Diamond Diagonal Number Pattern in PHP

Beginner

⏱️ 9 min read

📚 Updated: Aug 2026

🎯 3 Code Examples

🚀 Live Preview

Top + Bottom Halves

Diamond diagonal number pattern in PHP (1 1, 2 2, 3 3, 4 4, 5)

Overview

What You’ll Learn

The diamond diagonal pattern prints Program 53’s diagonal mirror rows for $i = 1..$n, then mirrors them vertically with a second loop $i = $n-1..1. For $n = 5 you get nine rows forming a full X diamond. This tutorial covers both outer loops, $i==$j and $i==$k logic, live preview, worked PHP examples, edge cases, and O(n²) complexity.

Main Diagonal

$i == $j

Left loop prints digit j when row i equals column j.

Mirror Diagonal

$i == $k

Right loop prints digit $k when $i == $k; runs $k = $n-1..1.

Space Fill

else " "

All non-diagonal positions print a space — only two digits per row (except overlap).

Vertical Mirror

2nd loop

Second outer loop runs $i = $n-1..1 — same row logic, reversed order.

Live Preview

3–12 for n

Pick size n and draw the full diamond diagonal pattern instantly in the browser.

O(n²)

Complexity

2n-1 rows × 2n-1 cells per row — total work grows as n².

Introduction

A diamond diagonal number pattern is Program 53 mirrored vertically. The top half prints rows 1..$n; the bottom half repeats rows $n-1..1. Row 1 and the last row both show 1 1; the middle row shows a single center digit.

In PHP: first outer loop for ($i = 1; $i <= $n; $i++), then second for ($i = $n - 1; $i >= 1; $i--). Each iteration runs the same inner diagonal loops and ends with echo PHP_EOL.

Why it matters?

It combines diagonal row printing with vertical mirroring — a classic diamond pattern technique.

Key Highlights

Outer loop i

$i = 1..$n picks each row number.

Main + mirror

Left loop prints when $i==$j; mirror loop prints when $i==$k.

Top + Bottom Halves

Two loops per row — main diagonal half, then mirror diagonal half.

Series Foundation

Builds on Program 53 diagonal mirror; continue to Program 55 diagonal-fill triangle.

In short: top loop $i=1..$n, bottom loop $i=$n-1..1; each row uses $i==$j and $i==$k inner loops, then echo PHP_EOL.

Foundation

📝 Problem & Approach

Given $n = 5, print nine lines — top half rows 1..5, then bottom half rows 4..1.

PHP

// $n = 5 (conceptual output)
// 1       1
//  2     2 
//   3   3  
//    4 4   
//     5

Inputs & Outputs

Item	Type	Description
`$n`	int	Pattern size — top-half row count; full output has `2n-1` rows (typically n ≥ 1).
`i, j`	int	Row index `i`; column `j` for main diagonal, column `k` for mirror diagonal.
Printed output	text	`2n-1` rows total and `2n-1` cells per row — diamond dimensions.

Minimal workflow

Pseudocode

for i from 1 to n: print diagonal row ($i==$j, $i==$k)
for i from n-1 down to 1: print same diagonal row
each row: j loop + k loop, then newline

Approach comparison

Approach	Idea	Best for
Two diagonal loops	`if ($i==$j) echo $j else echo " "`; mirror loop with `$i==$k`	Fixed width — 2n-1 cells per row
Vertical mirror	Second outer loop `$i = $n-1..1` repeats row logic in reverse	Full diamond — `2n-1` rows total
fgets(STDIN) input	`(int) $input` after `is_numeric($input)`	User-chosen pattern size
Star diagonals	Print * on diagonals instead of numbers	Visual X-shape without digits — Example 3

Cheat Sheet

⚡ Quick Reference

Goal	Pattern
Set size	`$n = 5;`
Top half loop	`for ($i = 1; $i <= $n; $i++)`
Bottom half loop	`for ($i = $n - 1; $i >= 1; $i--)`
Main diagonal	`for ($j = 1; $j <= $n; $j++)` — print when `$i == $j`
Mirror diagonal	`for ($k = $n - 1; $k >= 1; $k--)` — print when `$i == $k`
Non-diagonal cell	`echo " ");`
Row break	`echo PHP_EOL;` after both halves
Program 53 contrast	Program 53 prints only the top n rows; this adds a second outer loop to mirror vertically

Compare

📋 Top Half vs Bottom Half vs Row Logic

How outer row selection, main diagonal loop, mirror diagonal loop, and row breaks work together.

Top half

for ($i = 1; $i <= $n; $i++)

Prints rows 1 through n — same as Program 53.

