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Java Program to Display Numbers and Sum of First N Natural Numbers
• Last Updated : 09 Nov, 2020

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

1. Start for loop initialization with i = 1.
2. Write testing condition as i <= N.
3. Add increment statement as i++ or i+=1.
4. Initialize a variable sum with 0.
5. Start adding i with the sum at each iteration of for loop and print i.
6. Print sum at the end for loop.

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 Number ` `import` `java.io.*; ` `class` `GFG { ` `    ``public` `static` `void` `main(String[] args) ` `    ``{ ` `        ``int` `N = ``10``; ` `        ``int` `sum = ``0``; ` `        ``System.out.print(``"First "` `+ N + ``" Numbers = "``); ` ` `  `        ``// 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 + ``" "``); ` `            ``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)

Alternate Approach

1. Start for loop initialization with i = 1.
2. Write testing condition as i <= N.
3. Add increment statement as i++ or i+=1.
4. Start Printing i for each iteration.
5. 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 Number ` `import` `java.io.*; ` ` `  `class` `GFG { ` `   `  `    ``public` `static` `void` `main(String[] args) ` `    ``{ ` `        ``int` `N = ``5``; ` `        ``System.out.print(``"First "` `+ N + ``" Numbers = "``); ` ` `  `        ``// 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)

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