By the end of this article, you'll thoroughly understand Big O notation. You'll also know how to use it in the real world, and even the mathematics behind it!

In computer science, time complexity is the computational complexity that describes the amount of time it takes to run an algorithm.

Big O notation is a method for determining how fast an algorithm is. Using Big O notation, we can learn whether our algorithm is fast or slow. This knowledge lets us design better algorithms.

This article is written using agnostic Python. That means it will be easy to port the Big O notation code over to Java, or any other language. If the code isn't agnostic, there's Java code accompanying it.

Table of Contents

1. ❓ How Do We Measure How Long an Algorithm Takes to Run?
2. 🤔 What Is Big O Notation?
3. 🟢 Constant Time
4. 🔵 Logarithmic Time
5. 🟡 Linear Time
6. 🔴 Polynomial Time
7. ❌ Exponential Time
8. 😌 Simplifying Big O notation
9. ☁ Other Big O Times to Learn (But Not Essential)
10. 🥇 O(n log n)
11. 👿 O(n!)
12. 🧮 How to Calculate Big O Notation for Our Own Algorithms with Examples
13. 🤯 Big O Notation Cheat Sheet
14. 🎓 How to Calculate Big O Notation of a Function (Discrete Maths)
15. 👋 Summary

❓ How Do We Measure How Long an Algorithm Takes to Run?

We could run an algorithm 10,000 times and measure the average time taken.

$ python3 -m timeit '[print(x) for x in range(100)]'
100 loops, best of 3: 11.1 msec per loop 
$ python3 -m timeit '[print(x) for x in range(10)]'
1000 loops, best of 3: 1.09 msec per loop
# We can see that the time per loop changes depending on the input!

Say we have an algorithm that takes a shopping list and prints out every item on the shopping list. If the shopping list has 3 items, it'll execute quickly. If it has 10 billion items, it'll take a long time.

What is the “perfect” input size to get the “perfect” measure of how long the algorithm takes?

Other things we need to consider:

• Different processor speeds exist.
• Languages matter. Assembly is faster than Scratch; how do we consider this?

For this reason, we use Big O (pronounced Big Oh) notation.

🤔 What Is Big O Notation?

Big O is a formal notation that describes the behaviour of a function when the argument tends towards the maximum input. It was invented by Paul Bachmann, Edmund Landau and others between 1894 and 1820s. Popularised in the 1970s by Donald Knuth. Big O takes the upper bound. The worst-case results in the worst execution of the algorithm. For our shopping list example, the worst-case is an infinite list.

Instead of saying the input is 10 billion, or infinite - we say the input is n size. The exact size of the input doesn't matter, only how our algorithm performs with the worst input. We can still work out Big O without knowing the exact size of an input.

Big O is easy to read once we learn this table:

Where the further right they are, the longer it takes. n is the size of the input. Big O notation uses these functions to describe algorithm efficiency.

In our shopping list example, in the worst-case of our algorithm it prints out every item in the list sequentially. Since there are n items in the list, it takes O(n) time to complete the algorithm.

Other asymptotic (time-measuring) notations are:

Informally this is:

• Big Omega (best case)
• Big Theta (average case)
• Big O (worst case)

Let's walk through every single column in our "The Big O Notation Table".

🟢 Constant Time

Constant algorithms do not scale with the input size, they are constant no matter how big the input. An example of this is addition. 1 + 2 takes the same time as 500 + 700. They may take more physical time, but we do not add more steps in the algorithm for addition of big numbers. The underlying algorithm doesn't change at all.

We often see constant as O(1), but any number could be used and it would still be constant. We sometimes change the number to a 1, because it doesn't matter at all about how many steps it takes. What matters is that it takes a constant number of steps.

Constant time is the fastest of all Big O time complexities. The formal definition of constant time is:

It is upper-bounded by a constant

An example is:

def OddOrEven(n):
    return "Even" if n % 2 else "Odd"

Or in Java:

boolean isEven(double num) { return ((num % 2) == 0); }

In most programming languages, all integers have limits. Primitive operations (such as modulo, %) are all upper-bounded by this limit. If we go over this limit, we get an overflow error.

