Program to reverse the rows in a 2d Array
Last Updated :
28 Jul, 2025
Given a 2D array arr[][] of size M x N integers where M is the number of rows and N is the number of columns. The task is to reverse every row of the given 2D array.
Example:
Input: arr[][] =
{ {1, 2, 3},
{4, 5, 6},
{7, 8, 9} }
Output:
3 2 1
6 5 4
9 8 7
Input: arr[][] =
{ {1, 2},
{4, 5},
{7, 8},
{9, 10} }
Output:
2 1
5 4
8 7
10 9
Approach:
- For every row in the given 2D array do the following:
- Initialise the start index as 0 and end index as N-1.
- Iterate loop till start index is less than ending index, swap the value at these indexes and update the index as:
swap(arr[i][start], arr[i][end])
start++;
end--;
2. Do the above operation for all the rows in the 2D array.
Below is the implementation of the above approach:
C++
// C++ implementation of the above approach
#include <bits/stdc++.h>
using namespace std;
const int M = 3;
const int N = 3;
// A utility function
// to swap the two element
void swap(int* a, int* b)
{
int temp = *a;
*a = *b;
*b = temp;
}
// Function to reverse
// the given 2D arr[][]
void reverseArray(int arr[M][N])
{
// Traverse each row of arr[][]
for (int i = 0; i < M; i++) {
// Initialise start and end index
int start = 0;
int end = N - 1;
// Till start < end, swap the element
// at start and end index
while (start < end) {
// Swap the element
swap(&arr[i][start],
&arr[i][end]);
// Increment start and decrement
// end for next pair of swapping
start++;
end--;
}
}
// Print the arr[][] after
// reversing every row
for (int i = 0; i < M; i++) {
for (int j = 0; j < N; j++) {
cout << arr[i][j] << ' ';
}
cout << endl;
}
}
// Driver Code
int main()
{
int arr[][3] = { { 1, 2, 3 },
{ 4, 5, 6 },
{ 7, 8, 9 } };
// Function call
reverseArray(arr);
return 0;
}
// Java implementation of the above approach
class GFG{
static int M = 3;
static int N = 3;
// Function to reverse
// the given 2D arr[][]
static void reverseArray(int arr[][])
{
// Traverse each row of arr[][]
for (int i = 0; i < M; i++) {
// Initialise start and end index
int start = 0;
int end = N - 1;
// Till start < end, swap the element
// at start and end index
while (start < end) {
// Swap the element
int temp = arr[i][start];
arr[i][start] = arr[i][end];
arr[i][end] = temp;
// Increment start and decrement
// end for next pair of swapping
start++;
end--;
}
}
// Print the arr[][] after
// reversing every row
for (int i = 0; i < M; i++) {
for (int j = 0; j < N; j++) {
System.out.print(arr[i][j] + " ");
}
System.out.println();
}
}
// Driver Code
public static void main(String[] args)
{
int arr[][] = { { 1, 2, 3 },
{ 4, 5, 6 },
{ 7, 8, 9 } };
// Function call
reverseArray(arr);
}
}
// This code is contributed by PrinciRaj1992
# Python3 implementation of the above approach
M = 3;N = 3;
# Function to reverse
# the given 2D arr[][]
def reverseArray(arr) :
# Traverse each row of arr[][]
for i in range(M) :
# Initialise start and end index
start = 0;
end = N - 1;
# Till start < end, swap the element
# at start and end index
while (start < end) :
# Swap the element
arr[i][start], arr[i][end] = arr[i][end], arr[i][start];
# Increment start and decrement
# end for next pair of swapping
start += 1;
end -= 1;
# Print the arr[][] after
# reversing every row
for i in range(M) :
for j in range(N) :
print(arr[i][j],end= ' ');
print();
# Driver Code
if __name__ == "__main__" :
arr = [ [ 1, 2, 3 ],
[ 4, 5, 6 ],
[ 7, 8, 9 ] ];
# Function call
reverseArray(arr);
# This code is contributed by AnkitRai01
// C# implementation of the above approach
using System;
class GFG{
static int M = 3;
static int N = 3;
// Function to reverse
// the given 2D [,]arr
static void reverseArray(int [,]arr)
{
// Traverse each row of [,]arr
for (int i = 0; i < M; i++) {
// Initialise start and end index
int start = 0;
int end = N - 1;
// Till start < end, swap the element
// at start and end index
while (start < end) {
// Swap the element
int temp = arr[i,start];
arr[i, start] = arr[i, end];
arr[i, end] = temp;
// Increment start and decrement
// end for next pair of swapping
start++;
end--;
}
}
// Print the [,]arr after
// reversing every row
for (int i = 0; i < M; i++) {
for (int j = 0; j < N; j++) {
Console.Write(arr[i, j] + " ");
}
Console.WriteLine();
}
}
// Driver Code
public static void Main(String[] args)
{
int [,]arr = { { 1, 2, 3 },
{ 4, 5, 6 },
{ 7, 8, 9 } };
// Function call
reverseArray(arr);
}
}
// This code is contributed by PrinciRaj1992
<script>
// JavaScript implementation of the above approach
const M = 3;
const N = 3;
// Function to reverse
// the given 2D arr
function reverseArray(arr) {
// Traverse each row of arr
for (i = 0; i < M; i++) {
// Initialise start and end index
var start = 0;
var end = N - 1;
// Till start < end, swap the element
// at start and end index
while (start < end) {
// Swap the element
var temp = arr[i][start];
arr[i][start] = arr[i][end];
arr[i][end] = temp;
// Increment start and decrement
// end for next pair of swapping
start++;
end--;
}
}
// Print the arr after
// reversing every row
for (i = 0; i < M; i++) {
for (j = 0; j < N; j++) {
document.write(arr[i][j] + " ");
}
document.write("<br/>");
}
}
// Driver Code
var arr = [ [ 1, 2, 3 ],
[ 4, 5, 6 ],
[ 7, 8, 9 ] ];
// Function call
reverseArray(arr);
// This code is contributed by todaysgaurav
</script>
Time Complexity: O(n*n)
Auxiliary Space: O(1)
Approach#2: Using list comprehension
This approach uses list comprehension to reverse the rows in the input 2D array. It first initializes an empty list and then iterates over each row of the input array, reversing each row using the [::-1] slicing notation and appending the reversed row to the empty list using the append() method. Finally, it prints the reversed array using nested loops.
