# Insertion Sort: C

In my first post in my “learning in public” push, I’m going to be talking about the insertion sort algorithm and an implemention in C. The general idea of insertion sort is to build a sorted array in place, at the beginning of the original array, shifting the unsorted elements toward the end of the array as necessary. It relies on the fact that an array of length 1 is sorted by definition.

I’m going to show and walk through some pseudocode, touch on the efficiency of the algorithm, and then show an implementation in C.

## Pseudocode

NB: I use zero-indexed arrays and half-open array slicing in this pseudocode.

A = Array(...)
N = A.length
for i from 1 to N:
A[0:i] is sorted
key = array[i]
j = i - 1  // index of last element in the sorted portion
while j >= 0 and A[j] > key:
A[j + 1] = A[j]
j = j - 1
A[j + 1] = key


### Example

• Start with A = [3 1 2]
• First iteration: i = 1
• A[0:i] --> A[0:1] --> [3] is sorted by definition
• key = A[i] --> A[1] --> 1
• j = i - 1 --> 0
• Loop condition: 0 >= 0 and 3 > 1 --> TRUE
• A[j + 1] = A[j] --> A[1] = 3, A = [3 3 2]
• j = j - 1 --> -1
• Loop condition: FALSE (j < 0)
• A[j + 1] = key --> A[0] = 1, A = [1 3 2]
• A = [1 3 2]
• Second iteration: i = 2
• A[0:i] --> A[0:2] --> [1 3] is already sorted
• key = A[i] --> A[2] --> 2
• j = i - 1 --> 1
• Loop condition: 1 >= 0 and 3 > 2 --> TRUE
• A[j + 1] = A[j] --> A[2] = 3, A = [1 3 3]
• j = j - 1 -- > 0
• Loop condition: FALSE (!(1 > 2))
• A[j + 1] = key --> A[1] = 2, A = [1 2 3]
• A = [1 2 3]
• i == N so no third iteration

## Efficiency

• Worst-case
• Array is reverse sorted
• O(n2)
• Best-case
• Ω(n)

## Implementation

insertionSort() takes as input an integer, n, the length of the array, and an array of integers, arr. It sorts the array in place, so it doesn’t return anything.

void insertionSort(int n, int arr[])
{
for (int i = 1; i < n; i++)
{
int key = arr[i];
int j = i - 1;

while (j >= 0 && arr[j] > key)
{
arr[j + 1] = arr[j];
j--;
}
arr[j + 1] = key;
}
}