Because of this upper-bound, it satisfies O(1).

🔵 Logarithmic Time

Here's a quick explainer of what a logarithm is.

Log_{3}^{9}

What is being asked here is “3 to what power gives us 9?” This is 3 to the power of 2 gives us 9, so the whole expression looks like:

Log_{3}^{9} = 2

A logarithmic algorithm halves the list every time it’s run.

Let's look at binary search. Given the below sorted list:

a = [1, 2, 3, 4, 5, 6 , 7, 8, 9, 10]

We want to find the number "2".

We implement Binary Search as:

def binarySearch(alist, item):
    first = 0
    last = len(alist)-1
    found = False

    while first <= last and not found:
        midpoint = (first + last)//2
        if alist[midpoint] == item:
            found = True
        else:
            if item < alist[midpoint]:
            last = midpoint-1
            else:
                first = midpoint+1

    return found

In English this is:

• Go to the middle of the list
• Check to see if that element is the answer
• If it's not, check to see if that element is more than the item we want to find
• If it is, ignore the right-hand side (all the numbers higher than the midpoint) of the list and choose a new midpoint.
• Start over again, by finding the midpoint in the new list.

The algorithm halves the input every single time it iterates. Therefore it is logarithmic. Other examples include:

• Fibonacci number calculations
• Searching a Binary Search Tree
• Searching AVL trees

🟡 Linear Time

Linear time algorithms mean that every single element from the input is visited exactly once, O(n) times. As the size of the input, N, grows our algorithm's run time scales exactly with the size of the input.

Linear running time algorithms are widespread. Linear runtime means that the program visits every element from the input. Linear time complexity O(n) means that as the input grows, the algorithms take proportionally longer to complete.2 Apr 2019

Remember our shopping list app from earlier? The algorithm ran in O(n) which is linear time!

Linear time is where every single item in a list is visited once, in a worst-case scenario.

To read out our shopping list, our algorithm has to read out each item. It can't half the list, or add more items that we didn't add. It has to list all n items, one at a time.

shopping_list = ["Bread", "Butter", "The Nacho Libre soundtrack from the 2006 film Nacho Libre", "Reusable Water Bottle"]
for item in shopping_list:
    print(item)

Let's look at another example.

The largest item of an unsorted array

Given the list:

a = [2, 16, 7, 9, 8, 23, 12]

How do we work out what the largest item is?

We need to program it like this:

a = [2, 16, 7, 9, 8, 23, 12]
max_item = a[0]
for item in a:
    if item > max_item:
        max_item = item

We have to go through every item in the list, 1 by 1.

🔴 Polynomial Time

Polynomial time is a polynomial function of the input. A polynomial function looks like n² or n³ and so on.

If one loop through a list is O(n), 2 loops must be O(n^2). For each loop, we go over the list once. For each item in that list, we go over the entire list once. Resulting in n² operations.

a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for i in a:
    for x in a:
        print("x")

For each nesting on the same list, that adds an extra +1 onto the powers.

So a triple nested loop is O(n^3).

Bubblesort is a good example of an O(n²⁾ algorithm. The sorting algorithm takes the first number and swaps it with the adjacent number if they are in the wrong order. It does this for each number, until all numbers are in the right order - and thus sorted.

def bubbleSort(arr):
    n = len(arr)

    # Traverse through all array elements
    for i in range(n):

        # Last i elements are already in place
        for j in range(0, n-i-1):

            # traverse the array from 0 to n-i-1
            # Swap if the element found is greater
            # than the next element
            if arr[j] > arr[j+1] :
                arr[j], arr[j+1] = arr[j+1], arr[j]

# Driver code to test above
arr = [64, 34, 25, 12, 22, 11, 90]

bubbleSort(arr)

As a side note, my professor refers to any algorithm with a time of polynomial or above as:

A complete and utter disaster! This is a disaster! A catastrophe!

