Java Concentric Number Diamond Pattern

Beginner

7 min read

Updated: Sep 2026

3 programs

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Full concentric number diamond from 5 down to 1 and back to 5 in Java programming

Definition

What Is This Pattern?

A concentric number diamond prints the Program 46 top half (i = k..1), then mirrors it with i = 2..k — a full symmetric square of rings.

Remember

Rule: top i = k..1, then bottom i = 2..k
      each row: left j = k..1, right j = 2..k; print max(i, j)

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     ← k = 5

Unlike Program 46 (top half only), here a second outer loop completes the diamond.

Approach

How to Solve It

Print rows for i = k..1, then repeat the same row logic for i = 2..k.

Method	Idea	Best for
Two outer halves	Top `k..1`, bottom `2..k`; each row uses `max(i, j)`	Learning, interviews, exams
printRow helper	One method for both halves — same output, less duplication	Cleaner code once the row rule is clear

Pseudocode

for i from k down to 1:     // top
    print row for i
for i from 2 to k:          // bottom
    print row for i

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

Cheat sheet

Goal	Pattern
Top half	`for (int i = k; i >= 1; i--)`
Bottom half	`for (int i = 2; i <= k; i++)`
Left / right	`j = k..1` then `j = 2..k`
Cell value	`j > i ? j : i` or `Math.max(i, j)`
Total size	`2k − 1` rows and columns

Printing Numbers vs Starting a New Line

API	Effect	Use for
`System.out.print`	Stays on the same line	Each number (with a trailing space)
`System.out.println`	Ends the current line	After both half-loops of a row finish

Print values without a newline, then end the row once.

Try it

Live Preview

Change k and the concentric diamond updates instantly.

Trace

Worked Walkthrough — `k = 5`

Trace which i values print in each half and why the center row appears once.

Half	`i` values	Rows
Top	`5, 4, 3, 2, 1`	5 rows (includes the center)
Bottom	`2, 3, 4, 5`	4 rows (skips `i = 1`)
Center row	`i = 1`	`5 4 3 2 1 2 3 4 5`

Total rows = 5 + 4 = 9 = 2k − 1. Starting the bottom at 2 avoids a double center.

Code

Java Programs

Three complete programs: fixed k = 5, Scanner input, and a printRow helper. Use View Output to reveal sample results.

Example 1 — Fixed `k = 5`

Hard-coded size — top half then mirrored bottom half.

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--)
                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();
        }
    }
}

How It Works

1. Top half. Same as Program 46 — i from k down to 1.

2. Bottom half. i from 2 to k reprints the rings growing outward again.

3. Cell rule. Each value is still max(i, j) on left and right halves.

When i = 1: center row 5 4 3 2 1 2 3 4 5. When i = 5 (bottom): every cell is 5 again.

Example 2 — k Input

Read k at runtime. Same two-half core.

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();
        if (k < 1) return;

        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

1. Prompt and guard. Read k; exit early if it is less than 1.

2. Same core. Only the source of k changes from a literal to user input.

3. Safer input tip. Prefer:

Safer input

if (!sc.hasNextInt()) {
    System.out.println("Enter a positive integer.");
    return;
}
int k = sc.nextInt();
if (k < 1) {
    System.out.println("Enter a positive integer.");
    return;
}

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

1. Extract the row. printRow owns left half, right half, and println.

2. Short mains. Top and bottom loops only choose which i to pass.

3. Math.max. Same cell rule as the ternary — easier to read in a helper.

Edge Cases & Pitfalls

Check these before calling the solution done.

println inside

Column of numbers

If println is inside either half-loop, each value lands on its own line. Use print for values; println only after both halves.

Bottom from 1

Double center

Starting the bottom half at i = 1 reprints the center row and breaks the diamond.

Skip bottom

Looks like Program 46

Omitting the second outer loop prints only the top half.

No println

One long line

Forgetting println() after the row glues every number onto one endless line.

k = 1

Single cell

Output is just 1 — the bottom loop never runs.

Bad input

Use hasNextInt

nextInt() throws on letters — prefer hasNextInt() and require a positive integer.

Analysis

Time and Space Complexity

Program	Time	Extra space
Concentric diamond (Examples 1–3)	`O(k²)`	`O(1)`

2k − 1 rows each print 2k − 1 values — still quadratic. Only loop counters are stored.

Remember

Key Takeaways

Rule: each cell prints max(i, j) on left and right halves.
Two halves: top k..1, bottom 2..k — skip i = 1 on the way back.
DRY tip: extract printRow so both outer loops stay short.
Complexity: O(k²) time; O(1) extra space.

One line: print Program 46’s top half, then mirror with i = 2..k using the same max(i, j) row.

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.

Next: Powers of 11 Pattern

Move on to the powers of 11 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)

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

Java Concentric Number Diamond Pattern

What Is This Pattern?

How to Solve It

Pseudocode

Cheat sheet

Printing Numbers vs Starting a New Line

Live Preview

Worked Walkthrough — k = 5

Java Programs

Example 1 — Fixed k = 5

How It Works

Example 2 — k Input

How It Works

Example 3 — printRow Helper

How It Works

Edge Cases & Pitfalls

Column of numbers

Double center

Looks like Program 46

One long line

Single cell

Use hasNextInt

Time and Space Complexity

Key Takeaways

Frequently Asked Questions

Did you know?

Related Topics

Program 46

Program 48

Program 44

Number Patterns Hub

Next: Powers of 11 Pattern

About the author

Worked Walkthrough — `k = 5`

Example 1 — Fixed `k = 5`