Imagine you have a magical book that lets you look up any word instantly. You don’t need to flip through pages or search for the right letter section. You just say the word, and poof, the meaning appears immediately.

In programming, we have something very similar to this magical book: dictionaries (also called hashmaps). They are special data structures where you can store things in a way that makes finding them super fast, no matter how much you’ve stored.

But how does this work?

What’s a Dictionary in Programming?

A dictionary in programming is like a real-life dictionary, but instead of words and meanings, it stores keys and values. You give it a key (something unique like a number, a name, or a word), and it will give you back its value (like a description, number, or some other information related to the key).

For example:

fruit_colors = {
    "Apple": "Red",
    "Banana": "Yellow",
    "Grapes": "Purple"
}

• Key: "Apple"
• Value: "Red"

The Magic Behind O(1) Lookup

So, how does the dictionary give you answers so quickly? The secret is something called hashing.

Let’s think about it this way: if you had a huge pile of toys and someone said, "Find me the red car" you would have to look through the whole pile one by one, that would take time, especially if there are lots of toys.

Now, imagine if every toy had a magical sticker with an exact spot where it should go on a shelf. Every time you put a toy away, it goes straight to its specific spot. So, if you wanted the red car, you would know exactly where it is without looking through everything.

This is exactly how a hashmap works. When you add something to a dictionary, a special function (called a hash function) creates a magical "sticker" (called a hash), which tells the dictionary where to store the value.

When you ask for that value later using its key, the dictionary uses the same "sticker" to go straight to the spot and grab the value in one step. This quick access, no matter how much stuff you have, is called constant time or O(1) in Big O notation. It means the lookup time doesn’t grow even if you have millions of items stored.

Checking If a Key Exists in a Dictionary

When working with dictionaries, one common task is checking if a specific key exists. For example, in our magical toy shelf, one might want to know if a red car is already on the shelf before adding another one.

There are two ways you might write this check in Python:

• if key in dictionary
• if key in dictionary.keys()

At first glance, both seem to do the same thing however, there is a big difference in efficiency, and here is why.

if key in dictionary – The Fast, Direct Way
When you write if key in dictionary, Python automatically looks for the key in the dictionary’s "shelf" of keys. It knows exactly where to check, thanks to the hashing system we talked about earlier. This lookup is done in O(1) time, meaning it is super fast.

if key in dictionary.keys() – The Slower, Extra Work Way
On the other hand, if you write if key in dictionary.keys(), you are explicitly asking Python to first pull out all the keys from the dictionary and store them in a special "view" object. Then, Python checks if the key is in that view. While this still works, it is slower because you are adding an extra step: creating the view of all the keys.

Always use if key in dictionary. It is the faster, more efficient way to check if a key exists in a dictionary.

Dictionaries in programming are powerful tools, allowing you to store and retrieve data quickly, thanks to the magic of hashing. With constant-time lookups (O(1)), they enable you to efficiently manage large amounts of data without worrying about performance issues. Whether you are checking if a key exists or retrieving a value, dictionaries are your go-to structure for fast and effective operations.

Have any experiences with using dictionaries? Share your thoughts and tips in the comments below!