# Reverse an array in groups of given size | Set 3 (Single traversal)

Given an array, reverse every sub-array formed by consecutive k elements.

Examples:

Input: arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], k = 3.
Output: [3, 2, 1, 6, 5, 4, 9, 8, 7, 10]

Input: arr = [1, 2, 3, 4, 5, 6, 7], k = 5.
Output: [5, 4, 3, 2, 1, 7, 6]

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach:

1. We will use two pointers technique to solve this problem.
2. First, we will initialize our 1st pointer d with value k-1 (d = k-1) and a variable m with value 2 (m = 2).
3. Now, we will iterate the array with our 2nd pointer i and check

• If i < d, Swap (arr[i], arr[d]) and decrement d by 1. Otherwise,
• Make d = k * m – 1, i = k * (m – 1) – 1 and m = m + 1.

Below is the implementation of the above approach.

## C++

 `// C++ program to reverse every sub-array ` `// formed by consecutive k elements ` `#include ` `using` `namespace` `std; ` ` `  `// Function to reverse every sub-array  ` `// formed by consecutive k elements  ` `void` `ReverseInGroup(``int` `arr[], ``int` `n, ``int` `k) ` `{ ` `    ``if``(n < k) ` `    ``{ ` `        ``k = n; ` `    ``} ` ` `  `    ``// Initialize variables ` `    ``int` `d = k - 1, m = 2; ` `    ``int` `i = 0; ` `         `  `    ``for``(i = 0; i < n; i++) ` `    ``{ ` `        `  `       ``if` `(i >= d) ` `       ``{  ` `           ``// Update the variables ` `           ``d = k * (m); ` `           ``if``(d >= n) ` `           ``{ ` `               ``d = n;          ` `           ``}      ` `           ``i = k * (m - 1) - 1; ` `           ``m++; ` `       ``} ` `       ``else` `       ``{ ` `           ``int` `t = arr[i]; ` `           ``arr[i] = arr[d]; ` `           ``arr[d] = t; ` `       ``} ` `       ``d = d - 1;  ` `    ``} ` `    ``return``; ` `} ` ` `  `// Driver code  ` `int` `main()  ` `{  ` `    ``int` `arr[] = { 1, 2, 3, 4, 5, 6, 7 };  ` `    ``int` `k = 3;  ` `    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr);  ` ` `  `    ``ReverseInGroup(arr, n, k);  ` ` `  `    ``for``(``int` `i = 0; i < n; i++)  ` `       ``cout << arr[i] << ``" "``;  ` ` `  `    ``return` `0;  ` `} ` ` `  `// This code is contributed by Code_Mech `

## C

 `// C program to reverse every sub-array ` `// formed by consecutive k elements ` `#include ` ` `  `// Function to reverse every sub-array  ` `// formed by consecutive k elements  ` `void` `ReverseInGroup(``int` `arr[], ``int` `n, ``int` `k) ` `{ ` `    ``if``(n= d) ` `        ``{    ` `            ``// Update the variables ` `            ``d = k * (m); ` `            ``if``(d>=n) ` `            ``{ ` `                ``d = n;              ` `            ``}            ` `            ``i = k * (m - 1)-1; ` `            ``m++; ` `             `  `        ``} ` `        ``else` `        ``{ ` `            ``int` `t = arr[i]; ` `            ``arr[i] = arr[d]; ` `            ``arr[d] = t; ` `        ``} ` ` `  `     ``d = d - 1;       ` `        `  `    ``} ` `    ``return``; ` `         `  `} ` ` `  `// Driver code  ` `int` `main()  ` `{  ` `    ``int` `arr[] = {1, 2, 3, 4, 5, 6, 7};  ` `    ``int` `k = 3;  ` `   `  `    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr);  ` `   `  `    ``ReverseInGroup(arr, n, k);  ` `   `  `    ``for` `(``int` `i = 0; i < n; i++)  ` `        ``printf``(``"%d "``, arr[i]);  ` `   `  `    ``return` `0;  ` `} `

