Data Structures and Algorithms

by Albert Major

1. Dictionaries

What is a Dictionary?

(recap)

• A dictionary is a collection of key-value pairs.

• Similar to lists, but instead of using index numbers, it uses keys.

• Example:

  my_dict = {"apple": 4, "pear": 5, "banana": 7}
  

• Keys can be any immutable type:

  • Numbers (integers, floats)
  • Strings
  • Tuples

• Values can be any data type.

Accessing Values

• Use square brackets to get a value:

  print(my_dict["apple"])  # Output: 4
  

• If a key doesn’t exist, Python raises a KeyError:

  print(my_dict["orange"])  # KeyError
  

Adding and Updating Values

• Adding a new key-value pair:

  my_dict["orange"] = 66
  

• Updating an existing value:

  my_dict["orange"] = 55  # Now the value is updated to 55
  

• Keys must be unique—you cannot have duplicate keys.

Difference Between Lists and Dictionaries

• Lists: Ordered and indexed by position (0,1,2…).