Algorithm
1. Initialize an empty list.
2. For each row in the input array:
a. Reverse the row using the slicing notation [::-1].
b. Append the reversed row to the empty list using the append() method.
3. Print the reversed array using nested loops.
C++
#include <algorithm> // Include this header for the reverse function
#include <iostream>
#include <vector>
int main()
{
// Input array
std::vector<std::vector<int> > arr
= { { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9 } };
// Reverse the rows in the array using a loop and vector
for (int i = 0; i < arr.size(); ++i) {
std::vector<int>& row = arr[i];
std::reverse(row.begin(), row.end());
}
// Print the reversed array
for (int i = 0; i < arr.size(); ++i) {
for (int j = 0; j < arr[i].size(); ++j) {
std::cout << arr[i][j] << " ";
}
std::cout << std::endl;
}
return 0;
}
import java.util.ArrayList;
import java.util.Collections;
public class GFG {
public static void main(String[] args) {
// Input array
ArrayList<ArrayList<Integer>> arr = new ArrayList<ArrayList<Integer>>();
arr.add(new ArrayList<Integer>() {{
add(1);
add(2);
add(3);
}});
arr.add(new ArrayList<Integer>() {{
add(4);
add(5);
add(6);
}});
arr.add(new ArrayList<Integer>() {{
add(7);
add(8);
add(9);
}});
// Reverse the rows in the array using a loop and ArrayList
for (int i = 0; i < arr.size(); i++) {
Collections.reverse(arr.get(i));
}
// Print the reversed array
for (int i = 0; i < arr.size(); i++) {
for (int j = 0; j < arr.get(i).size(); j++) {
System.out.print(arr.get(i).get(j) + " ");
}
System.out.println();
}
}
}
# Input array
arr = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
# Reverse the rows in the array using list comprehension
arr = [row[::-1] for row in arr]
# Print the reversed array
for i in range(len(arr)):
for j in range(len(arr[i])):
print(arr[i][j], end=" ")
print()
using System;
using System.Collections.Generic;
class Program
{
static void Main()
{
// Input 2D array
List<List<int>> arr = new List<List<int>>
{
new List<int> {1, 2, 3},
new List<int> {4, 5, 6},
new List<int> {7, 8, 9}
};
// Reverse the rows in the array using a loop and List.Reverse method
for (int i = 0; i < arr.Count; ++i)
{
List<int> row = arr[i];
row.Reverse();
}
// Print the reversed array
for (int i = 0; i < arr.Count; ++i)
{
for (int j = 0; j < arr[i].Count; ++j)
{
Console.Write(arr[i][j] + " ");
}
Console.WriteLine();
}
}
}
// Reverse function is available in JavaScript, no need to include any header
// Input array
let arr = [
[1, 2, 3],
[4, 5, 6],
[7, 8, 9]
];
// Reverse the rows in the array using a loop and array methods
for (let i = 0; i < arr.length; ++i) {
let row = arr[i];
row.reverse(); // Reverse the row
}
// Print the reversed array
for (let i = 0; i < arr.length; ++i) {
for (let j = 0; j < arr[i].length; ++j) {
console.log(arr[i][j] + " ");
}
console.log(""); // Print a new line after each row
}
Time complexity: O(mn), where m is the number of rows and n is the number of columns in the input array. The code iterates over each element of the input array once to reverse the rows and again to print the reversed array.
Auxiliary Space: O(mn), where m is the number of rows and n is the number of columns in the input array. The code initializes an empty list of size m and then appends n elements to it for each row of the input array, resulting in a total of m*n elements.
