Dictionaries#

Learning Objectives

Leverage dictionaries to store more complex data, reference data by keys, and manipulate data stored

Dictionaries are a data structure in Python that are used to organize data into collections. Unlike lists, the data in a dictionary is not accessed based on its position. Instead, it is organized into pairs of keys and values. The key is used to access the corresponding value. A dictionary key can be a different data type, such as a string, integer, float, or tuple.

To create a dictionary, you use curly brackets: {}.
To store values in a dictionary, you first specify the key, followed by a colon and the corresponding value, seperated by a comma.
```
          For example, animals = {"bears": 10, "lions": 1, "tigers": 2}
```

The above block represents a dictionary with three key-value pairs, stored in the variable animals. The key "bears" points to the integer value 10, while the key "lions" points to the integer value 1, and "tigers" points to the integer 2.

To access the values, the key can be referenced, like this: animals[“bears”]. This returns the integer 10, since that is the corresponding value for this key.

Initializing Dictionaries#

empty_dict = {}

print("Dictionary: {}".format(empty_dict))
print("Type: {}".format(type(empty_dict)))

Dictionary: {}
Type: <class 'dict'>

animals = {"bears": 10, "lions": 1, "tigers": 2}

print("Number of bears: {}".format(animals['bears']))

Number of bears: 10

The in keyword can be used to check if a key is present in a dictionary. If the key is found in the dictionary, it returns True, otherwise it returns `False’.

print("lion in dictionary: {}".format("lions" in animals),end="\n\n")

print("zebra in dictionary: {}".format("zebras" in animals))

lion in dictionary: True

zebra in dictionary: False

Mutability of Dictionaries#

Dictionaries are mutable, meaning they can be modified by adding, removing, and replacing elements.

A new key-value pair can be added to a dictionary by assigning a value to a new key.
Value of an existing key in the dictionary is also modified the same way.
The del keyword helps in removing elements from a dictionary.

# Adding a new key-value pair: zebra - 25
animals['zebras'] = 25
print("Dictionary after adding Zebras: {}".format(animals),end="\n\n")

# Changing value of existing key: lions - 50
animals['lions'] = 50
print("Lion population changes: {}".format(animals),end="\n\n")

# Removing 'tigers' from the dictionary
del(animals['tigers'])
print("Final dictionary: {}".format(animals),end="\n\n")

Dictionary after adding Zebras: {'bears': 10, 'lions': 1, 'tigers': 2, 'zebras': 25}

Lion population changes: {'bears': 10, 'lions': 50, 'tigers': 2, 'zebras': 25}

Final dictionary: {'bears': 10, 'lions': 50, 'zebras': 25}

In Python, we can iterate over dictionaries using a for loop, similar to strings, lists, and tuples. However, since dictionaries have key-value pairs, the for loop will iterate over the sequence of keys in the dictionary. To access the corresponding values associated with the keys, we can use the keys as indexes.

Alternatively, we can use the items() method on the dictionary, which returns a tuple for each element in the dictionary where the first element in the tuple is the key and the second element is the value.

print(animals,end="\n\n")

# Using '.items()' method   
for animal, count in animals.items():
    print("There are {} {} in the zoo.".format(count,animal))

{'bears': 10, 'lions': 50, 'zebras': 25}

There are 10 bears in the zoo.
There are 50 lions in the zoo.
There are 25 zebras in the zoo.

If we only wanted to access the keys in a dictionary, we can use the keys() method on the dictionary, which returns a list of all the keys in the dictionary.

Similarly, if we only wanted the values, we can use the values() method, which returns a list of all the values in the dictionary.

# Using '.keys()' method to obtain the keys of the dictionary
for animal in animals.keys():
    print("The zoo has {} in it.".format(animal))

The zoo has bears in it.
The zoo has lions in it.
The zoo has zebras in it.

total = 0

# Using '.values' method to obtain the values of the dictionary
for amount in animals.values():
    total += amount 

print("There are total {} animals in the zoo.".format(total))

There are total 85 animals in the zoo.

