Concentric Number Diamond Pattern in Java

Beginner

⏱️ 8 min read

📚 Updated: Aug 2026

🎯 3 Code Examples

🚀 Live Preview

Nested Loops + Mirror

Concentric number diamond pattern in Java (5 5 5 … down to 5 4 3 2 1 2 3 4 5)

Overview

What You’ll Learn

The concentric number diamond prints a full symmetric shape: top half i = k..1, bottom mirror i = 2..k, each cell from max(i, j). This tutorial covers both outer loops, live preview, algorithm steps, worked Java examples, edge cases, and complexity.

Shape Rule

max(i,j)

Print j when j > i; otherwise i — same rule as Program 46, applied to both halves.

Top Half

i = k..1

for (i = k; i >= 1; i--) shrinks toward the center row.

Bottom Mirror

i = 2..k

for (i = 2; i <= k; i++) expands back out — skips i=1 to avoid duplicating center.

Row Columns

j = k..1, 2..k

Each row uses left j=k..1 and right j=2..k — 2k-1 values per row.

Live Preview

k = 3–8

Pick a value for k and draw the full concentric number diamond in the browser.

O(k²)

Complexity

Prints (2k-1)² values total — e.g. 81 for k=5; memory stays O(1).

Introduction

A concentric number diamond pattern shrinks toward the center, then expands back symmetrically. With k = 5, you get 2k-1 = 9 rows; the center row is 5 4 3 2 1 2 3 4 5.

In Java you run a top-half loop (i = k..1), then a bottom mirror (i = 2..k). Each row prints max(i, j) over left and right column loops.

Why it matters?

It extends Program 46 with one mirror loop — the classic two-halves diamond technique.

Key Highlights

Top Half

i = k..1 — shrink toward center.

Bottom Mirror

i = 2..k — expand without repeating center.

max(i,j) Rule

Same cell rule in every row of both halves.

Series Foundation

Follow Program 46 concentric square; continue to Program 48 powers of 11.

In short: top loop i=k..1, bottom loop i=2..k, each row prints max(i,j), then println().

Foundation

📝 Problem & Approach

Given k = 5, print a full diamond: top half i=k..1, bottom mirror i=2..k, each cell max(i,j).

Java

// k = 5 (conceptual shape)
// 5 5 5 5 5 5 5 5 5
// 5 4 4 4 4 4 4 4 5
// 5 4 3 3 3 3 3 4 5
// 5 4 3 2 2 2 3 4 5
// 5 4 3 2 1 2 3 4 5
// 5 4 3 2 2 2 3 4 5
// 5 4 3 3 3 3 3 4 5
// 5 4 4 4 4 4 4 4 5
// 5 5 5 5 5 5 5 5 5

Inputs & Outputs

Item	Type	Description
`k`	int	Maximum value and row count (typically ≥ 1).
Printed output	text	`2k-1 rows` rows, each with `2k-1 rows` space-separated numbers.

Minimal workflow

Pseudocode

for i from k down to 1:
    for j from k down to 1:
        print max(i, j)
    for j from 2 to k:
        print max(i, j)
    print newline

Approach comparison

Approach	Idea	Best for
Two outer loops + max rule	`j>i ? j : i` in both halves	Learning and interviews
User-input k	`sc.nextInt();`	Flexible console programs
printRow helper	`Math.max(i,j)` in both loops	Cleaner production-style code

Cheat Sheet

⚡ Quick Reference

Goal	Pattern
Walk rows	`for (i = k; i >= 1; i--)`
Top half	`for (j = k; j >= 1; j--)`
Bottom mirror	`for (j = 2; j <= k; j++)`
Value rule	`j > i ? j : i` or `Math.max(i, j)`
End the row	`System.out.println();`
Program 46 contrast	Concentric square prints top k rows only; this diamond mirrors with i=2..k

Compare

📋 Top Half vs Bottom Half vs Combined

Same diamond row — how top and bottom outer loops reuse the max(i,j) rule.