Similar Reads
Basics & Prerequisites
Data Structures
Array Data StructureIn this article, we introduce array, implementation in different popular languages, its basic operations and commonly seen problems / interview questions. An array stores items (in case of C/C++ and Java Primitive Arrays) or their references (in case of Python, JS, Java Non-Primitive) at contiguous
3 min read
String in Data StructureA string is a sequence of characters. The following facts make string an interesting data structure.Small set of elements. Unlike normal array, strings typically have smaller set of items. For example, lowercase English alphabet has only 26 characters. ASCII has only 256 characters.Strings are immut
2 min read
Hashing in Data StructureHashing is a technique used in data structures that efficiently stores and retrieves data in a way that allows for quick access. Hashing involves mapping data to a specific index in a hash table (an array of items) using a hash function. It enables fast retrieval of information based on its key. The
2 min read
Linked List Data StructureA linked list is a fundamental data structure in computer science. It mainly allows efficient insertion and deletion operations compared to arrays. Like arrays, it is also used to implement other data structures like stack, queue and deque. Here’s the comparison of Linked List vs Arrays Linked List:
2 min read
Stack Data StructureA Stack is a linear data structure that follows a particular order in which the operations are performed. The order may be LIFO(Last In First Out) or FILO(First In Last Out). LIFO implies that the element that is inserted last, comes out first and FILO implies that the element that is inserted first
2 min read
Queue Data StructureA Queue Data Structure is a fundamental concept in computer science used for storing and managing data in a specific order. It follows the principle of "First in, First out" (FIFO), where the first element added to the queue is the first one to be removed. It is used as a buffer in computer systems
2 min read
Tree Data StructureTree Data Structure is a non-linear data structure in which a collection of elements known as nodes are connected to each other via edges such that there exists exactly one path between any two nodes. Types of TreeBinary Tree : Every node has at most two childrenTernary Tree : Every node has at most
4 min read
Graph Data StructureGraph Data Structure is a collection of nodes connected by edges. It's used to represent relationships between different entities. If you are looking for topic-wise list of problems on different topics like DFS, BFS, Topological Sort, Shortest Path, etc., please refer to Graph Algorithms. Basics of
3 min read
Trie Data StructureThe Trie data structure is a tree-like structure used for storing a dynamic set of strings. It allows for efficient retrieval and storage of keys, making it highly effective in handling large datasets. Trie supports operations such as insertion, search, deletion of keys, and prefix searches. In this
15+ min read
Algorithms
Searching AlgorithmsSearching algorithms are essential tools in computer science used to locate specific items within a collection of data. In this tutorial, we are mainly going to focus upon searching in an array. When we search an item in an array, there are two most common algorithms used based on the type of input
2 min read
Sorting AlgorithmsA Sorting Algorithm is used to rearrange a given array or list of elements in an order. For example, a given array [10, 20, 5, 2] becomes [2, 5, 10, 20] after sorting in increasing order and becomes [20, 10, 5, 2] after sorting in decreasing order. There exist different sorting algorithms for differ
3 min read
Introduction to RecursionThe process in which a function calls itself directly or indirectly is called recursion and the corresponding function is called a recursive function. A recursive algorithm takes one step toward solution and then recursively call itself to further move. The algorithm stops once we reach the solution
14 min read
Greedy AlgorithmsGreedy algorithms are a class of algorithms that make locally optimal choices at each step with the hope of finding a global optimum solution. At every step of the algorithm, we make a choice that looks the best at the moment. To make the choice, we sometimes sort the array so that we can always get
3 min read
Graph AlgorithmsGraph is a non-linear data structure like tree data structure. The limitation of tree is, it can only represent hierarchical data. For situations where nodes or vertices are randomly connected with each other other, we use Graph. Example situations where we use graph data structure are, a social net
3 min read
Dynamic Programming or DPDynamic Programming is an algorithmic technique with the following properties.It is mainly an optimization over plain recursion. Wherever we see a recursive solution that has repeated calls for the same inputs, we can optimize it using Dynamic Programming. The idea is to simply store the results of
3 min read
Bitwise AlgorithmsBitwise algorithms in Data Structures and Algorithms (DSA) involve manipulating individual bits of binary representations of numbers to perform operations efficiently. These algorithms utilize bitwise operators like AND, OR, XOR, NOT, Left Shift, and Right Shift.BasicsIntroduction to Bitwise Algorit
4 min read
Advanced
Segment TreeSegment Tree is a data structure that allows efficient querying and updating of intervals or segments of an array. It is particularly useful for problems involving range queries, such as finding the sum, minimum, maximum, or any other operation over a specific range of elements in an array. The tree
3 min read
Pattern SearchingPattern searching algorithms are essential tools in computer science and data processing. These algorithms are designed to efficiently find a particular pattern within a larger set of data. Patten SearchingImportant Pattern Searching Algorithms:Naive String Matching : A Simple Algorithm that works i
2 min read
GeometryGeometry is a branch of mathematics that studies the properties, measurements, and relationships of points, lines, angles, surfaces, and solids. From basic lines and angles to complex structures, it helps us understand the world around us.Geometry for Students and BeginnersThis section covers key br
2 min read
Interview Preparation
Practice Problem