## Java

 `// Java program to reverse every sub-array ` `// formed by consecutive k elements ` `class` `GFG{ ` `     `  `// Function to reverse every sub-array  ` `// formed by consecutive k elements  ` `static` `void` `ReverseInGroup(``int` `arr[],  ` `                           ``int` `n, ``int` `k) ` `{ ` `    ``if``(n < k) ` `    ``{ ` `        ``k = n; ` `    ``} ` ` `  `    ``// Initialize variables ` `    ``int` `d = k - ``1``, m = ``2``; ` `    ``int` `i = ``0``; ` `         `  `    ``for``(i = ``0``; i < n; i++) ` `    ``{ ` `       ``if` `(i >= d) ` `       ``{  ` `            `  `           ``// Update the variables ` `           ``d = k * (m); ` `           ``if``(d >= n) ` `           ``{ ` `               ``d = n;              ` `           ``}          ` `           ``i = k * (m - ``1``) - ``1``; ` `           ``m++; ` `       ``} ` `       ``else` `       ``{ ` `           ``int` `t = arr[i]; ` `           ``arr[i] = arr[d]; ` `           ``arr[d] = t; ` `       ``} ` `       ``d = d - ``1``;      ` `    ``} ` `    ``return``; ` `} ` ` `  `// Driver code  ` `public` `static` `void` `main(String[] args)  ` `{ ` `    ``int` `arr[] = { ``1``, ``2``, ``3``, ``4``, ``5``, ``6``, ``7` `};  ` `    ``int` `k = ``3``;  ` `    ``int` `n = arr.length;  ` `     `  `    ``ReverseInGroup(arr, n, k);  ` `     `  `    ``for``(``int` `i = ``0``; i < n; i++)  ` `       ``System.out.printf(``"%d "``, arr[i]);  ` `}  ` `} ` ` `  `// This code is contributed by sapnasingh4991 `

## C#

 `// C# program to reverse every sub-array ` `// formed by consecutive k elements ` `using` `System; ` `class` `GFG{ ` `     `  `// Function to reverse every sub-array  ` `// formed by consecutive k elements  ` `static` `void` `ReverseInGroup(``int` `[]arr,  ` `                           ``int` `n, ``int` `k) ` `{ ` `    ``if``(n < k) ` `    ``{ ` `        ``k = n; ` `    ``} ` ` `  `    ``// Initialize variables ` `    ``int` `d = k - 1, m = 2; ` `    ``int` `i = 0; ` `         `  `    ``for``(i = 0; i < n; i++) ` `    ``{ ` `        ``if` `(i >= d) ` `        ``{  ` `                 `  `            ``// Update the variables ` `            ``d = k * (m); ` `            ``if``(d >= n) ` `            ``{ ` `                ``d = n;              ` `            ``}          ` `            ``i = k * (m - 1) - 1; ` `            ``m++; ` `        ``} ` `        ``else` `        ``{ ` `            ``int` `t = arr[i]; ` `            ``arr[i] = arr[d]; ` `            ``arr[d] = t; ` `        ``} ` `        ``d = d - 1;      ` `    ``} ` `    ``return``; ` `} ` ` `  `// Driver code  ` `public` `static` `void` `Main()  ` `{ ` `    ``int` `[]arr = { 1, 2, 3, 4, 5, 6, 7 };  ` `    ``int` `k = 3;  ` `    ``int` `n = arr.Length;  ` `     `  `    ``ReverseInGroup(arr, n, k);  ` `     `  `    ``for``(``int` `i = 0; i < n; i++)  ` `        ``Console.Write(arr[i] + ``" "``);  ` `}  ` `} ` ` `  `// This code is contributed by Code_Mech `

Output:

```3 2 1 6 5 4 7
```

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Improved By : sapnasingh4991, Code_Mech