Top half

i = k..1

Left segment j=k..1 before the mirror

Bottom mirror

i = 2..k

Right segment j=2..k completes each row

Value rule

max(i,j)

Print j when j>i; otherwise print i

Learning tip

trace i=3,j=2

Dry-run cell (3,2): max(3,2) → prints 3

Context

When This Pattern Shows Up

Reach for this pattern when teaching max(i,j) inside mirrored column loops.

First lab exercise
Classic follow-up after concentric diamonds and series patterns.
Nested-loop warm-up
Outer/inner bound practice with an immediate visual check.
Console I/O practice
Combine loops with Scanner for a flexible row count.
Gateway to variants
Compare with Program 46 (concentric square), then continue to Program 48 (powers of 11).
Not a UI layout tool
This is a console teaching pattern — not how you build modern app screens.

Key benefit: one small program that locks in nested loops, output sequencing, and O(k²) thinking.

Interactive

🔮 Live Preview

Choose a value for k and draw the concentric number diamond in the browser.

Hands-On

Examples Gallery

Three complete Java programs — fixed k, Scanner input, and a printRow helper variant. Click View Output to reveal sample console results.

📚 Getting Started

Print nine rows for k=5 — top half plus mirrored bottom.

Example 1 — Fixed `k = 5`

Two outer loops — top i=k..1 and bottom i=2..k — with the same inner max rule.

Java

public class ConcentricNumberDiamond {
    public static void main(String[] args) {
        int k = 5;

        for (int i = k; i >= 1; i--) {
            for (int j = k; j >= 1; j--) {
                if (j > i) System.out.print(j + " ");
                else System.out.print(i + " ");
            }
            for (int j = 2; j <= k; j++) {
                if (j > i) System.out.print(j + " ");
                else System.out.print(i + " ");
            }
            System.out.println();
        }

        for (int i = 2; i <= k; i++) {
            for (int j = k; j >= 1; j--) {
                if (j > i) System.out.print(j + " ");
                else System.out.print(i + " ");
            }
            for (int j = 2; j <= k; j++) {
                if (j > i) System.out.print(j + " ");
                else System.out.print(i + " ");
            }
            System.out.println();
        }
    }
}

How It Works

The first loop prints rows 5 down to 1; the second loop prints rows 2 up to 5. Starting the bottom at i = 2 keeps the center row from printing twice.

📈 Practical Variant

Let the user choose k at runtime.

Example 2 — User Input Version

Read k with Scanner.nextInt() (check hasNextInt() in real apps).

Java

import java.util.Scanner;

public class ConcentricNumberDiamondInput {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter k (e.g., 5): ");
        int k = sc.nextInt();

        for (int i = k; i >= 1; i--) {
            for (int j = k; j >= 1; j--) System.out.print((j > i ? j : i) + " ");
            for (int j = 2; j <= k; j++) System.out.print((j > i ? j : i) + " ");
            System.out.println();
        }

        for (int i = 2; i <= k; i++) {
            for (int j = k; j >= 1; j--) System.out.print((j > i ? j : i) + " ");
            for (int j = 2; j <= k; j++) System.out.print((j > i ? j : i) + " ");
            System.out.println();
        }

        sc.close();
    }
}

How It Works

Same nested-loop core as Example 1; only the source of k changes. Non-numeric input throws InputMismatchException with nextInt() — check hasNextInt() for safer labs.

⚡ Readability Variant

Extract row printing into a helper to avoid duplicating four inner loops.

Example 3 — printRow Helper

One printRow(i, k) method — both outer loops call it.

Java

public class ConcentricNumberDiamondHelper {
    static void printRow(int i, int k) {
        for (int j = k; j >= 1; j--) System.out.print(Math.max(i, j) + " ");
        for (int j = 2; j <= k; j++) System.out.print(Math.max(i, j) + " ");
        System.out.println();
    }

    public static void main(String[] args) {
        int k = 5;

        for (int i = k; i >= 1; i--) printRow(i, k);
        for (int i = 2; i <= k; i++) printRow(i, k);
    }
}

How It Works

printRow encapsulates the left/right inner loops and row break — both outer loops stay short and readable.