Program 19#

Write a function that takes a permission value in octal notation and returns a string representing the permissions in the symbolic notation.

Instructions:

Define a function named octal_to_symbolic(permission) that takes one parameter:

      permission: an integer value representing the permission in octal notation.

In the function, convert the octal permission value to a string using the built-in str() function, which returns a string representing the octal notation of the given integer.
Extract the last three characters from the string. These represent the permission values for others, group, and owner.
Create a dictionary that maps each permission value to its symbolic notation: '4' to 'r', '2' to 'w', and '1' to 'x'.
Using string slicing and dictionary lookups, convert each permission value to its corresponding symbolic notation.
Return a string that represents the permission in symbolic notation.

def octal_to_string(octal):
    
    """ 
    Convert an octal permission number to a string representation

    Parameter:
        octal (int): An octal number representing file permission bits

    Returns:
        str: A string representation of file permission bits
    
    """

    octal_in_str = str(octal)
    permission_list = []
    x = 1
    j = 0

    check_dict = {"0": "---", "1": "--x", "2": "-w-", "3": "-wx", "4": "r--", "5": "r-x", "6": "rw-", "7": "rwx"}

    for characters in octal_in_str:
        permission_list.append(check_dict[characters])
    
    output_str = "".join(permission_list)

    return output_str

# Should be rwxr-xr-x
print("Run 1")
print(octal_to_string(755),end="\n\n")

# Should be rw-r--r--
print("Run 2")
print(octal_to_string(644),end="\n\n")

# Should be rwxr-x---
print("Run 3")
print(octal_to_string(750),end="\n\n")

# Should be rw-------
print("Run 4")
print(octal_to_string(600))

Run 1
rwxr-xr-x

Run 2
rw-r--r--

Run 3
rwxr-x---

Run 4
rw-------

Program 20#

Write a Python function called email_list that takes one argument:

users: a dictionary where each key is a domain name (string), and each value is a list of usernames (strings) for that domain.

The function should create and return a list of complete email addresses for all users in the dictionary, in the format "username@domain.com".

For example, if the input dictionary is:

        {
            'gmail' : ['diana.prince', 'bruce.wayne']
            'yahoo' : ['clark.kent', 'lois.lane', 'peter.parker']

        }

The output list should be:

        ['diana.prince@gmail.com', 'bruce.wayne@gmail.com', 'clark.kent@yahoo.com', 'lois.lane@yahoo.com', 'peter.parker@yahoo.com']

def email_list(domains):
    
    """ 
    Parameters:
        domains (dict): A dictionary containing domain names as keys 
                 and a list of users as values
 
    Returns:
        emails (list): A list of complete email addresses for all the 
        users in the input dictionary
    
    """
    emails = []

    for domain in domains.keys():
        for user in domains[domain]:
            x = user + "@" + domain
            emails.append(x)
    return(emails)

print("Run 1")
print(email_list({"gmail.com": ["clark.kent", "diana.prince", "peter.parker"], "yahoo.com": ["barbara.gordon", "jean.grey"], "hotmail.com": ["bruce.wayne"]}))

Run 1
['clark.kent@gmail.com', 'diana.prince@gmail.com', 'peter.parker@gmail.com', 'barbara.gordon@yahoo.com', 'jean.grey@yahoo.com', 'bruce.wayne@hotmail.com']

Program 21#

Write a function named add_prices that returns the total price of all the groceries in the dictionary.

def add_prices(basket):
    
    """
    Parameters:
        basket (dict): A dictionary containing the name of
        items as keys and their prices as values.
        
    Returns:
        The total price of all the items in the basket.
    """

    total = 0

    total = sum(list(basket.values()))

    return round(total,2)

groceries = {"bananas": 1.56, "apples": 2.50, "oranges": 0.99, "bread": 4.59, 
"coffee": 6.99, "milk": 3.39, "eggs": 2.98, "cheese": 5.44}

# Should print 28.44
print("Run 1")
print(add_prices(groceries)) 

Run 1
28.44

Fundamentals of Python

Dictionaries

Contents