Dictionaries and key-value pairs in Intro to Computing - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ trace

intermediate

Trace the dictionary update

What is the final content of the dictionary data after running the code below?

Intro to Computing

data = {'a': 1, 'b': 2}
data['c'] = data['a'] + data['b']
data['a'] = 5

A {'a': 5, 'b': 2, 'c': 7}

B {'a': 1, 'b': 2, 'c': 3}

C {'a': 5, 'b': 2, 'c': 3}

D {'a': 1, 'b': 2, 'c': 7}

💡 Hint

Remember that dictionary values can be updated after they are used to calculate other values.

The key 'c' is assigned the sum of values of 'a' and 'b' at that moment, which is 1 + 2 = 3. Then 'a' is updated to 5, but 'c' remains 3.

🧠 Conceptual

intermediate

Understanding dictionary keys

Which of the following can be used as a key in a dictionary?

A A string like 'name'

B A list like [1, 2, 3]

C A dictionary like {'x': 1}

D A set like {1, 2, 3}

💡 Hint

Dictionary keys must be immutable types.

Dictionary keys must be immutable (unchangeable). Strings are immutable, but lists, dictionaries, and sets are mutable and cannot be keys.

❓ Comparison

advanced

Compare dictionary creation methods

Which option creates a dictionary with keys 1, 2, 3 and values their squares?

A {x: x**2 for x in [1, 2, 3]}

B dict((x, x**2) for x in [1, 2, 3])

C {(x, x**2) for x in [1, 2, 3]}

D dict[x, x**2 for x in [1, 2, 3]]

💡 Hint

Look for the syntax that correctly creates key-value pairs.

Option B has invalid syntax (missing ':' after key 'x'). Option B uses the dict() constructor with a generator of (key, value) tuples, creating {1: 1, 2: 4, 3: 9}. Option B creates a set of tuples. Option B has invalid syntax.

❓ identification

advanced

Identify the error type

What error will this code raise?

my_dict = {'x': 10, 'y': 20}
value = my_dict['z']

Intro to Computing

my_dict = {'x': 10, 'y': 20}
value = my_dict['z']

A IndexError

B TypeError

C SyntaxError

D KeyError

💡 Hint

The key 'z' does not exist in the dictionary.

Accessing a dictionary with a key that does not exist raises a KeyError.

🚀 Application

expert

Count unique values in a dictionary

Given the dictionary data = {'a': 1, 'b': 2, 'c': 1, 'd': 3}, which code snippet correctly counts how many unique values are present?

Intro to Computing

data = {'a': 1, 'b': 2, 'c': 1, 'd': 3}

A len(set(data.values()))

B len(data)

C len(set(data.keys()))

D len(data.values())

💡 Hint

Keys and values are different; unique values means unique dictionary values.

To count unique values, convert the dictionary's values to a set (which removes duplicates) and count its length.