But the thing with large time complexities is that they often show us that something can be quickened.

For instance, a problem I had. Given a sentence, how many of those words appear in the English Dictionary? We can imagine the O(n²⁾ method. One for loop through the sentence, another through the dictionary.

dictionary = ["a", "an"] # imagine if this was the dictionary
sentence = "hello uu988j my nadjjrjejas is brandon nanndwifjasj banana".split(" ")

counter = 0
for word in sentence:
    for item in dictionary:
        if word == item:
            counter = counter + 1

O(n^2)! A disaster! But, knowing that this is a disaster means we can speed it up. Dictionaries are sorted by default. What if we sort our list of words in the sentence, and checked each word that way? We only need to loop through the dictionary once. And if the word we want to check is less than the word we're on in the dictionary, we switch to the second word in the list.

Now our algorithm is O(n log n). We recognise that this isn't a disaster, so we can move on! Knowing time complexities isn't only useful in interviews. It's an essential tool to improve our algorithms.

We can hasten many polynomial algorithms we construct using knowledge of algorithmic design.

❌ Exponential Time

Exponential time is 2^n, where 2 depends on the permutations involved.

This algorithm is the slowest of them all. You saw how my professor reacted to polynomial algorithms. He was jumping up and down in furiosity at exponential algorithms!

An example of this is say we have a password consisting only of numbers (so that’s 10 numbers, 0 through to 9). we want to crack a password which has a length of n, so to bruteforce through every combination we'll have:10ⁿ

Combinations to work through.

One example of exponential time is to find all the subsets of a set.

>>> subsets([''])
['']
>>> subsets(['x'])
['', 'x']
>>> subsets(['a', 'b'])
['', 'a', 'b', 'ab']

We can see that when we have an input size of 2, the output size is 2² = 4.

Now, let's code up subsets.

from itertools import chain, combinations

def subsets(iterable):
    s = list(iterable)
    return chain.from_iterable(combinations(s, r) for r in range(len(s)+1))

Taken from the documentation for itertools. What's important here is to see that it exponentially grows depending on the input size. Java code can be found here.

Exponential algorithms are horrific, but like polynomial algorithms we can learn a thing or two. Let's say we have to calculate 10^4. We need to do this:

10 * 10 * 10 * 10 = 10² * 10²

We have to calculate 10² twice! What if we store that value somewhere and use it later so we do not have to recalculate it? This is the principle of Dynamic Programming, which you can read about here.

When we see an exponential algorithm, dynamic programming can often be used to speed it up.

Again, knowing time complexities allows us to build better algorithms.

Here's our Big O notation graph where the numbers are reduced so we can see all the different lines.

You can find the code for this graph here.

😌 Simplifying Big O notation

Rarely will time complexity be as easy as counting how many for loops we have. What if our algorithm looks like O(n + n^2)? We can simplify our algorithm using these simple rules:

Drop the constants

If we have an algorithm described as O(2n), we drop the 2 so it becomes O(n).

Drop the non-dominant terms

O(n² + n) becomes O(n²). Only keep the larger one in Big O.

If we have a sum such as O(b² + a) we can’t drop either without knowledge of what b and a are.

Is that it?

Yup! The hardest part is figuring out what our program's complexity is first. Simplifying is the easy part! Just remember the golden rule of Big O notation:

"What is the worst-case scenario here?"

☁ Other Big O Times to Learn (But Not Essential)

🥇 O(n log n)

This is the fastest time possible for a comparison sort. We cannot get any faster unless we use some special property, like Radix sort. O(n log n) is the fastest comparison sort time.

It's rather famous, because Mergesort runs in O(n log n). Mergesort is a great algorithm not only because it sorts fast, but because the idea is used to build other algorithms.

Mergesort is used to teach divide & conquer algorithms. And for good reason, it's a fantastic sorting algorithm that has roots outside of sorting.

