Drill 13 Solution

Question 1

Given this dictionary, key_to_value,

key_to_val = {}

key_to_val["a"] = 100
key_to_val["a"] = 500
key_to_val["b"] = 20

How many keys are inside the dict?

( ) Error: You cannot use the same key twice in a dictionary.
( ) 1
( ) 3
( ) 2

Answer

2
When we define key_to_val, it has 0 keys. We add "a" and it gets one key, then we update the value of "a", and it still has one key. We add "b" and it gets a second key.

Question 2

Suppose we have a list of names:

names = ["Joe", "Bob", "Mary", "Joe", "Bob", "Bob"]

Fill in the blank so that we create a dictionary that maps a
name to the number of times it appears in our list of names.

name_to_count = {} # our dict mapping names to count
for name in names:
 if name in ___________:
    name_to_count[name] = name_to_count[name] + 1
 else:
    name_to_count[name] = 1

Answer

name_to_count

Question 3

Given the following code:

dept = "CS"

prof_to_dept["Kathi"] = dept
prof_to_dept["Tim"] = "CS" 
prof_to_dept["Bob"] = "English" 

dept = "English"

What does prof_to_dept["Kathi"] evaluate to?

( ) "CS"
( ) "English"
( ) Error: Cannot change value in a dict.

Answer

"CS"

Question 4

Let's say we want to keep track of the courses that we're taking this semester at Brown.

student_to_courses = {}

leo_courses = ["CS111", "ENGL400"]
student_to_courses["Leo"] = leo_courses
student_to_courses["Naomi"] = ["CS111"]
student_to_courses["Jane"] = ["CS17"]

leo_courses.append("CS127")

What will the line of code student_to_courses["Leo"] evaluate to?

( ) ["CS111", "ENGL400"]
( ) ["CS127", "CS111", "ENGL400"]
( ) ["ENGL400", "CS127", "CS111"]
( ) ["CS111", "ENGL400", "CS127"]

Answer

["CS111", "ENGL400", "CS127"]

When leo_courses is updated, so is student_to_courses["Leo"], because they are two names for the same list.

Question 5

Let's say we want to keep track of the number of fruits in a shopping cart.

cart = {}

grape = 2
cart["apple"] = 1
cart["grape"] = grape
cart["melon"] = 3

grape = 10

What will the line of code cart["grape"] evaluate to?

( ) 10
( ) 2
( ) 3
( ) error: cannot change the value in a dictionary

Answer

2

The key "grape" is initially mapped to a value of 2. Even though the name grape changes to 10, it does not change the original key-value mapping of grape to 2.

Question 6

Suppose we have a list of fruits:

fruits = ["apple", "apple", "banana", "apple"]

Will the following piece of code successfully create a dictionary that maps each fruit in fruits to the number of times it appears in the list?

fruit_to_count = {} # our dict mapping fruits to count
for fruit in fruits:
    fruit_to_count[fruit] = 0
    fruit_to_count[fruit] += 1

( ) Yes
( ) No

Answer

No

Because the code does not check whether a fruit already exists in the dictionary, it overrides each fruit value to 0 and increments it by 1 in each iteration of the loop.

Question 7

Assume we have a dictionary that maps student names to their heights. Fill in the blanks in the following function that returns the maximum height in students.

def get_max_student_height(students: dict):
  max_height = 0
  for BLANK1 in BLANK2:
    max_height = max(max_height, students[name])
  return max_height

print(get_max_student_height({"rohan" : 70, "leo": 75})

Answer

BLANK1: name

BLANK2: students