In-Place Algorithm
In-place has more than one definition. One strict definition is.
An in-place algorithm is an algorithm that does not need an extra space and produces an output in the same memory that contains the data by transforming the input ‘in-place’. However, a small constant extra space used for variables is allowed.
A more broad definition is,
In-place means that the algorithm does not use extra space for manipulating the input but may require a small though non-constant extra space for its operation. Usually, this space is O(log n), though sometimes anything in O(n) (Smaller than linear) is allowed.
A Not In-Place Implementation of reversing an array
C++
// An Not in-place C++ program to reverse an array#include <bits/stdc++.h>using namespace std; /* Function to reverse arr[] from start to end*/void revereseArray(int arr[], int n){ // Create a copy array and store reversed // elements int rev[n]; for (int i=0; i<n; i++) rev[n-i-1] = arr[i]; // Now copy reversed elements back to arr[] for (int i=0; i<n; i++) arr[i] = rev[i];} /* Utility function to print an array */void printArray(int arr[], int size){ for (int i = 0; i < size; i++) cout << arr[i] << " "; cout << endl;} /* Driver function to test above functions */int main() { int arr[] = {1, 2, 3, 4, 5, 6}; int n = sizeof(arr)/sizeof(arr[0]); printArray(arr, n); revereseArray(arr, n); cout << "Reversed array is" << endl; printArray(arr, n); return 0;} |
Java
// An Not in-place Java program// to reverse an arrayimport java.util.*; class GFG{ /* Function to reverse arr[] from start to end*/ public static void revereseArray(int []arr, int n) { // Create a copy array // and store reversed // elements int []rev = new int[n]; for (int i = 0; i < n; i++) rev[n - i - 1] = arr[i]; // Now copy reversed // elements back to arr[] for (int i = 0; i < n; i++) arr[i] = rev[i]; } /* Utility function to print an array */ public static void printArray(int []arr, int size) { for (int i = 0; i < size; i++) System.out.print(arr[i] + " "); System.out.println(""); } // Driver code public static void main(String[] args) { int arr[] = {1, 2, 3, 4, 5, 6}; int n = arr.length; printArray(arr, n); revereseArray(arr, n); System.out.println("Reversed array is"); printArray(arr, n); }} // This code is contributed// by Harshit Saini |
Python3
# An Not in-place Python program # to reverse an array ''' Function to reverse arr[] from start to end '''def revereseArray(arr, n): # Create a copy array # and store reversed # elements rev = n * [0] for i in range(0, n): rev[n - i - 1] = arr[i] # Now copy reversed # elements back to arr[] for i in range(0, n): arr[i] = rev[i] # Driver codeif __name__ == "__main__": arr = [1, 2, 3, 4, 5, 6] n = len(arr) print(*arr) revereseArray(arr, n); print("Reversed array is") print(*arr) # This code is contributed# by Harshit Saini |
C#
// An Not in-place C# program// to reverse an arrayusing System; class GFG{ /* Function to reverse arr[] from start to end*/ public static void revereseArray(int[] arr, int n) { // Create a copy array // and store reversed // elements int[] rev = new int[n]; for (int i = 0; i < n; i++) rev[n - i - 1] = arr[i]; // Now copy reversed // elements back to arr[] for (int i = 0; i < n; i++) arr[i] = rev[i]; } /* Utility function to print an array */ public static void printArray(int[] arr, int size) { for (int i = 0; i < size; i++) Console.Write(arr[i] + " "); Console.Write("\n"); } // Driver code public static void Main() { int[] arr = {1, 2, 3, 4, 5, 6}; int n = arr.Length; printArray(arr, n); revereseArray(arr, n); Console.WriteLine("Reversed array is"); printArray(arr, n); }} // This code is contributed by Ita_c. |
Javascript
<script>// An Not in-place Javascript program// to reverse an array /* Function to reverse arr[] from start to end*/ function revereseArray(arr,n) { // Create a copy array // and store reversed // elements let rev = new Array(n); for (let i = 0; i < n; i++) rev[n - i - 1] = arr[i]; // Now copy reversed // elements back to arr[] for (let i = 0; i < n; i++) arr[i] = rev[i]; } /* Utility function to print an array */ function printArray(arr,size) { for (let i = 0; i < size; i++) document.write(arr[i] + " "); document.write("<br>"); } // Driver code let arr=[1, 2, 3, 4, 5, 6]; let n = arr.length; printArray(arr, n); revereseArray(arr, n); document.write("Reversed array is<br>"); printArray(arr, n); // This code is contributed by rag2127</script> |
Output:
1 2 3 4 5 6 Reversed array is 6 5 4 3 2 1
This needs O(n) extra space and is an example of a not-in-place algorithm.