Bottom half

for ($i = $n - 1; $i >= 1; $i--)

Mirrors top half — starts at n-1 to skip duplicate middle row.

Row logic

j loop: $i==$j
k loop: $i==$k ($k=$n-1..1)

Identical inner loops in both outer halves — main and mirror diagonals.

Learning tip

trace n=3

Full diamond: 5 rows — top 1..3, bottom 2..1.

Context

When This Pattern Shows Up

Reach for this pattern when teaching vertical mirroring, diagonal conditions, and full diamond output.

First lab exercise
Classic follow-up after Program 53 diagonal mirror — adds vertical mirroring.
Nested-loop warm-up
Outer/inner bound practice with an immediate visual check.
Console I/O practice
Combine loops with fgets(STDIN) for a flexible pattern size.
Gateway to variants
Compare with Program 53 (top half only), then continue to Program 55 diagonal-fill triangle.
Not a UI layout tool
This is a console teaching pattern — not how you build modern app screens.

Key benefit: one program that locks in vertical mirroring, diagonal conditions, and O(n²) thinking.

Interactive

🔮 Live Preview

Choose pattern size $n and draw the full diamond diagonal number pattern in the browser.

Hands-On

Examples Gallery

Three complete PHP programs — fixed $n = 5, fgets(STDIN) input, and a star-diagonal variant. Click View Output to reveal sample console results.

📚 Getting Started

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

Example 1 — Fixed `$n = 5`

Hard-coded size — top half loop 1..$n, bottom half $n-1..1, same diagonal row logic.

PHP

<?php
$n = 5;

for ($i = 1; $i <= $n; $i++) {
    for ($j = 1; $j <= $n; $j++) {
        if ($i == $j) echo $j;
        else echo " ";
    }
    for ($k = $n - 1; $k >= 1; $k--) {
        if ($i == $k) echo $k;
        else echo " ";
    }
    echo PHP_EOL;
}

for ($i = $n - 1; $i >= 1; $i--) {
    for ($j = 1; $j <= $n; $j++) {
        if ($i == $j) echo $j;
        else echo " ";
    }
    for ($k = $n - 1; $k >= 1; $k--) {
        if ($i == $k) echo $k;
        else echo " ";
    }
    echo PHP_EOL;
}

How It Works

The first outer loop prints rows 1..5 (Program 53 output). The second loop prints rows 4..1, mirroring vertically. Row 1 and row 9 both show 1 1.

📈 Practical Variant

Read $n with fgets(STDIN) for flexible output size.

Example 2 — fgets(STDIN) Input

Same diagonal conditions; size n comes from user input.

PHP

<?php
echo "Enter n: ";
$input = trim(fgets(STDIN));
if (!is_numeric($input)) {
    echo "Invalid input." . PHP_EOL;
    exit(1);
}
$n = (int) $input;

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

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

How It Works

Same two outer loops as Example 1; only the size comes from user input.

⚡ Character Variant

Print * on diagonals instead of row numbers.

Example 3 — Star Diagonals

Replace digits with * when $i==$j or $i==$k.

PHP

<?php
$n = 5;

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

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

How It Works

Same two outer loops and diagonal logic; digits become * on both halves.

🧠 How the Algorithm Builds the Diamond

Set up

echo is built in; use fgets(STDIN) when reading input. Set $n (e.g. 5).

Setup

Top half loop

for ($i = 1; $i <= $n; $i++) — prints n rows (Program 53 output).

Loop

Bottom half loop

for ($i = $n - 1; $i >= 1; $i--) — mirrors top half; starts at $n-1 to skip duplicate middle row.

Diagonals

Row logic (both halves)

Each row: j loop with $i==$j, k loop with $i==$k, then echo PHP_EOL.

Break

Full diamond complete

Total rows = 2n-1; total checks ≈ (2n-1)(2n-1) — O(n²) time, O(1) extra memory.

Trace

🔎 Worked Walkthrough — `n = 3` full diamond

Trace how top half (rows 1..3) and bottom half (rows 2..1) combine into five output lines.

Phase	Outer i	Output line
Top half	1	1 1
Top half	2	2 2
Top half	3	3
Bottom half	2	2 2
Bottom half	1	1 1

Five rows total for n=3. Bottom half starts at i=2, not 3, so the middle row is not duplicated.

Use Cases

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

1. Teaching Top + Bottom Halves

Clearest visual proof that outer and inner bounds interact.

Example: use fgets(STDIN) for dynamic n — see Example 2.

2. Mirror Loop Bounds

Mirror loop runs k = n-1 down to 1 so the center column is not duplicated.

