 + 2
8
>>> 3 * 3 + 2
11
If we wanted to do this with a larger set of numbers, that'd be a lot of repeated code! Let's write a function to capture this operation given any input number.
def foo(x):
return x * 3 + 2
This function, called foo
, takes in a single argument and will return the
result of multiplying that argument by 3 and adding 2.
Now we can call this function whenever we want this operation to be done:
>>> foo(1)
5
>>> foo(2)
8
>>> foo(1000)
3002
Applying a function to some arguments is done with a call expression.
Call expressions
A call expression applies a function, which may or may not accept arguments. The call expression evaluates to the function's return value. It has a familiar notation borrowed from Math.
The syntax of a function call:
add ( 2 , 3 )
  
operator operand operand
Every call expression requires a set of parentheses delimiting its commaseparated operands.
To evaluate a function call:
 Evaluate the operator
 Evaluate the operands (from left to right).
 Apply the operator to the operands (the values of the operands).
If an operand is a nested call expression, then these two steps are applied to that operand in order to evaluate it.
return
and print
Most functions that you define will contain a return
statement. The return
statement will give the result of some computation back to the caller of the
function and exit the function. For example, the function square
below takes
in a number x
and returns its square.
def square(x):
"""
>>> square(4)
16
"""
return x * x
When Python executes a return
statement, the function terminates immediately.
If Python reaches the end of the function body without executing a return
statement, it will automatically return None
.
In contrast, the print
function is used to display values in the Terminal.
This can lead to some confusion between print
and return
because calling a
function in the Python interpreter will print out the function's return value.
However, unlike a return
statement, when Python evaluates a print
expression, the function does not terminate immediately.
def what_prints():
print('Hello World!')
return 'Exiting this function.'
print('CS 88 is awesome!')
>>> what_prints()
Hello World!
'Exiting this function.'
Notice also that
return
will preserve the quotes.
Control
Boolean Operators
You briefly encountered Boolean operators last week, but here you will see there is a close connection between them and conditionals.
Python supports three boolean operators: and
, or
, and not
:
>>> a = 4
>>> a < 2 and a > 0
False
>>> a < 2 or a > 0
True
>>> not (a > 0)
False
and
evaluates toTrue
only if both operands evaluate toTrue
. If at least one operand isFalse
, thenand
evaluates toFalse
.or
evaluates toTrue
if at least one operand evaluates toTrue
. If all operands areFalse
, thenor
evaluates toFalse
.
What do you think the following expression evaluates to? Try it out in the Python interpreter.
>>> True and not False or not True and False
It is difficult to read complex expressions, like the one above, and understand how a program will behave. Using parentheses can make your code easier to understand. Just so you know, Python interprets that expression in the following way:
>>> (True and (not False)) or ((not True) and False)
This is because boolean operators, like arithmetic operators, have an order of operation:
not
has the highest priorityand
or
has the lowest priority
To make your code more readable, and
and or
work on more than just booleans (True
,
False
). Other Python values can be considered "falsey," including 0
,
None
, and ''
(the empty string). All other values are considered "truthy."
Short Circuiting
What do you think will happen if we type the following into Python?
1 / 0
Try it out in Python! You should see a ZeroDivisionError
. But what about this expression?
True or 1 / 0
It evaluates to True
because Python's and
and or
operators shortcircuit. That is, they don't necessarily evaluate every operand.
Operator  Evaluates from left to right until:  Example 

AND  The first "falsey" value  False and 1 / 0 evaluates to False 
OR  The first "truthy" value  True or 1 / 0 evaluates to True 
If and
and or
do not shortcircuit, they just return the last
value. This means that and
and or
don't always return booleans when using truthy
and falsey values.
Here's a little example:
def divides(x, y):
"""
return True if x divides y
"""
return x and y % x == 0
If Statements
You can review the syntax of if
statements in
Section 1.5.4
of Composing Programs.
Tip: We sometimes see code that looks like this:
if x > 3: return True else: return False
This can be written more concisely as
return x > 3
. If your code looks like the code above, see if you can rewrite it more clearly!
While Loops
You can review the syntax of while
loops in
Section 1.5.5
of Composing Programs.
Error Messages
By now, you've probably seen a couple of error messages. They might look intimidating, but error messages are very helpful for debugging code. The following are some common types of errors:
Error Types  Descriptions 