An In-Place Implementation of Reversing an array.
C++
// An in-place C++ program to reverse an array#include <bits/stdc++.h>using namespace std; /* Function to reverse arr[] from start to end*/void revereseArray(int arr[], int n){ for (int i=0; i<n/2; i++) swap(arr[i], arr[n-i-1]);} /* Utility function to print an array */void printArray(int arr[], int size){ for (int i = 0; i < size; i++) cout << arr[i] << " "; cout << endl;} /* Driver function to test above functions */int main() { int arr[] = {1, 2, 3, 4, 5, 6}; int n = sizeof(arr)/sizeof(arr[0]); printArray(arr, n); revereseArray(arr, n); cout << "Reversed array is" << endl; printArray(arr, n); return 0;} |
Java
// An in-place Java program// to reverse an arrayimport java.util.*; class GFG{ public static int __(int x, int y) {return x;} /* Function to reverse arr[] from start to end*/ public static void revereseArray(int []arr, int n) { for (int i = 0; i < n / 2; i++) arr[i] = __(arr[n - i - 1], arr[n - i - 1] = arr[i]); } /* Utility function to print an array */ public static void printArray(int []arr, int size) { for (int i = 0; i < size; i++) System.out.print(Integer.toString(arr[i]) + " "); System.out.println(""); } // Driver code public static void main(String[] args) { int []arr = new int[]{1, 2, 3, 4, 5, 6}; int n = arr.length; printArray(arr, n); revereseArray(arr, n); System.out.println("Reversed array is"); printArray(arr, n); }} // This code is contributed // by Harshit Saini |
Python3
# An in-place Python program# to reverse an array ''' Function to reverse arr[] from start to end'''def revereseArray(arr, n): for i in range(0, int(n / 2)): arr[i], arr[n - i - 1] = arr[n - i - 1], arr[i] # Driver codeif __name__ == "__main__": arr = [1, 2, 3, 4, 5, 6] n = len(arr) print(*arr) revereseArray(arr, n) print("Reversed array is") print(*arr) # This code is contributed # by Harshit Saini |
C#
// An in-place C# program// to reverse an arrayusing System; class GFG{ public static int __(int x, int y) {return x;} /* Function to reverse arr[] from start to end*/ public static void revereseArray(int []arr, int n) { for (int i = 0; i < n / 2; i++) arr[i] = __(arr[n - i - 1], arr[n - i - 1] = arr[i]); } /* Utility function to print an array */ public static void printArray(int []arr, int size) { for (int i = 0; i < size; i++) Console.Write(arr[i] + " "); Console.WriteLine(""); } // Driver code public static void Main(String[] args) { int []arr = new int[]{1, 2, 3, 4, 5, 6}; int n = arr.Length; printArray(arr, n); revereseArray(arr, n); Console.WriteLine("Reversed array is"); printArray(arr, n); }} /* This code is contributed by PrinciRaj1992 */ |
Javascript
<script>// An in-place Javascript program// to reverse an array function __(x,y) { return x; } /* Function to reverse arr[] from start to end*/ function revereseArray(arr,n) { for (let i = 0; i < n / 2; i++) arr[i] = __(arr[n - i - 1], arr[n - i - 1] = arr[i]); } /* Utility function to print an array */ function printArray(arr,size) { for (let i = 0; i < size; i++) document.write(arr[i] + " "); document.write("<br>"); } // Driver code let arr=[1, 2, 3, 4, 5, 6]; let n = arr.length; printArray(arr, n); revereseArray(arr, n); document.write("Reversed array is<br>"); printArray(arr, n); // This code is contributed by avanitrachhadiya2155</script> |
Output:
1 2 3 4 5 6 Reversed array is 6 5 4 3 2 1
This needs O(1) extra space for exchanging elements and is an example of an in-place algorithm.
Which Sorting Algorithms are In-Place and which are not?
In Place: Bubble sort, Selection Sort, Insertion Sort, Heapsort.
Not In-Place: Merge Sort. Note that merge sort requires O(n) extra space.
What about QuickSort? Why is it called In-Place?
QuickSort uses extra space for recursive function calls. It is called in-place according to broad definition as extra space required is not used to manipulate input, but only for recursive calls.
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