Dictionaries

Overview

Teaching: 15 min
Exercises: 15 min

Questions

• What is a dictionary, and how do I use it?

Objectives

• Explain how dictionaries work.

• Learn about dictionary operations.

A dictionary allows you to keep data associated to a custom key

• Different from lists
  • You access elements in lists by their position, but in a dictionary the keys don’t need to be ordered.
• Different from sets
  • Sets contain only unique values with no associated keys, while dictionaries map keys to values.
• A very fast data structure
  • Lookups, insertions, and deletions in dictionaries are very fast (average-case O(1)).

Basic Operations

• Creation: Use curly braces {} or the dict() constructor.
• Access: Use square brackets (dict[key]) or the get() method.
• Modification: Add or update entries using assignment (dict[key] = value).
• Deletion: Use the del statement or the pop() method.
• Keys: Use dict.keys() to get all keys in the dictionary.
• Values: Use dict.values() to get all values in the dictionary.

# A simple example of a dictionary
# The keys can be strings or numbers (ints or floats)
# With the values in the dictionary you have a bit more freedom 
data = {'a': "Hello", 'b': 2, 'c': ["apple", "banana", "cherry"]}
print(len(data))         # Output: 3
print(data['a'])         # Output: Hello

Example: DNA Codon Table

Consider a simplified DNA codon table where each codon (a triplet of nucleotides) maps to an amino acid.

# Example input here
codon_to_amino = {
    "ATG": "Methionine",
    "TTT": "Phenylalanine",
    "TTC": "Phenylalanine",
    "TAA": "Stop",
    "TAG": "Stop",
    "TGA": "Stop"
}
# Access amino acid for codon 'ATG'
print("Codon ATG codes for:", codon_to_amino["ATG"])

Codon ATG codes for: Methionine

Initialising

What does the following program print?

codon_dict = {"ATG": "Methionine", "TAA": "Stop", "TAG": "Stop"}
codon_dict["ATG"] = "Start"  # Change: ATG now maps to Start
codon_dict["TGA"] = "Stop"
print("Updated codon dictionary:", codon_dict)

Hint: The order of keys may vary.

Fill in the Blanks

Fill in the blanks so that the program below retrieves the correct amino acid for the given codon.

codon_translation = {"GGT": "Glycine", "GGC": "Glycine", "GGA": "Glycine", "GGG": "Glycine"}
amino_acid = codon_translation[______]
print("Amino acid for GGT:", amino_acid)

Expected output: Amino acid for GGT: Glycine

Adding a New Codon

Extend the dictionary by adding the codon "CCC" for "Proline" to the dictionary below.

codon_dict = {"ATG": "Methionine", "TAA": "Stop"}
# Add code here
print(codon_dict)

Expected output: {‘ATG’: ‘Methionine’, ‘TAA’: ‘Stop’, ‘CCC’: ‘Proline’}

Key Points

• A dictionary stores values accessible by unique keys.

• Dictionaries may contain values of different types.