Dictionaries and key-value pairs in Intro to Computing - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a dictionary in computing?

A dictionary is a collection that stores data in pairs called key-value pairs. Each key is unique and is used to find its matching value, like looking up a word in a real dictionary to find its meaning.

beginner

Explain the term 'key' in a dictionary.

A key is a unique identifier used to access a value in a dictionary. Think of it like a label on a box that tells you what's inside.

beginner

How does a dictionary differ from a list?

A dictionary uses keys to find values, while a list uses numbered positions (indexes). It's like a phone book (dictionary) versus a numbered seating chart (list).

intermediate

What happens if you try to use a key that does not exist in a dictionary?

You get an error or a special response saying the key is not found, similar to looking up a word that isn't in a dictionary.

intermediate

Can keys in a dictionary be changed after creation?

No, keys are fixed and must be unique. However, the values they point to can be changed. It's like a label on a jar—you can't change the label, but you can change what's inside the jar.

What does a dictionary store?

A Pairs of keys and values

B Only values

C Only keys

D Ordered list of items

A dictionary stores data as pairs of keys and values, where each key is unique.

Which of these is a valid key in a dictionary?

A An index number

B A unique label

C A repeated value

D A number only

Keys must be unique labels to identify values in a dictionary.

If you want to find a value in a dictionary, you use the:

A Value

B Index

C Key

D Position

You use the key to look up the matching value in a dictionary.

What happens if you try to access a key that does not exist?

A You get an error or 'not found' message

B You get the first value

C You get a random value

D The dictionary adds the key automatically

Accessing a non-existent key usually causes an error or a 'not found' response.

Can dictionary keys be changed after they are created?

A Keys can be changed only if values are numbers

B Yes, keys can be changed anytime

C Keys change automatically

D No, keys cannot be changed

Keys are fixed and cannot be changed once set; only values can be updated.

Describe what a dictionary is and how it stores data.

Think about how a phone book or a real dictionary works.

Explain the difference between a dictionary and a list.

Compare a phone book to a numbered seating chart.

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