SyntaxError  Contained improper syntax (e.g. missing a colon after an if statement or forgetting to close parentheses/quotes) 
IndentationError  Contained improper indentation (e.g. inconsistent indentation of a function body) 
TypeError  Attempted operation on incompatible types (e.g. trying to add a function and a number) or called function with the wrong number of arguments 
ZeroDivisionError  Attempted division by zero 
Using these descriptions of error messages, you should be able to get a better idea of what went wrong with your code. If you run into error messages, try to identify the problem before asking for help. You can often Google unfamiliar error messages to see if others have made similar mistakes to help you debug.
For example:
>>> square(3, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: square() takes 1 positional argument but 2 were given
Note:
 The last line of an error message tells us the type of the error. In the
example above, we have a
TypeError
.  The error message tells us what we did wrong — we gave
square
2 arguments when it can only take in 1 argument. In general, the last line is the most helpful.  The second to last line of the error message tells us on which line the
error occurred. This helps us track down he error. In the example above,
TypeError
occurred atline 1
.
Required Questions
What Would Python Display (Part 1)?
Question 1: WWPD: Control
Use Ok to test your knowledge with the following "What Would Python Display?" questions:
python3 ok q control u
>>> def xk(c, d):
... if c == 4:
... return 6
... elif d >= 4:
... return 6 + 7 + c
... else:
... return 25
>>> xk(10, 10)
______23
>>> xk(10, 6)
______23
>>> xk(4, 6)
______6
>>> xk(0, 0)
______25
>>> def how_big(x):
... if x > 10:
... print('huge')
... elif x > 5:
... return 'big'
... elif x > 0:
... print('small')
... else:
... print("nothin'")
>>> how_big(7)
______'big'
>>> how_big(12)
______huge
>>> how_big(1)
______small
>>> how_big(1)
______nothin'
>>> n = 3
>>> while n >= 0:
... n = 1
... print(n)
______2
1
0
1
Hint: Make sure your
while
loop conditions eventually evaluate to a false value, or they'll never stop! TypingCtrlC
will stop infinite loops in the interpreter.
>>> positive = 28
>>> while positive:
... print("positive?")
... positive = 3
______Infinite Loop
>>> positive = 9
>>> negative = 12
>>> while negative:
... if positive:
... print(negative)
... positive += 3
... negative += 3
______12
9
6
Question 2: WWPD: Veritasiness
Use Ok to test your knowledge with the following "What Would Python Display?" questions:
python3 ok q short_circuiting u
>>> True and 13
______13
>>> False or 0
______0
>>> not 10
______False
>>> not None
______True
>>> True and 1 / 0 and False
______Error (ZeroDivisionError)
>>> True or 1 / 0 or False
______True
>>> True and 0
______0
>>> False or 1
______1
>>> 1 and 3 and 6 and 10 and 15
______15
>>> 0 or False or 2 or 1 / 0
______2
>>> not 0
______True
>>> (1 + 1) and 1
______1
>>> 1/0 or True
______Error
>>> (True or False) and False
______False
Coding Practice
Question 3: Fix the Bug
The following snippet of code doesn't work! Figure out what is wrong and fix the bugs.
def both_positive(x, y):
"""Returns True if both x and y are positive.
>>> both_positive(1, 1)
False
>>> both_positive(1, 1)
True
"""
return x and y > 0 # You can replace this line!
return x > 0 and y > 0
The original line (return x and y > 0
) will check that two things are
true:
x
y > 0
When will x
be considered True? In Python, any number that is not 0
is considered True. Thus, the first doctest will fail: x = 1
and 1 != 0
, and y = 1 > 0
, so both clauses are True.
Use OK to test your code:
python3 ok q both_positive
Question 4: 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(n):
"""Sum all the digits of n.
>>> sum_digits(10) # 1 + 0 = 1
1
>>> sum_digits(4224) # 4 + 2 + 2 + 4 = 12
12
>>> sum_digits(1234567890)
45
>>> x = sum_digits(123) # make sure that you are using return rather than print
>>> x
6
"""
"*** YOUR CODE HERE ***"
total = 0
while n > 0:
total, n = total + n % 10, n // 10
return total
Use OK to test your code:
python3 ok q sum_digits
Optional Questions
What Would Python Display (Part 2)?
Question 5: WWPD: What If?
Use Ok to test your knowledge with the following "What Would Python Display?" questions:
python3 ok q what_if 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'
More Coding Practice
Question 6: 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.
If k
is larger than n, only multiply up to n consecutive numbers!
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 ***"
total, stop = 1, nk
while n > stop:
total, n = total*n, n1
return total
Use OK to test your code:
python3 ok q falling
Question 7: 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 ***"
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
Use OK to test your code:
python3 ok q double_eights