Example: compare row 3 with n=5 — digits at columns 3 and 3 on main and mirror diagonals.

3. Console Formatting Drills

Practice PHP_EOL vs print for multi-line vs single-line output.

Example: use echo "*" when $i==$j or $i==$k — see Example 3.

4. Star Diagonals

Print * when $i==$j or $i==$k instead of the row number.

Example: print n=5 with * on diagonals and compare the X shape.

5. Complexity Intuition

Fixed width 2n-1 makes O(n²) concrete for beginners.

Example: count cells for n=5 → 9 rows × 9 cells = 81 prints.

6. Input Validation Labs

Pair the pattern with fgets(STDIN) and positive-row checks.

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

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

Advantages

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

1. Instant Visual Feedback
Wrong mirror loop start shows immediately — center column may print twice.
2. Minimal Concepts
Only loops and console output — no arrays or math libraries.
3. Easy to Extend
Change $n, use fgets(STDIN), or print * on diagonals instead of numbers.
4. O(1) Extra Memory
Streaming output needs no storage beyond loop counters.

Pro Tip: learn the $i==$j and $i==$k conditions first; then try fgets(STDIN) input and the star variant in Example 3.

Usage Tips

Small habits that keep number-pattern code clean.

1. Name Variables Clearly
Use $n for pattern size and i/j/k for row/column indices.
2. Prefer fgets(STDIN)
Avoid crashes when the user types letters instead of a number.
3. Row Break After Inner Loop
Run left diagonal loop, run mirror loop, then echo PHP_EOL.
4. Use print for Diagonal Cells
Use echo $j or echo " " in the main loop; same for the mirror loop; one echo PHP_EOL per row after both halves.
5. Dry-Run One Small n
Trace $n = 5, $i = 3 on paper — expect digits at both diagonal positions.

Pro Tip: if the center column prints twice, check whether the mirror loop starts at n instead of n-1.

Common Pitfalls

Mistakes that commonly break diamond diagonal number patterns.

1. echo PHP_EOL Inside an Inner Loop
Each cell lands on its own line — you get a column, not an X-shaped row.
→ Use print inside both inner loops; echo PHP_EOL only after they finish.
2. Bottom Half Starts at n
Starting the second outer loop at $i = $n duplicates the middle row — the diamond gets an extra widest line.
→ Start the bottom half at $i = $n - 1 and count down to 1.
3. Forgetting the Row Break
Omitting echo PHP_EOL after both inner loops glues all rows onto one line.
→ Always call echo PHP_EOL after both diagonal loops complete.
4. Mirror Loop Starts at n
Starting at k = n duplicates the center column within a row.
→ Start the inner mirror loop at k = n - 1 and count down to 1.
5. Unchecked fgets(STDIN) Input
Letters or empty input throw invalid input without validation.
→ Call is_numeric($input) before casting to (int).
6. Hard-coding 5 Everywhere
Using literal 5 in loop bounds instead of variable $n breaks dynamic input.
→ Use one $n variable for both outer loops and inner bounds.

Edge Cases

Check these inputs before calling the solution done.

n = 1

Single line

Output is one line: 1 — bottom half loop does not run.

n = 0

Empty pattern

Loop never runs — print nothing or show a message.

Negative

`n < 0`

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

Large n

Large values produce wide rows — fine for labs; use smaller n for quick demos.

Bad input

Non-numeric fgets(STDIN) input

Unchecked fgets(STDIN) input leaves $n unset — call is_numeric($input) first.

Compact

Single-line form

Use conditional spacing to avoid trailing spaces — see Example 3.

Drill

🎯 Practice Problems

Try these variations to lock in the pattern.

1. Change n

Try n = 3, 6, or 8
Verify digits appear only on $i==$j and $i==$k positions

2. Star diagonals

Print * instead of numbers on diagonals
Compare number X vs star X output

3. Bottom half bounds

Try bottom loop $i = $n..1 instead of $n-1..1
Observe middle-row duplication

4. Next in series

Continue with Program 55 diagonal-fill triangle
Compare full diamond vs Program 53 top half only

Important

Notes

Cell count. Total checks ≈ (2n-1)² (e.g. 81 cells for n=5).
print stays on the line; PHP_EOL advances — mix them carefully.
Validate n > 0 for interactive programs; n = 1 prints a single digit.
Main diagonal: if ($i==$j) echo $j else echo " "; mirror: if ($i==$k) echo $k else echo " ".

Quick Takeaway: top loop $i=1..$n, bottom loop $i=$n-1..1; each row uses $i==$j and $i==$k inner loops, then echo PHP_EOL.

