Understanding
Dictionaries in Programming

What is a Dictionary?

A dictionary is a data structure that stores data in the form of key-value pairs. Each key in the dictionary is unique and is used to access its corresponding value. Think of a dictionary like a real-world glossary or contact list: you look up a word (key) and find its meaning (value).

Why Use Dictionaries?

• Fast lookups using keys.
• Efficient for storing associated data (e.g., ID → Name).
• Dynamic size – can grow as you add entries.

Example 1: Creating a Dictionary

# Create an empty dictionary and add key-value pairs
dictionary = {}

dictionary["name"] = "Alice"
dictionary["age"] = 30
dictionary["city"] = "Wonderland"

print(dictionary)

Output:

{"name": "Alice", "age": 30, "city": "Wonderland"}

How does a dictionary store its data?

A dictionary uses a special structure called a hash table internally. This allows it to locate values very quickly using the keys.

Question:

Can dictionary keys be repeated?

Answer: No, keys must be unique. If you assign a new value to an existing key, it will overwrite the previous value.

Example 2: Accessing and Modifying Values

# Access a value using its key
name = dictionary["name"]
print(name)

# Modify an existing value
dictionary["age"] = 31
print(dictionary["age"])

Output:

Alice
31

Example 3: Removing Keys

# Remove a key-value pair
remove dictionary["city"]

print(dictionary)

Output:

{"name": "Alice", "age": 31}

Question:

What happens if you try to access a key that doesn’t exist?

Answer: It will usually result in an error or a null/undefined value depending on the language or environment. Some implementations allow safe access with default values.

Example 4: Iterating Over a Dictionary

# Iterate over keys and values
for each key in dictionary:
    value = dictionary[key]
    print(key + ": " + toString(value))

Output:

name: Alice
age: 31

Use-Cases of Dictionaries

• Storing configuration settings (e.g., {"theme": "dark", "language": "en"})
• Mapping IDs to user data
• Counting occurrences of elements

Question:

Are dictionaries ordered?

Answer: In many modern implementations, yes – items retain the order they were added. But in some older or lower-level systems, order is not guaranteed.

Summary

• Dictionaries store data as key-value pairs.
• They allow fast access and updates using keys.
• Keys must be unique and typically immutable (e.g., strings, numbers).

Practice Challenge

Create a dictionary that stores student names as keys and their test scores as values. Then:

• Update one of the scores
• Remove one student
• Print the final list