🕒 Running Time Wednesday! Understanding Big O (Made Simple)

It’s Running Time Wednesday!

Hey friends 👋

Like I stated last week, today, we are learning something that every developer should understand — running time, also known as Big O notation.

It sounds fancy, but it’s just a way to measure how fast (or slow) your code runs as your input grows.

Let’s break it down.

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What’s Running Time?

Running time tells you how the performance of your code changes as the amount of data increases.

For example, sorting 10 items is quick. Sorting 10, 000 items? That’s where efficiency comes into play.

So, Big O helps you predict how your program behaves when things scale (up or down).

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Why “Big O”?

“Big O” describes the order of growth, in essence, how the number of operations grows compared to the size of the input (n).

We’re not counting seconds; we’re counting how many steps your algorithm takes as n increases.

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Common Big O Notations

Notation	Name	Description	Example
O(1)	Constant time	Doesn’t depend on input size	Accessing an element by index
O(log n)	Logarithmic	Grows slowly as input increases	Binary search
O(n)	Linear	Grows directly with input	Looping through an array
O(n log n)	Linearithmic	Common in efficient sorting	Merge sort, quicksort
O(n²)	Quadratic	Grows fast with nested loops	Comparing every pair in a list
O(2ⁿ)	Exponential	Doubles with every new element	Recursive Fibonacci
O(n!)	Factorial	Blows up quickly	Generating all permutations

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Quick Examples

1️⃣ O(1) — Constant

function getFirst(arr) {
  return arr[0]; // One step, no matter how big arr is
}

2️⃣ O(n) — Linear

function printAll(arr) {
  for (let item of arr) console.log(item);
  // Takes more time as arr grows
}

3️⃣ O(n²) — Quadratic

function printPairs(arr) {
  for (let i of arr) {
    for (let j of arr) {
      console.log(i, j);
    }
  }
  // For 10 items = 100 operations
}

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Visualising Growth

O(1)     → constant line
O(log n) → slow curve
O(n)     → steady climb
O(n²)    → sharp rise
O(2ⁿ)    → rocket 🚀

You don’t need maths to get it — just imagine how the number of steps go up for larger inputs.

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Try It Yourself in CodePen

👉 Play with Running Time Examples

• Pick a function type – O(1), O(n), or O(n²)
• Click “Run Test.” The chart shows how long each one takes for different array sizes.

Constant time stays flat, linear grows steadily, and quadratic shoots up fast.

Try tweaking it
In the JavaScript panel, find this line:

const sizes = [100, 500, 1000, 2000];

Change it to something bigger:

const sizes = [1000, 5000, 10000, 20000];

Then click Run again.
You’ll see the chart climb faster — that’s how running time scales with data.

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🙋🏽‍♀️ Wrap up!

Here’s the takeaway:

• Running time = how your algorithm scales (go up or down).
• Think of Big O as a way to predict how an algorithm will handle a huge amount of data. It's about growth, not how fast it runs right now.
• Aim for O(1) or O(log n) when possible, but clarity comes first.

Next Wednesday, we’ll connect this to space complexity, that is, how much memory your program uses.

Till then, write clean code and stay curious!

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That’s it for today’s midweek mini tutorial!

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See you, same time, next Wednesday🚀