Mergesort works by breaking down the list of numbers into individual numbers:

And then sorting each list, before merging them:

def mergeSort(alist):
    print("Splitting ",alist)
    if len(alist)>1:
        mid = len(alist)//2
        lefthalf = alist[:mid]
        righthalf = alist[mid:]

        mergeSort(lefthalf)
        mergeSort(righthalf)

        i=0
        j=0
        k=0
        while i < len(lefthalf) and j < len(righthalf):
            if lefthalf[i] <= righthalf[j]:
                alist[k]=lefthalf[i]
                i=i+1
            else:
                alist[k]=righthalf[j]
                j=j+1
            k=k+1

        while i < len(lefthalf):
            alist[k]=lefthalf[i]
            i=i+1
            k=k+1

        while j < len(righthalf):
            alist[k]=righthalf[j]
            j=j+1
            k=k+1
    print("Merging ",alist)

alist = [54,26,93,17,77,31,44,55,20]
mergeSort(alist)
print(alist)

Read more on Mergesort here.

👿 O(n!)

Notice the le10 at the top? This one is so large, it makes all other times look constant!

This time complexity is often used as a joke, referring to Bogo Sort. I have yet to find a real life (not-a-joke) algorithm that runs in O(n!) that isn't an algorithm calculating O(6!) or the likes.

🧮 How to Calculate Big O Notation for Our Own Algorithms with Examples

Our own algorithms will normally be based on some famous algorithm that already has a Big O notation. If it's not, do not worry! Working out the Big O of our algorithm is easy.

Just think:

"What is the absolute worst input for my program?"

Take, for instance, a sequential searching algorithm.

def search(listInput, toFind):
    for counter, item in enumerate(listInput):
        if toFind == item:
            return (counter, item)
    return "did not find the item!"

The best input would be:

search(["apples"], "apples")

But the worst input is if the item was at the end of a long list.

search(["apples", "oranges", "The soundtrack from the 2006 film Nacho Libre", "Shrek"], "Shrek")

The worst-case scenario is O(n), because we have to go past every item in the list to find it.

What if our search algorithm was binary search? We learnt that binary search divides the list into half everytime. This sounds like log n!

What if our binary search looks for an object, and then looks to find other similar objects?

# here we want to find the film shrek, find its IMDB rating and find other films with that IMDB rating. We are using binary search, then sequential search
toFind = {title: "Shrek", IMDBrating: None}
ret = search(toFind)
ret = search(ret['IMDBrating'])

We find Shrek with an IMDB score of 7.8. But we're only sorted on the title, not the IMDB rating. We have to use sequential search to find all other films with the same rating.

Binary search is O(log n) and sequential search is O(n), this makes our algorithm O(n log n). This isn't a disaster, so we can sure it's not a terrible algorithm.

Even in the instances where our algorithms are not strictly related to other algorithms, we can still compare them to things we know. O(log n) means halfing. O(n²⁾ means a nested for loop.

One last thing, we don't always deal with n. Take this below algorithm:

x = [1, 2, 3, 4, 5]
y = [2, 6]
y = iter(y)
counter = 0
total = 0.0
while counter != len(x):
    # cycles through the y list.
    # multiplies 2 by 1, then 6 by 2. Then 2 by 3. 
    total = total + x[counter] * next(y)
    counter += 1
print(total)

We have 2 inputs, x and y. Our notation is then O(x + y). Sometimes we cannot make our notation smaller without knowing more about the data.

🤯 Big O Notation Cheat Sheet

I made this little infographic for you! The "add +1 for every nested for loop" depends on the for loop, as we saw earlier. But explaining that all over again would take up too much space 😅

🎓 How to Calculate Big O Notation of a Function (Discrete Maths)

Unfortunately, Dev does not support MathJax :-( So this part of the article cannot be included :'( click here to view the full article

👋 Summary

Big O represents how long an algorithm takes but sometimes we care about how much memory (space complexity) an algorithm takes too. If you're ever stuck, come back to this page and check out the infographics!