# In-place vs out-of-place algorithms

Posted in /  Vinay Khatri
Last updated on December 7, 2023

## In-Place algorithms

This is a category of the algorithms that do not consume any extra space in order to solve a given task. They generally override the given input with the output. We can say that the auxiliary space complexity of these algorithms is O(1). These algorithms may sometimes require a very small space but that space should not depend on size of input. The algorithms that work recursively and require call stack memory are generally not considered in-place algorithms.

Below is an example of an in-place algorithm to reverse a given array:

```#include <bits/stdc++.h>
using namespace std;

void reverseArray(int arr[], int start, int end)
{
while (start < end)
{
int temp = arr[start];
arr[start] = arr[end];
arr[end] = temp;
start++;
end--;
}
}

int main()
{
int arr[] = {1, 2, 3};

int n = sizeof(arr) / sizeof(arr);

reverseArray(arr, 0, n-1);

for (int i = 0; i < n; i++)
cout << arr[i] << " ";

return 0;
}```

#### Output

`3 2 1`

### Out-of-Place algorithms

These algorithms require extra memory to accomplish a given task. The time complexity of these algorithms is never constant and depends on the size of the input. Below is an algorithm to reverse a given array using extra space. The auxiliary space complexity of the algorithm is O(N).

```#include <iostream>
using namespace std;
int main()
{
int original_arr[] = {1, 2, 3};

int len = sizeof(original_arr)/sizeof(original_arr);

int copied_arr[len], i, j;

for (i = 0; i < len; i++)
{
copied_arr[i] = original_arr[len - i - 1];
}
for(int i=0;i<len;i++)
{
cout<<copied_arr[i]<<" ";
}
return 0;
}```

#### Output

`3 2 1`

## FAQs

An in-place algorithm is one that does not consume any extra memory apart from the array used for sorting.

An out-of-place algorithm is one that consumes extra memory depending upon the size of the input.

Insertion sort and selection sort are two examples of in-place of algorithms.

The best examples of out-of-place algorithms are Merge sort and counting sort.

The fastest sorting algorithm is Quicksort.