PHP Concentric Number Diamond Pattern

Beginner
5 min read
Updated: Sep 2026
3 programs
Live preview
Full concentric number diamond pattern 5 down to 1 and back out in PHP programming
Definition

What Is This Pattern?

A concentric number diamond is Program 46’s square mirrored top-to-bottom: layers from $k down to 1, then back out to $k.

Remember
Rule: top i=k..1, bottom i=2..k; cell = (j > i ? j : i)

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

Same row logic as Program 46 (left $k..1, right 2..$k). Add a second outer loop $i = 2..$k to complete the diamond.

Approach

How to Solve It

Reuse the square’s row builder twice — once going in, once going out.

MethodIdeaBest for
Two-half diamondTop $k..1, bottom 2..$k, same cell ruleLearning, interviews, exams
Input $kSame logic with a user-chosen peakPractice / demos

Pseudocode

Pseudocode
for i from k down to 1:
    print left half j = k..1 and right half j = 2..k
    (value = j if j > i else i)
for i from 2 to k:
    print the same row rule again
    print newline after each row

Cheat sheet

GoalPattern
Top halffor ($i = $k; $i >= 1; $i--)
Bottom halffor ($i = 2; $i <= $k; $i++)
Left / right$j = $k..1 / $j = 2..$k
Cell valueecho ($j > $i ? $j : $i) . " ";
End the rowecho PHP_EOL;

Printing Numbers vs Starting a New Line

APIEffectUse for
echo ($j > $i ? $j : $i) . " "Stays on the same lineEach number plus a trailing space
echo PHP_EOLEnds the current lineAfter both half-loops finish a row

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

Try it

Live Preview

Change $k and the concentric diamond updates instantly.

Whole numbers from 1 to 7. Size = (2k - 1) × (2k - 1). Tap a chip or type a value — the preview redraws as you go.

Live result k = 5 · 9 × 9
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
Trace

Worked Walkthrough — $k = 3

Top half walks in; bottom half walks out (skip repeating the center).

Half$iPrinted row
Top33 3 3 3 3
Top23 2 2 2 3
Top13 2 1 2 3
Bottom23 2 2 2 3
Bottom33 3 3 3 3

Total lines = 2*$k - 1. Cells for $k = 5 = 9 × 9 = 81.

Code

PHP Programs

Three complete programs: fixed $k = 5, user-input $k, and a compact $k = 3 demo. Use View Output for sample results.

Example 1 — Fixed $k = 5

Top half then bottom half — same row builder twice.

PHP
<?php
$k = 5;

for ($i = $k; $i >= 1; $i--) {
    for ($j = $k; $j >= 1; $j--)
        echo ($j > $i ? $j : $i) . " ";
    for ($j = 2; $j <= $k; $j++)
        echo ($j > $i ? $j : $i) . " ";
    echo PHP_EOL;
}

for ($i = 2; $i <= $k; $i++) {
    for ($j = $k; $j >= 1; $j--)
        echo ($j > $i ? $j : $i) . " ";
    for ($j = 2; $j <= $k; $j++)
        echo ($j > $i ? $j : $i) . " ";
    echo PHP_EOL;
}

How It Works

1. Top half. $i runs from 5 down to 1 — same concentric rows as Program 46.

2. Bottom half. $i runs from 2 up to 5 — mirrors outward without repeating the center.

3. Cell rule. Every print uses $j > $i ? $j : $i on left $k..1 and right 2..$k.

Example 2 — User Input ($k)

Read $k; both halves and width scale automatically.

PHP
<?php
echo "Enter k: ";
$input = trim(fgets(STDIN));
if (!is_numeric($input) || (int) $input < 1) {
    echo "Please enter a positive whole number." . PHP_EOL;
    exit(1);
}

$k = (int) $input;

for ($i = $k; $i >= 1; $i--) {
    for ($j = $k; $j >= 1; $j--)
        echo ($j > $i ? $j : $i) . " ";
    for ($j = 2; $j <= $k; $j++)
        echo ($j > $i ? $j : $i) . " ";
    echo PHP_EOL;
}

for ($i = 2; $i <= $k; $i++) {
    for ($j = $k; $j >= 1; $j--)
        echo ($j > $i ? $j : $i) . " ";
    for ($j = 2; $j <= $k; $j++)
        echo ($j > $i ? $j : $i) . " ";
    echo PHP_EOL;
}

How It Works

1. Validate k. Check is_numeric; require $k >= 1.

2. Same diamond. Size becomes (2*$k - 1) × (2*$k - 1).

Example 3 — Compact $k = 3

A smaller fixed demo — five lines, easy to check by hand.

