Print first N natural numbers using an iterative approach i.e. using for loop. For loop has three parameters initialization, testing condition, and increment/decrement.
Input: N = 10
Output: First 10 Numbers = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
Sum of first 10 Natural Number = 55
Input: N = 5
Output: First 5 Numbers = 1, 2, 3, 4, 5
Sum of first 5 Natural Number = 15
Approach
- Start for loop initialization with i = 1.
- Write testing condition as i <= N.
- Add increment statement as i++ or i+=1.
- Initialize a variable sum with 0.
- Start adding i with the sum at each iteration of for loop and print i.
- Print sum at the end for loop.Â
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Below is the implementation of above approach
Java
// Java Program to Display Numbers// from 1 to N Using For Loop and// sum of First N Natural Numberimport java.io.*;class GFG {Â Â Â Â public static void main(String[] args)Â Â Â Â {Â Â Â Â Â Â Â Â int N = 10;Â Â Â Â Â Â Â Â int sum = 0;Â Â Â Â Â Â Â Â System.out.print("First " + N + " Numbers = ");Â
        // we initialize the value of the variable i        // with 1 and increment each time with 1        for (int i = 1; i <= N; i++) {Â
            // print the value of the variable as            // long as the code executes            System.out.print(i + " ");            sum += i;        }        System.out.println();        System.out.println("Sum of first " + N                           + " Natural Number = " + sum);    }} |
Output
First 10 Numbers = 1 2 3 4 5 6 7 8 9 10 Sum of first 10 Natural Number = 55
Time Complexity: O(n)
Auxiliary Space: O(1) because constant space for variables is being used
Alternate Approach
- Start for loop initialization with i = 1.
- Write testing condition as i <= N.
- Add increment statement as i++ or i+=1.
- Start Printing i for each iteration.
- Print sum using first N natural number formula at the end of for loop.
Below is the implementation of the above approach
Java
// Java Program to Display Numbers// from 1 to N Using For Loop and// sum of First N Natural Numberimport java.io.*;Â
class GFG {Â Â Â Â Â Â Â public static void main(String[] args)Â Â Â Â {Â Â Â Â Â Â Â Â int N = 5;Â Â Â Â Â Â Â Â System.out.print("First " + N + " Numbers = ");Â
        // we initialize the value of the variable i        // with 1 and increment each time with 1        for (int i = 1; i <= N; i++) {                       // print the value of the variable as            // long as the code executes            System.out.print(i + " ");        }        System.out.println();        System.out.println("Sum of first " + N                           + " Natural Number = " + (N*(N+1))/2);    }} |
Output
First 5 Numbers = 1 2 3 4 5 Sum of first 5 Natural Number = 15
Time Complexity: O(n)
Auxiliary Space: O(1) as it is using constant space for variables
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