Oğuzhan Olguncu / October 23, 2020 / 3 min read

javascripttutorialcomputerscience

Today, we are going to discover how **Selection Sort** works and discuss its complexity using Big O Notation. Selection sort may not be one of the fastest, but one of the easiest sort to write down.

```
Modern Times -- 8.5
The Godfather: Part II -- 9.0
The Shawshank Redemption -- 9.2
The Silence of the Lambs -- 8.6
Twelve Angry Men -- 8.9
```

Now, let's suppose you want to sort movie ratings in IMDB, from most to least. How would you do it?

```
Modern Times -- 8.5 The Shawshank Redemption -- 9.2
The Godfather: Part II -- 9.0 --->
The Shawshank Redemption -- 9.2
The Silence of the Lambs -- 8.6
Twelve Angry Men -- 8.9
Modern Times -- 8.5 The Shawshank Redemption -- 9.2
The Godfather: Part II -- 9.0 ---> The Godfather: Part II -- 9.0
/*DELETED*/
The Silence of the Lambs -- 8.6
Twelve Angry Men -- 8.9
Modern Times -- 8.5 The Shawshank Redemption -- 9.2
/*DELETED*/ ---> The Godfather: Part II -- 9.0
/*DELETED*/ Twelve Angry Men -- 8.9
The Silence of the Lambs -- 8.6
Twelve Angry Men -- 8.9
```

So, it's time to talk about its complexity. Each time we look for an element cost us **O(n)** but, since we have to do this operation for each element, we need to do it n times which costs us **O(n x n)** meaning **O(n ^{2})**

```
const findSmallest = (arr) => {
let smallest = arr[0];
let smallestIndex = 0;
for (let i = 0; i < arr.length; i++) {
if (arr[i] < smallest) {
smallest = arr[i];
smallestIndex = i;
}
}
return smallestIndex;
};
```

First, we need to find the smallest or highest to sort. To do that, we write this simple function, that takes an array as an argument and chooses the first element as its pivot, then iterates over an array. If any element is smaller than our **smallest** we swap the values. Finally, when we're done, we return the value.

```
const selectionSort = arr => {
const newArray = []
const arrayLength = arr.length
for(let i = 0; i < arrayLength; i++)
newArray.push(...arr.splice(findSmallest(arr),1)) // Removing smallest from the array
return newArray // and destructring it since splice returns an array.
// then pushing it into our sorted array.
}
selectionSort([10,2,99,6,1,7]) --> Returns: 1,2,6,7,10,99
```

This function makes use of our `findSmallest()`

. Whenever we find the smallest value, we push it to our `newArray`

and delete from the existing one. Three dots used for destructuring since splice returns an array. By the way, `splice()`

manipulates original array and returns desired output.

Thanks for reading.

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