Alternate elements of an array
Last Updated :
04 Dec, 2024
Given an array arr[], the task is to print every alternate element of the array starting from the first element.
Examples:
Input: arr[] = [10, 20, 30, 40, 50]
Output: 10 30 50
Explanation: Print the first element (10), skip the second element (20), print the third element (30), skip the fourth element(40) and print the fifth element(50).
Input: arr[] = [-5, 1, 4, 2, 12]
Output: -5 4 12
Iterative Approach
The idea is to start iterating from index 0, print the element at that index, and then increment the index by 2 to move to the next alternate element. Keep on printing the elements till we reach the end of the array.
C++
// Iterate C++ Program to print alternate elements
// of the array
#include <iostream>
#include <vector>
using namespace std;
vector<int> getAlternates(vector<int> &arr) {
vector<int> res;
// Iterate over all alternate elements
for(int i = 0; i < arr.size(); i += 2) {
res.push_back(arr[i]);
}
return res;
}
int main() {
vector<int> arr = {10, 20, 30, 40, 50};
vector<int> res = getAlternates(arr);
for(int x: res)
cout << x << " ";
}
// Iterate C Program to print alternate elements
// of the array
#include <stdio.h>
void getAlternates(int arr[], int n) {
// Iterate over all alternate elements
for(int i = 0; i < n; i += 2) {
printf("%d ", arr[i]);
}
}
int main() {
int arr[] = {10, 20, 30, 40, 50};
int size = sizeof(arr) / sizeof(arr[0]);
getAlternates(arr, size);
return 0;
}
// Iterate Java Program to print alternate elements
// of the array
import java.util.*;
class GfG {
static ArrayList<Integer> getAlternates(int[] arr) {
ArrayList<Integer> res = new ArrayList<>();
// Iterate over all alternate elements
for (int i = 0; i < arr.length; i += 2) {
res.add(arr[i]);
}
return res;
}
public static void main(String[] args) {
int[] arr = {10, 20, 30, 40, 50};
ArrayList<Integer> res = getAlternates(arr);
for (int x : res) {
System.out.print(x + " ");
}
}
}
# Iterate Python Program to print alternate elements
# of the array
def getAlternates(arr):
res = []
# Iterate over all alternate elements
for i in range(0, len(arr), 2):
res.append(arr[i])
return res
if __name__ == "__main__":
arr = [10, 20, 30, 40, 50]
res = getAlternates(arr)
print(" ".join(map(str, res)))
// Iterate C# Program to print alternate elements
// of the array
using System;
using System.Collections.Generic;
class GfG {
static List<int> getAlternates(int[] arr) {
List<int> res = new List<int>();
// Iterate over all alternate elements
for (int i = 0; i < arr.Length; i += 2) {
res.Add(arr[i]);
}
return res;
}
static void Main() {
int[] arr = { 10, 20, 30, 40, 50 };
List<int> res = getAlternates(arr);
foreach (int x in res) {
Console.Write(x + " ");
}
}
}
// Iterate JavaScript Program to print alternate elements
// of the array
function getAlternates(arr) {
let res = [];
// Iterate over all alternate elements
for (let i = 0; i < arr.length; i += 2) {
res.push(arr[i]);
}
return res;
}
// Driver Code
const arr = [10, 20, 30, 40, 50];
const res = getAlternates(arr);
console.log(res.join(" "));
Time Complexity: O(n), where n is the number of elements in arr[].
Auxiliary Space: O(1)
Recursive Approach
We can also print the alternate elements using recursion. We start from index = 0, that is the first element of the array and print its value. We then call the recursive function again with the (index + 2) as the current index.
C++
// Recursive C++ Program to print alternate elements
// of the array
#include <iostream>
#include <vector>
using namespace std;
// Recursive function to store all alternate elements
void getAlternatesRec(vector<int> &arr, int idx, vector<int>& res) {
if(idx < arr.size()) {
res.push_back(arr[idx]);
getAlternatesRec(arr, idx + 2, res);
}
}
vector<int> getAlternates(vector<int> &arr) {
vector<int> res;
getAlternatesRec(arr, 0, res);
return res;
}
int main() {
vector<int> arr = {10, 20, 30, 40, 50};
vector<int> res = getAlternates(arr);
for(int x: res)
cout << x << " ";
}
// Recursive Java Program to print alternate elements
// of the array
import java.util.ArrayList;
class GfG {
// Recursive function to store all alternate elements
static void getAlternatesRec(int[] arr, int idx,
ArrayList<Integer> res) {
if (idx < arr.length) {
res.add(arr[idx]);
getAlternatesRec(arr, idx + 2, res);
}
}
static ArrayList<Integer> getAlternates(int[] arr) {
ArrayList<Integer> res = new ArrayList<>();
getAlternatesRec(arr, 0, res);
return res;
}
public static void main(String[] args) {
int[] arr = {10, 20, 30, 40, 50};
ArrayList<Integer> res = getAlternates(arr);
for (int x : res) {
System.out.print(x + " ");
}
}
}
# Recursive Python Program to print alternate elements
# of the array
# Recursive function to store all alternate elements
def getAlternatesRec(arr, idx, res):
if idx < len(arr):
res.append(arr[idx])
getAlternatesRec(arr, idx + 2, res)
def getAlternates(arr):
res = []
getAlternatesRec(arr, 0, res)
return res
if __name__ == "__main__":
arr = [10, 20, 30, 40, 50]
res = getAlternates(arr)
print(" ".join(map(str, res)))
// Recursive C# Program to print alternate elements
// of the array
using System;
using System.Collections.Generic;
class GfG {
// Recursive function to store all alternate elements
static void getAlternatesRec(int[] arr, int idx,
List<int> res) {
if (idx < arr.Length) {
res.Add(arr[idx]);
getAlternatesRec(arr, idx + 2, res);
}
}
static List<int> getAlternates(int[] arr) {
List<int> res = new List<int>();
getAlternatesRec(arr, 0, res);
return res;
}
static void Main() {
int[] arr = { 10, 20, 30, 40, 50 };
List<int> res = getAlternates(arr);
foreach (int x in res) {
Console.Write(x + " ");
}
}
}
// Recursive JavaScript Program to print alternate
// elements of the array
// Recursive function to store all alternate elements
function getAlternatesRec(arr, idx, res) {
if (idx < arr.length) {
res.push(arr[idx]);
getAlternatesRec(arr, idx + 2, res);
}
}
function getAlternates(arr) {
let res = [];
getAlternatesRec(arr, 0, res);
return res;
}
// Driver Code
let arr = [10, 20, 30, 40, 50];
let res = getAlternates(arr);
console.log(res.join(" "));
Time Complexity: O(n), where n is the number of elements in arr[].
Auxiliary Space: O(n), for recursive call stack.
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