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Dictionaries

Dictionaries in Python are data structures that store data in key-value pairs, allowing efficient data retrieval and modification based on unique keys.

What Is a Dictionary?​

  • Key-Value Pairs: Each entry in a dictionary consists of a key and its associated value.
  • Access by Keys: Unlike lists, dictionaries are accessed via keys, not indices.
  • Mutable: Dictionaries can be modified by adding, changing, or deleting elements.
  • Key Types: Keys can be of various immutable data types like strings, integers, floats, and tuples.
  • Value Types: Values can be of any data type, including lists and other dictionaries.

Creating an Empty Dictionary:

x = {}
print(type(x))  # Output: <class 'dict'>

Initializing Dictionaries​

You can create a dictionary with initial key-value pairs:

file_counts = {
    "jpg": 10,
    "txt": 14,
    "csv": 2,
    "py": 23
}
print(file_counts)

Output:

{'jpg': 10, 'txt': 14, 'csv': 2, 'py': 23}

Accessing Dictionary Values​

  • By Key:

    print(file_counts["txt"])  # Output: 14

  • Checking for a Key:

    print("jpg" in file_counts)   # Output: True
    print("html" in file_counts)  # Output: False

Modifying Dictionaries​

  • Adding a New Key-Value Pair:

    file_counts["cfg"] = 8
    print(file_counts)

    Output:

    {'jpg': 10, 'txt': 14, 'csv': 2, 'py': 23, 'cfg': 8}

  • Updating an Existing Key's Value:

    file_counts["csv"] = 17
    print(file_counts)

    Output:

    {'jpg': 10, 'txt': 14, 'csv': 17, 'py': 23, 'cfg': 8}

  • Deleting a Key-Value Pair:

    del file_counts["cfg"]
    print(file_counts)

    Output:

    {'jpg': 10, 'txt': 14, 'csv': 17, 'py': 23}

Iterating Over Dictionaries​

  • Iterating Over Keys:

    for extension in file_counts:
        print(extension)

    Output:

    jpg
    txt
    csv
    py

  • Iterating Over Key-Value Pairs:

    for ext, count in file_counts.items():
        print(f"There are {count} files with the .{ext} extension")

    Output:

    There are 10 files with the .jpg extension
    There are 14 files with the .txt extension
    There are 17 files with the .csv extension
    There are 23 files with the .py extension

  • Iterating Over Values:

    for count in file_counts.values():
        print(count)

    Output:

    10
    14
    17
    23

Counting Elements with Dictionaries​

Dictionaries are ideal for counting the frequency of elements.

Example: Counting Letters in a String

def count_letters(text):
    result = {}
    for letter in text:
        if letter not in result:
            result[letter] = 0
        result[letter] += 1
    return result

Usage:

print(count_letters("tenant"))

Output:

{'t': 2, 'e': 1, 'n': 2, 'a': 1}
print(count_letters("a long string with a lot of letters"))

Output:

{'a': 2, ' ': 7, 'l': 3, 'o': 3, 'n': 2, 'g': 2, 's': 2, 't': 5, 'r': 2, 'i': 2, 'w': 1, 'h': 1, 'f': 1, 'e': 3}

Dictionaries vs. Lists​

Similarities​

  • Collections: Both store collections of elements.
  • Iteration: Both can be iterated over using loops.
  • Mutability: Both are mutable; elements can be added, modified, or removed.

Differences​

FeatureDictionariesLists
Access MethodVia unique keysVia index positions
OrderUnordered (Python 3.7+ maintains insertion order)Ordered
Key TypesImmutable types (strings, integers, tuples, etc.)Indices are integers starting from 0
Lookup SpeedFaster for large datasets (constant time complexity)Slower as list size increases (linear time)
SyntaxCurly braces {} with colons : between keys/valuesSquare brackets []

When to Use Dictionaries vs. Lists​

  • Use Dictionaries when:
    • You need to associate values with unique keys.
    • Fast lookup, insertion, and deletion are required.
    • The dataset is large, and quick access is important.
  • Use Lists when:
    • You have an ordered collection of items.
    • You need to access elements by their position.
    • Order and sequence of elements are important.

Dictionary Methods and Operations​

Common Operations​

  • Get the Number of Items:

    len(dictionary)

  • Access a Value by Key:

    value = dictionary[key]

  • Set a Value by Key:

    dictionary[key] = value

  • Delete a Key-Value Pair:

    del dictionary[key]

Useful Methods​

  • .get(key, default): Returns the value for a key if it exists, else returns a default value.

    value = dictionary.get(key, default_value)

  • .keys(): Returns a view object of all keys.

    keys = dictionary.keys()

  • .values(): Returns a view object of all values.

    values = dictionary.values()

  • .items(): Returns a view object of all key-value pairs.

    items = dictionary.items()

  • Append to a List in a Dictionary:

    dictionary[key].append(new_value)

  • Update with Another Dictionary:

    dictionary.update(other_dictionary)

  • Clear All Items:

    dictionary.clear()

  • Copy a Dictionary:

    new_dict = dictionary.copy()

Practical Examples​

Summing Values in a Dictionary​

def sum_server_use_time(server):
    total_use_time = 0.0
    for user_time in server.values():
        total_use_time += user_time
    return round(total_use_time, 2)

file_server = {
    "EndUser1": 2.25,
    "EndUser2": 4.5,
    "EndUser3": 1,
    "EndUser4": 3.75,
    "EndUser5": 0.6,
    "EndUser6": 8
}

print(sum_server_use_time(file_server))  # Output: 20.1

Generating Full Names from a Dictionary​

def list_full_names(employee_dictionary):
    full_names = []
    for last_name, first_names in employee_dictionary.items():
        for first_name in first_names:
            full_names.append(f"{first_name} {last_name}")
    return full_names

print(list_full_names({
    "Ali": ["Muhammad", "Amir", "Malik"],
    "Devi": ["Ram", "Amaira"],
    "Chen": ["Feng", "Li"]
}))

Output:

['Muhammad Ali', 'Amir Ali', 'Malik Ali', 'Ram Devi', 'Amaira Devi', 'Feng Chen', 'Li Chen']

Inverting a Dictionary​

def invert_resource_dict(resource_dictionary):
    inverted = {}
    for resource_group, resources in resource_dictionary.items():
        for resource in resources:
            if resource in inverted:
                inverted[resource].append(resource_group)
            else:
                inverted[resource] = [resource_group]
    return inverted

resources = {
    "Hard Drives": ["IDE HDDs", "SCSI HDDs"],
    "PC Parts": ["IDE HDDs", "SCSI HDDs", "High-end video cards", "Basic video cards"],
    "Video Cards": ["High-end video cards", "Basic video cards"]
}

print(invert_resource_dict(resources))

Output:

{
    'IDE HDDs': ['Hard Drives', 'PC Parts'],
    'SCSI HDDs': ['Hard Drives', 'PC Parts'],
    'High-end video cards': ['PC Parts', 'Video Cards'],
    'Basic video cards': ['PC Parts', 'Video Cards']
}

Resources​