🧠 How the Algorithm Prints Rows

Set up

System.out is built in; use Scanner when reading input. Set k (fixed or from input).

Setup

Top half loop

for (i = k; i >= 1; i--) — shrinks toward the center row.

Top

max(i,j) per row

Left j=k..1 and right j=2..k print max(i,j), then println().

Cells

Bottom mirror loop

for (i = 2; i <= k; i++) — expands back out; skips center duplicate.

Mirror

Concentric number diamond pattern complete

Total values printed ≈ (2k-1)² — O(k²) time, O(1) extra memory.

Trace

🔎 Worked Walkthrough — why bottom half starts at `i = 2`

Trace the mirror boundary to see why the center row is not duplicated.

Check	Result	Prints
Top half ends	`i = 1`	center row printed once
Bottom half starts	`i = 2`	avoids duplicate center
Total rows	`2k-1`	9 when k=5

Full diamond: 2k-1 = 9 rows and 9 values per row when k=5.

Use Cases

Where this tiny pattern (and its loop structure) shows up beyond the homework prompt.

1. Teaching Nested Loops

Clearest visual proof that outer and inner bounds interact.

Example: use Math.max(i,j) instead of if-else — see Example 3.

2. Pattern Series Base

Foundation for concentric layouts, symmetric grids, and distance-based rules.

Example: swap max for min(i,j) to explore a different shape.

3. Console Formatting Drills

Practice System.out.print vs row newline without complex math.

Example: put System.out.println() inside the inner loop by mistake.

4. Character Substitution

Swap numbers for letters or stars once the max rule works.

Example: print row numbers with leading spaces for alignment.

5. Complexity Intuition

Grid totals make O(k²) concrete for beginners.

Example: count values for k=5 → 9 rows × 9 values = 81 prints.

6. Input Validation Labs

Pair the pattern with Scanner and positive-row checks.

Example: reject k <= 0 and re-prompt.

Pro Tip: when an interviewer asks for patterns, explain the outer/inner roles first — then write the loops. The story matters as much as the code.

Advantages

Why this pattern earns a permanent spot in beginner Java courses.

1. Instant Visual Feedback
Wrong mirror bounds (starting right half at 1) duplicate the center value.
2. Minimal Concepts
Only loops and console output — no arrays or math libraries.
3. Easy to Extend
Change k, swap max for min, or extract printRow for cleaner code.
4. O(1) Extra Memory
Streaming output needs no storage beyond loop counters.

Pro Tip: learn the top and bottom mirror loops first; then try the printRow helper in Example 3.

Usage Tips

Small habits that keep number-pattern code clean.

1. Name Bounds Clearly
Use k for the outer bound and keep i/j for row/column — or rename to row/col.
2. Prefer Scanner
Avoid crashes when the user types letters instead of a number.
3. Keep System.out.println() Outside
Only call System.out.println() after the inner loop finishes the row.
4. Use Ternary for Compact Code
Compact: System.out.print(Math.max(i, j) + " "); in both inner loops.
5. Dry-Run One Small n
Trace k = 3 on paper before coding larger demos.

Pro Tip: if the output is a vertical list of numbers per line, you almost certainly put System.out.println() inside the inner loop.

Common Pitfalls

Mistakes that commonly break concentric number diamond patterns.

