dictionary.md

Python Dictionary Notes

Definition and Characteristics

• Dictionaries store data in key-value pairs.
• They are collections that are:
  • Unordered (in Python < 3.7; ordered as of Python 3.7+).
  • Changeable (mutable; can be modified after creation).
  • Do not allow duplicate keys (latest value overrides earlier ones for duplicate keys).
• Syntax: Dictionaries are written using curly braces {}.

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Examples and Usage

Basic Structure

info = {
    "key": "value",
    "subject": ["math", "science", "english"],
    "topics": ("dictionaries", "list"),
    "name": "noushad",
    "learning": "python",
    "age": 24,
    "marks": 90,
    12.33: 2.3
}

Accessing Dictionary Items

• Use the key to access values:

  print(info["name"])  # Output: noushad
  print(info["topics"])  # Output: ('dictionaries', 'list')

Updating Values

• Modify an existing key's value:

  info["name"] = "noushad ramim"
  print(info["name"])  # Output: noushad ramim

Creating an Empty Dictionary

• Add key-value pairs to an initially empty dictionary:

  null_dict = {}
  null_dict["name"] = "noushad"
  print(null_dict)  # Output: {'name': 'noushad'}

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Nested Dictionaries

• A dictionary can contain another dictionary:

  student = {
      "name": "kazi jetu",
      "subject": {
          "math": 90,
          "science": 80,
          "math": 96  # Latest value is retained
      }
  }

• Access nested values:

  print(student["subject"]["math"])  # Output: 96

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Dictionary Methods

Keys and Values

• Retrieve all keys:

  print(student.keys())  # Output: dict_keys(['name', 'subject'])
  print(list(student.keys()))  # Convert keys to a list

• Retrieve all values:

  print(student.values())  # Output: dict_values(['kazi jetu', {'math': 96, 'science': 80}])
  print(list(student.values()))  # Convert values to a list

Key-Value Pairs

• Retrieve all key-value pairs:

  print(student.items())  # Output: dict_items([('name', 'kazi jetu'), ('subject', {...})])
  print(list(student.items()))  # Convert pairs to a list

• Access a specific pair:

  pairs = list(student.items())
  print(pairs[0])  # Output: ('name', 'kazi jetu')

Accessing Keys Safely

• Direct key access (throws error if the key doesn’t exist):

  print(student["name"])

• Safe key access using get() (returns None if key doesn’t exist):

  print(student.get("name"))  # Output: kazi jetu

Updating a Dictionary

• Add or update key-value pairs:

  student.update({"city": "dhaka"})
  new_dict = {"name": "noushad"}
  student.update(new_dict)
  print(student)  # Output: {'name': 'noushad', 'subject': {'math': 96, 'science': 80}, 'city': 'dhaka'}

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• Length of a Dictionary: Use len() to get the number of keys.
• Duplicates: If duplicate keys are present, the last value is retained.
• Mutable Nature: Dictionaries allow adding, updating, and deleting items dynamically.