PHP
<?php
$k = 3;

for ($i = $k; $i >= 1; $i--) {
    for ($j = $k; $j >= 1; $j--)
        echo ($j > $i ? $j : $i) . " ";
    for ($j = 2; $j <= $k; $j++)
        echo ($j > $i ? $j : $i) . " ";
    echo PHP_EOL;
}

for ($i = 2; $i <= $k; $i++) {
    for ($j = $k; $j >= 1; $j--)
        echo ($j > $i ? $j : $i) . " ";
    for ($j = 2; $j <= $k; $j++)
        echo ($j > $i ? $j : $i) . " ";
    echo PHP_EOL;
}

How It Works

1. Same rules. Grow in, then out; cell value is always $j > $i ? $j : $i.

2. Quick check. Center row is 3 2 1 2 3; first and last rows are all 3s.

Edge Cases & Pitfalls

Check these before calling the solution done.

i = 1 twice

Bottom loop starts at $i = 1

The center row prints twice. Start the bottom half at $i = 2.

no bottom

Forget the second outer loop

You get Program 46’s square only. Add for ($i = 2; $i <= $k; $i++).

j = 1

Right half starts at $j = 1

The center digit doubles on every row. Keep $j = 2..$k.

k = 1

Single cell

Output is just 1 — the bottom loop never runs.

PHP_EOL

echo PHP_EOL inside a half-loop

That breaks the row. Call echo PHP_EOL only after both inner loops.

Bad input

Casting without checks

Prefer is_numeric after trim(fgets(STDIN)) so non-numeric input does not silently become 0.

Analysis

Time and Space Complexity

ProgramTimeExtra space
Fixed / compact (Examples 1, 3)O($k²)O(1)
User input (Example 2)O($k²)O(1)

The grid is (2*$k - 1) × (2*$k - 1), so total prints are about 4*$k² — quadratic in $k.

Remember

Key Takeaways

  • Rule: top $k..1, bottom 2..$k; cell = $j > $i ? $j : $i.
  • Shape: Program 46’s square plus a mirror — size (2*$k - 1)².
  • echo vs PHP_EOL: numbers stay on the line; echo PHP_EOL advances after both halves.
  • Next step: Program 48 prints the powers-of-11 sequence (1, 11, 121, …).

One line: print concentric rows inward to 1, then mirror them outward to finish the diamond.

Frequently Asked Questions

A full concentric number diamond: outer layer k, values decrease to 1 at the center, then increase back to k — a (2k-1)×(2k-1) grid.
The first prints the top half ($i = $k down to 1). The second prints the bottom half ($i = 2 up to $k) to mirror the shape.
Program 46 prints only the top half (k rows). Program 47 adds the bottom half loop to form a complete diamond.
When column $j is still outside the current row layer $i, print $j. Otherwise print $i (the current layer value).
Each row has 2*$k - 1 numbers. With $k = 5, width and height are both 9.
Change $k or read it from input — both outer loops and row width adjust automatically — see Example 2.
O(k²) because the grid has about (2k-1)² cells and each is printed once.
The center prints 1 — the deepest layer of the concentric pattern.
Use trim(fgets(STDIN)) and check is_numeric($input) before casting to int — see Example 2.

Did you know?

Top half: for ($i = $k; $i >= 1; $i--). Bottom half: for ($i = 2; $i <= $k; $i++). Each cell: $j > $i ? $j : $i. Grid size = 2k - 1 rows and columns.

Next: Powers of 11 Pattern

Print 1, 11, 121, 1331, 14641 with a running multiply-by-11 state.

Program 48 tutorial →

About the author

Mari Selvan M P
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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PHP Number Pattern 46 — Concentric Number Square PHP Number Pattern 48 — Powers of 11 Pattern