1. System.out.println() Inside the Inner Loop
Each number lands on its own line — you get a column, not a symmetric row.
→ Use System.out.print for each value; System.out.println() only after both inner loops finish.
2. Bottom Half Starts at i = 1
Starting the mirror loop at i = 1 prints the center row twice in the full diamond.
→ Keep for (i = 2; i <= k; i++) for the bottom half.
3. Wrong Right-Half Start (Inner Loop)
Starting the right inner loop at j = 1 prints the center value twice on every row.
→ Keep for (j = 2; j <= k; j++) so the center appears once per row.
4. Forgetting the Row Break
Omitting System.out.println() glues every row onto one endless line.
→ Always end the row after both inner loops complete.
5. Unchecked Scanner input
Letters or empty input throw InputMismatchException.
→ Prefer Scanner and re-prompt on failure.
6. Hard-coding 5 everywhere
Using literal 5 in loop bounds instead of variable k breaks dynamic input.
→ Use one k variable for the outer bound and both inner loops.

Edge Cases

Check these inputs before calling the solution done.

k = 1

Single row

Output is one row of 2k-1 rows numbers; for k=1 you get a single 1.

k = 0

Empty pattern

Outer loop never runs — print nothing or show a message.

Negative

`k < 0`

Treat as invalid; re-prompt instead of silent empty output.

Large n

Large k

Output grows with k rows and 2k-1 rows values per row — fine for labs, noisy for huge k.

Bad input

Non-numeric Scanner input

Unchecked Scanner leaves k unset — call sc.hasNextInt() first.

Compact

Math.max form

Use Math.max(i,j) for cleaner code — see Example 3.

Drill

🎯 Practice Problems

Try these variations to lock in the pattern.

1. Change k

Try k = 3 or k = 4 — count 2k-1 rows
Verify center row prints once

2. Compare with Program 46

Print only top half (drop second outer loop)
See how mirror completes the diamond

3. printRow refactor

Extract helper like Example 3
Add trailing-space trim with StringBuilder

4. Next in series

Continue with Program 48 powers of 11
Try min(i,j) for a different shape

Important

Notes

Value count. Total numbers printed ≈ (2k-1 rows)² (e.g. 9×9 = 81 for k=5).
print stays on the line; println advances — mix them carefully.
Validate k > 0 for interactive programs; k = 1 prints one value.
Bottom mirror must start at j = 2 so the center value is not duplicated.

Quick Takeaway: set k, loop i = k..1, print max(i,j) for left and right halves, then break the row.

Analysis

⏱️ Time and Space Complexity

Program	Time	Extra space
Nested loops (Examples 1–2)	`O(k²)`	`O(1)`
printRow helper (Example 3)	`O(k²)`	`O(1)`

Wrap Up

🎉 Conclusion

The concentric number diamond combines top and bottom mirror loops with max(i,j) — a natural step after concentric number squares. Master the fixed-k version first, then try user input and the printRow helper form in Example 3.

Practice the three examples above, then continue to Program 48 for the powers of 11 number pattern.

Every cell uses print — keep println() only after the inner column loop finishes.

View Examples Gallery Continue to Program 48

Pro Tips

💡 Best Practices

✅ Do

Explain top half, bottom mirror, and max(i,j) before coding
Use print(max(i,j) + " ") and println() after both inner loops
Validate k ≥ 1 for interactive programs
Check Scanner return value before using k
State O(k²) time when asked about complexity

❌ Don’t

Call System.out.println() between left and right halves (mid-row break)
Start the bottom half at i = 1 (duplicates center row)
Start the right half at j = 1 (duplicates center value)
Hard-code 5 instead of variable k
Ignore bad console input in user-facing demos
Skip the k = 1 edge case

Summary

Key Takeaways

Knowledge Unlocked

Five things to remember about this concentric number diamond pattern

Print the pattern the beginner-friendly way.

Core concepts

△ 01

Rule

Each cell prints max(i,j)

Definition

↻ 02

Top half

i = k..1

Code

↓ 03

Bottom mirror

i = 2..k

Logic

n 04

Total rows

2k-1 rows

I/O

O 05

Complexity

O(k²)

Analysis

❓ Frequently Asked Questions

Why does the pattern have 9 rows for k=5?

