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.

Practice

1. What is a dictionary in computing fundamentals?

easy

A. A sequence of characters forming a word

B. A list of numbers sorted in order

C. A type of loop used for iteration

D. A collection of key-value pairs for storing data

Solution

1. Step 1: Understand the definition of a dictionary

   A dictionary stores data as pairs where each key is linked to a value.

2. Step 2: Compare with other data types

   Unlike lists or strings, dictionaries use keys to quickly find values.

3. Final Answer:

   A collection of key-value pairs for storing data -> Option D

4. Quick Check:

   Dictionary = key-value pairs [OK]

Hint: Remember: dictionary = keys + values [OK]

Common Mistakes:

• Confusing dictionary with list or string
• Thinking dictionary is a type of loop
• Believing dictionary stores only numbers

2. Which of the following is the correct way to create a dictionary with keys 'name' and 'age'?

easy

A. ['name' = 'Alice', 'age' = 30]

B. {'name': 'Alice', 'age': 30}

C. {'name' -> 'Alice', 'age' -> 30}

D. ('name': 'Alice', 'age': 30)

Solution

1. Step 1: Identify correct dictionary syntax

   Dictionaries use curly braces {} with key:value pairs separated by commas.

2. Step 2: Check each option

   {'name': 'Alice', 'age': 30} uses correct syntax with colons and curly braces; others use wrong symbols or brackets.

3. Final Answer:

   {'name': 'Alice', 'age': 30} -> Option B

4. Quick Check:

   Dictionary syntax = curly braces + colons [OK]

Hint: Use curly braces and colons for dictionaries [OK]

Common Mistakes:

• Using square brackets instead of curly braces
• Using '=' or '->' instead of ':'
• Using parentheses instead of braces

3. What will be the output of the following code?

person = {'name': 'Bob', 'age': 25}
print(person['age'])

medium

A. Bob

B. Error

C. 25

D. 'age'

Solution

1. Step 1: Understand dictionary access by key

   Accessing person['age'] retrieves the value linked to key 'age'.

2. Step 2: Identify the value for key 'age'

   The value stored is 25, so print(person['age']) outputs 25.

3. Final Answer:

   25 -> Option C

4. Quick Check:

   person['age'] = 25 [OK]

Hint: Keys fetch their values directly from dictionary [OK]

Common Mistakes:

• Printing the key name instead of value
• Confusing key with value
• Expecting the whole dictionary to print

4. Identify the error in this dictionary code:

data = {'x': 10, 'y': 20, 'x': 30}
print(data['x'])

medium

A. Duplicate keys cause the last value to overwrite earlier ones

B. Syntax error due to missing comma

C. Keys must be numbers, not strings

D. Cannot print dictionary values directly

Solution

1. Step 1: Check for duplicate keys in dictionary

   The key 'x' appears twice; dictionaries keep only the last value for duplicates.

2. Step 2: Understand output of print statement

   Printing data['x'] shows 30, the last assigned value, overwriting 10.

3. Final Answer:

   Duplicate keys cause the last value to overwrite earlier ones -> Option A

4. Quick Check:

   Duplicate keys overwrite previous values [OK]

Hint: Duplicate keys keep only last value [OK]

Common Mistakes:

• Thinking duplicate keys cause syntax error
• Expecting both values to be stored
• Believing keys must be numbers

5. Given a list of students and their scores:

students = [('Anna', 85), ('Ben', 92), ('Cara', 85), ('Dan', 92)]

Which dictionary comprehension creates a dictionary mapping each student to their score?

hard

A. {name: score for name, score in students}

B. {score: name for name, score in students}

C. {name: score if score > 90 else 0 for name, score in students}

D. {score: name if name.startswith('A') else 'Unknown' for name, score in students}

Solution

1. Step 1: Understand the goal

   Create a dictionary with student names as keys and scores as values.

2. Step 2: Analyze each comprehension

   {name: score for name, score in students} correctly maps name: score. {score: name for name, score in students} reverses keys and values, causing score keys to overwrite. Options C and D add conditions changing values or keys incorrectly.

3. Final Answer:

   {name: score for name, score in students} -> Option A

4. Quick Check:

   Keys = names, Values = scores [OK]

Hint: Keys are unique; use names as keys to avoid overwriting [OK]

Common Mistakes:

• Using scores as keys causing overwrites
• Adding conditions that change keys incorrectly
• Confusing key-value order in comprehension