Java Program to Print Number Pattern

You can print a number pattern in Java using nested for loops; for example, for (int i = 1; i <= n; i++) { for (int j = 1; j <= i; j++) { System.out.print(j + " "); } System.out.println(); } prints numbers from 1 up to the current line number.

Examples

Inputn = 3

Output1 1 2 1 2 3

Inputn = 5

Output1 1 2 1 2 3 1 2 3 4 1 2 3 4 5

Inputn = 1

Output1

How to Think About It

To print a number pattern, think of each line as a row where you print numbers starting from 1 up to the line number. Use one loop to go through each line and a nested loop inside it to print the numbers for that line.

Algorithm

1

Get the number of lines to print (n).

2

Start a loop from 1 to n for each line.

3

Inside this loop, start another loop from 1 to the current line number.

4

Print the current number in the inner loop followed by a space.

5

After the inner loop ends, print a new line to move to the next line.

6

Repeat until all lines are printed.

Code

java

public class NumberPattern {
    public static void main(String[] args) {
        int n = 5; // number of lines
        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print(j + " ");
            }
            System.out.println();
        }
    }
}

Output

1 
1 2 
1 2 3 
1 2 3 4 
1 2 3 4 5 

Dry Run

Let's trace n=3 through the code

1

Start outer loop i=1

Inner loop runs j=1; prints '1 '

2

Print newline after inner loop

Output so far: 1

3

Outer loop i=2

Inner loop runs j=1, j=2; prints '1 2 '

4

Print newline after inner loop

Output so far: 1 1 2

5

Outer loop i=3

Inner loop runs j=1, j=2, j=3; prints '1 2 3 '

6

Print newline after inner loop

Output so far: 1 1 2 1 2 3

i (line)	j (number printed)	Output line
1	1	1
2	1 2	1 2
3	1 2 3	1 2 3

Why This Works

Step 1: Outer loop controls lines

The outer for loop runs from 1 to n, controlling how many lines are printed.

Step 2: Inner loop prints numbers

The inner for loop runs from 1 to the current line number i, printing numbers in sequence.

Step 3: New line after each row

After printing numbers for a line, System.out.println() moves the cursor to the next line.

Alternative Approaches

Using while loops

java

public class NumberPattern {
    public static void main(String[] args) {
        int n = 5;
        int i = 1;
        while (i <= n) {
            int j = 1;
            while (j <= i) {
                System.out.print(j + " ");
                j++;
            }
            System.out.println();
            i++;
        }
    }
}

This uses while loops instead of for loops; functionally the same but slightly longer.

Printing pattern in reverse order

java

public class NumberPattern {
    public static void main(String[] args) {
        int n = 5;
        for (int i = n; i >= 1; i--) {
            for (int j = 1; j <= i; j++) {
                System.out.print(j + " ");
            }
            System.out.println();
        }
    }
}

This prints the pattern starting from n numbers down to 1, reversing the pattern.

Complexity: O(n^2) time, O(1) space

Time Complexity

The nested loops cause the program to run in O(n^2) time because for each line i, it prints i numbers.

Space Complexity

The program uses constant extra space O(1) as it only prints output and uses a few variables.

Which Approach is Fastest?

All approaches have similar O(n^2) time; using for loops is more concise and readable than while loops.

Approach	Time	Space	Best For
Nested for loops	O(n^2)	O(1)	Simple and readable pattern printing
Nested while loops	O(n^2)	O(1)	When loop conditions are dynamic or less predictable
Reverse pattern with for loops	O(n^2)	O(1)	Printing patterns in descending order