Top half prints 5 rows (i=5..1) and bottom half prints 4 rows (i=2..5). Total = 2k-1 = 9.

Why does each row have 9 columns?

Left half k..1 plus right half 2..k gives k + (k-1) = 2k-1 values per row.

Why start the bottom half at i=2?

Starting at 2 avoids duplicating the center row (i=1) that already printed in the top half.

How are values chosen in each cell?

Print j when j>i; otherwise i — equivalent to max(i, j).

How is this different from Program 46?

Program 46 prints only the top k rows. This pattern adds a bottom mirror loop i=2..k for a full diamond.

Can I use a helper method for each row?

Yes. Extract printRow(i, k) and call it from both outer loops — see Example 3.

What is the time complexity?

O(k²) because 2k-1 rows each print about 2k-1 values.

How should I read k safely?

Use sc.hasNextInt() before sc.nextInt() so bad input does not throw InputMismatchException.

Did you Know? 🔊

Print the top half with i = k..1, then mirror with i = 2..k. Each cell uses max(i, j) over left j = k..1 and right j = 2..k — total rows = 2k-1.

Continue to Program 48

Move on to the 1, 11, 121, 1331, 14641 number pattern in the Java number-pattern series.

Program 48 tutorial →

About the author

Mari Selvan M P 🔗

Developer, cloud engineer, and technical writer

Experience 12 years building web and cloud systems
Focus Full Stack Development, AWS, and Developer Education

I write practical tutorials so students and working developers can learn by doing—from databases and APIs to deployment on AWS.

AWS Certified Developer – Associate (Credly)

12 people found this page helpful

Java Number Pattern 46 — Concentric Number Square Java Number Pattern 48 — Powers of 11 Pattern

Concentric Number Diamond Pattern in Java

What You’ll Learn

Shape Rule

Top Half

Bottom Mirror

Row Columns

Live Preview

O(k²)

Introduction

Why it matters?

Top Half

Bottom Mirror

max(i,j) Rule

Series Foundation

📝 Problem & Approach

Inputs & Outputs

Minimal workflow

Approach comparison

⚡ Quick Reference

📋 Top Half vs Bottom Half vs Combined

When This Pattern Shows Up

First lab exercise

Nested-loop warm-up

Console I/O practice

Gateway to variants

Not a UI layout tool

🔮 Live Preview

Examples Gallery

📚 Getting Started

Example 1 — Fixed k = 5

How It Works

📈 Practical Variant

Example 2 — User Input Version

How It Works

⚡ Readability Variant

Example 3 — printRow Helper

How It Works

🧠 How the Algorithm Prints Rows

Set up

Top half loop

max(i,j) per row

Bottom mirror loop

Concentric number diamond pattern complete

🔎 Worked Walkthrough — why bottom half starts at i = 2

Use Cases

1. Teaching Nested Loops

2. Pattern Series Base

3. Console Formatting Drills

4. Character Substitution

5. Complexity Intuition

6. Input Validation Labs

Advantages

1. Instant Visual Feedback

2. Minimal Concepts

3. Easy to Extend

4. O(1) Extra Memory

Usage Tips

1. Name Bounds Clearly

2. Prefer Scanner

3. Keep System.out.println() Outside

4. Use Ternary for Compact Code

5. Dry-Run One Small n

Common Pitfalls

1. System.out.println() Inside the Inner Loop

2. Bottom Half Starts at i = 1

3. Wrong Right-Half Start (Inner Loop)

4. Forgetting the Row Break

5. Unchecked Scanner input

6. Hard-coding 5 everywhere

Edge Cases

Single row

Empty pattern

k < 0

Large k

Non-numeric Scanner input

Math.max form

🎯 Practice Problems

1. Change k

2. Compare with Program 46

3. printRow refactor

Example 1 — Fixed `k = 5`

🔎 Worked Walkthrough — why bottom half starts at `i = 2`

2. Prefer `Scanner`

`k < 0`