Java Number Triangle Pattern (Alternating Direction)
Beginner
6 min read
Updated: Sep 2026
3 programs
Live preview
Definition
What Is This Pattern?
An alternating ascending/descending triangle fills numbers from 1 continuously, but flips print direction each row — odd rows ascend, even rows descend.
Remember
Rule: k starts at 1
for each row i: m = k + i - 1
odd i → print k upward
even i → print m downward
always k++ after each value
1
3 2
4 5 6
10 9 8 7
11 12 13 14 15 ← rows = 5
Unlike Program 50 (fixed digits with dual loops), here a running counter k drives Floyd-style filling with alternating direction.
Approach
How to Solve It
Keep a running counter k. For each row, compute the end m, then print ascending or descending by parity.
Method
Idea
Best for
Fixed rows
Hard-code rows; k/m logic
Labs and demos
Scanner input
Same logic; read rows at runtime
Interactive practice
No trailing space
Space before 2nd+ values only
Cleaner row formatting
Pseudocode
Pseudocode
k = 1
for i from 1 to rows:
m = k + i - 1
for j from 1 to i:
if i is odd: print k
else: print m; m = m - 1
k = k + 1
new line
Cheat sheet
Goal
Pattern
Start counter
int k = 1;
Row end
int m = k + i - 1;
Odd row
System.out.print(k + " ");
Even row
System.out.print(m-- + " ");
Advance
k++; after every printed value
Printing Numbers vs Starting a New Line
API
Effect
Use for
System.out.print
Stays on the same line
Each value in the row
System.out.println
Ends the line
After the inner loop finishes
Print values side by side, then break once per row.
Try it
Live Preview
Change the row count and the alternating triangle updates instantly.
Whole numbers from 3 to 9. Tap a chip or type a value — the preview redraws as you go.
Live resultrows = 5 · values = 15
1
3 2
4 5 6
10 9 8 7
11 12 13 14 15
Trace
Worked Walkthrough — rows = 5
Trace k, m, and row parity for each line.
i
Start k
m
Direction
Printed row
1
1
1
odd ↑
1
2
2
3
even ↓
3 2
3
4
6
odd ↑
4 5 6
4
7
10
even ↓
10 9 8 7
5
11
15
odd ↑
11 12 13 14 15
Always increment k after each print — even on even rows where you display m.
Code
Java Programs
Three complete programs: fixed rows = 5, Scanner input, and no trailing space. Use View Output to reveal sample results.
Example 1 — Fixed rows = 5
Hard-coded size — compute m = k + i - 1 and alternate print direction by row parity.
Java
public class AlternatingNumberTriangle {
public static void main(String[] args) {
int rows = 5;
int k = 1;
for (int i = 1; i <= rows; i++) {
int m = k + i - 1;
for (int j = 1; j <= i; j++) {
if (i % 2 == 1) {
System.out.print(k + " ");
} else {
System.out.print(m-- + " ");
}
k++;
}
System.out.println();
}
}
}
Output
1
3 2
4 5 6
10 9 8 7
11 12 13 14 15
How It Works
1. Running counter.k is the next number in sequence and never resets between rows.
2. Row end.m = k + i - 1 is the last value that belongs on this row.
3. Direction. Odd rows print k ascending; even rows print m downward. Always k++ after each value.
Example 2 — Scanner Input
Read rows at runtime. Same k/m logic.
Java
import java.util.Scanner;
public class AlternatingNumberTriangleInput {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.print("Enter the number of rows: ");
int rows = sc.nextInt();
if (rows < 1) return;
int k = 1;
for (int i = 1; i <= rows; i++) {
int m = k + i - 1;
for (int j = 1; j <= i; j++) {
if (i % 2 == 1) System.out.print(k + " ");
else System.out.print(m-- + " ");
k++;
}
System.out.println();
}
sc.close();
}
}
Output (when user enters 5)
Enter the number of rows: 5
1
3 2
4 5 6
10 9 8 7
11 12 13 14 15
How It Works
1. Prompt and guard. Read rows; exit early if it is less than 1.
2. Same logic. Only the source of rows changes — the counter and parity rules are identical.
3. Safer input tip. Prefer:
Safer input
if (!sc.hasNextInt()) {
System.out.println("Enter a positive integer.");
return;
}
int rows = sc.nextInt();
if (rows < 1) {
System.out.println("Enter a positive integer.");
return;
}
Example 3 — No Trailing Space
Same counter logic — space only before the second and later values on each row.
Java
public class AlternatingNumberTriangleNoTrail {
public static void main(String[] args) {
int rows = 5;
int k = 1;
for (int i = 1; i <= rows; i++) {
int m = k + i - 1;
for (int j = 1; j <= i; j++) {
if (j > 1) System.out.print(" ");
if (i % 2 == 1) System.out.print(k);
else System.out.print(m--);
k++;
}
System.out.println();
}
}
}
Output
1
3 2
4 5 6
10 9 8 7
11 12 13 14 15
How It Works
1. Same math. Still use m = k + i - 1 and odd/even direction.
2. Smarter spacing.if (j > 1) adds a space before later values — no trailing space after the last number.
3. Same shape. The triangle looks identical; only the end-of-row whitespace is cleaner.
Edge Cases & Pitfalls
Check these before calling the solution done.
Forgot k++
Stuck numbers
If you skip k++ on even rows, later rows reuse the wrong range. Always increment after every print.
Wrong m
Broken reverse
Use m = k + i - 1. Starting from k alone on even rows prints ascending instead of descending.
Reset k
Not continuous
Resetting k each row breaks the continuous fill — numbers must keep climbing across the whole triangle.
rows = 1
Single value
Output is just 1.
rows = 0
Empty output
The outer loop never runs. Guard interactive input with rows >= 1.
Bad input
Use hasNextInt
nextInt() throws on letters — prefer hasNextInt() and require a positive integer.
Analysis
Time and Space Complexity
Program
Time
Extra space
All three examples
O(n²)
O(1)
Total printed values for n rows: 1 + 2 + … + n = n(n+1)/2.
Remember
Key Takeaways
Rule: continuous k; odd rows print k, even rows print m downward.
Row end:m = k + i - 1 before the inner loop.
Always:k++ after every printed value — even on descending rows.
Complexity:O(n²) prints for n rows.
One line: fill with k; odd rows go up, even rows go down from m.
Frequently Asked Questions
Row 2 is even, so values 2 and 3 are printed in reverse order as 3 2.
If k is the next number to print, row i contains i values ending at m = k + i - 1.
Row 4 starts at k=7, so m=10. Even rows print m downward: 10, 9, 8, 7.
Yes. Use a variable rows in the outer loop — the same k/m logic works for any positive n.
Print a space only before the second and later values — see Example 3.
No. k is a running counter that continues across all rows.
O(n²) for n rows. Total prints are 1+2+...+n = n(n+1)/2.
Use sc.hasNextInt() before sc.nextInt() so bad input does not throw InputMismatchException.
🤔
Did you know?
Numbers fill continuously from 1, but each row alternates print direction — odd rows ascending (4 5 6), even rows descending (10 9 8 7). Compute row end with m = k + i - 1.
