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Practice

A Data Structure Comprising Keys and Values: Dictionary

In programming, you may encounter situations where you need to store a pairing of a person's name with their age or manage a product name alongside its price.

In cases where you need to manage data in pairs of key and value, Python uses a data structure called Dictionary.


What is a Dictionary?​

In a Dictionary, the key serves as an identifier (ID) for the data and the value represents the content of that data.

Basic Structure of a Dictionary
{
    "key1": "value1",
    "key2": "value2",
    "key3": "value3"
}

In the code above, "key1", "key2", and "key3" are keys, and "value1", "value2", and "value3" are the corresponding values.

A pair consisting of a key and its value, like key1 and value1, is known as an item. It is also referred to as an element or entry.


Creating a Dictionary​

A dictionary is created using curly brackets { }. Keys and values are separated by a colon (:), and multiple key-value pairs are separated by commas (,).

Example of Creating a Dictionary
# Dictionary to store a student's name, age, and major
student = {
    "name": "Alex",
    "age": 22,
    "major": "Computer Science"
}

In this code, "name", "age", and "major" are keys, and "Alex", 22, and "Computer Science" are the corresponding values.


Accessing Keys and Values in a Dictionary​

The biggest advantage of a dictionary is that, no matter how much data it contains, you can quickly access values using their corresponding keys.

It's similar to how knowing a zip code (key) allows you to quickly find an address (value).


Accessing Values Using Keys​

To retrieve the value corresponding to a specific key in a dictionary, you input the key as a string within square brackets [ ].

Example of Accessing Values in a Dictionary
student = {
    "name": "Alex",
    "age": 22,
    "major": "Computer Science"
}

# Output: Alex
print(student["name"])

# Output: Computer Science
print(student["major"])

If you request a value using a key that doesn't exist within the dictionary, Python raises a KeyError.

Example of Requesting Value with a Non-existing Key
student = {
    "name": "Alex",
    "age": 22,
    "major": "Computer Science"
}

# KeyError raised
print(student["address"])

Checking the Existence of a Key​

To determine if a specific key exists in a dictionary, use the in keyword.

Example of Checking Key Existence
student = {
    "name": "Alex",
    "age": 22,
    "major": "Computer Science"
}

# Check if key exists using the in keyword
if "age" in student:
    print("Age is:", student["age"])
else:
    print("Key not found")

In the code above, it checks whether the key "age" exists in the dictionary. If the key exists, it prints the value of age. If the key doesn't exist, it prints "Key not found".

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