Analysis

⏱️ Time and Space Complexity

Program	Time	Extra space
Fixed `$n = 5` (Example 1)	`O(n²)`	`O(1)`
fgets(STDIN) input (Example 2)	`O(n²)`	`O(1)`
Star diagonals (Example 3)	`O(n²)`	`O(1)`

Wrap Up

🎉 Conclusion

The diamond diagonal pattern combines Program 53 row logic with a vertical mirror loop — a natural step after single-half diagonal patterns. Master the fixed-$n version first, then try fgets(STDIN) input and the star-diagonal variant in Example 3.

Practice the three examples above, then continue to Program 55 for the diagonal-fill number triangle pattern.

Keep bottom half at $i = $n-1..1 and echo PHP_EOL after each row — one break per outer iteration.

View Examples Gallery Continue to Program 55

Pro Tips

💡 Best Practices

✅ Do

Explain top loop 1..$n, bottom loop $n-1..1, and diagonal checks before coding
Use echo $j or echo " " in left loop; same for mirror loop; then echo PHP_EOL
Validate n ≥ 1 for interactive programs
Check fgets(STDIN) return value before using $n
State O(n²) time when asked about complexity

❌ Don’t

Start bottom half at $i = $n (duplicates middle row)
Start mirror loop at k = n (duplicates center column)
Use $i == $j in the mirror loop by mistake
Hard-code 5 instead of variable $n
Ignore bad console input in user-facing demos
Skip the n = 1 edge case

Summary

Key Takeaways

Knowledge Unlocked

Five things to remember about this diamond diagonal pattern

Print Program 53 top half, then mirror rows $n-1..1 for a full X diamond.

Core concepts

△ 01

Rule

Full diamond has 2n-1 rows

Definition

↻ 02

Outer loops

Top: 1..$n; bottom: $n-1..1

Code

↓ 03

Diagonal checks

Main: $i==$j; mirror: $i==$k

Logic

n 04

Fixed width

2n-1 cells/row

I/O

O 05

Complexity

O(n²)

Analysis

❓ Frequently Asked Questions

How is this different from Program 53?

Program 53 prints only $n rows (the top half). Program 54 adds a second outer loop from $n-1 down to 1 to mirror the output vertically.

Why does the bottom half start at n-1?

Starting at $n would duplicate the middle row (the widest point). $n-1 down to 1 mirrors without repeating the center.

How many rows does the output have?

For size $n, the diamond has 2*$n-1 rows: $n for the top half and $n-1 for the bottom half.

Why does the middle row show only one number?

At $i=$n, both diagonals meet at the same position, so only one digit appears on the center row.

Can I change 5 to any n?

Yes. Use a variable $n — both outer loops reuse the same diagonal row logic.

How do I print stars instead of numbers?

Replace the printed digit with * when $i==$j or $i==$k in both outer loops — see Example 3.

What is the time complexity?

O(n²) for (2*$n-1) rows. Each row checks O(n) cells in two inner loops.

How should I read n safely?

Use trim(fgets(STDIN)) and is_numeric($input) before casting to int.

Did you Know? 🔊

Program 54 prints the diagonal mirror pattern (Program 53) for rows 1..$n, then repeats the same logic for rows $n-1..1 — forming a full diamond with numbers on both diagonals.

Continue to Program 55

Move on to the diagonal-fill number triangle pattern in the PHP number-pattern series.

Program 55 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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PHP Number Pattern 53 — Diagonal Mirror Number Pattern PHP Number Pattern 55 — Diagonal-Fill Number Triangle Pattern

Diamond Diagonal Number Pattern in PHP

What You’ll Learn

Main Diagonal

Mirror Diagonal

Space Fill

Vertical Mirror

Live Preview

O(n²)

Introduction

Why it matters?

Outer loop i

Main + mirror

Top + Bottom Halves

Series Foundation

📝 Problem & Approach

Inputs & Outputs

Minimal workflow

Approach comparison

⚡ Quick Reference

📋 Top Half vs Bottom Half vs Row Logic

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 $n = 5

How It Works

📈 Practical Variant

Example 2 — fgets(STDIN) Input

How It Works

⚡ Character Variant

Example 3 — Star Diagonals

How It Works

🧠 How the Algorithm Builds the Diamond

Set up

Top half loop

Bottom half loop

Row logic (both halves)

Full diamond complete

🔎 Worked Walkthrough — n = 3 full diamond

Use Cases

1. Teaching Top + Bottom Halves

2. Mirror Loop Bounds

3. Console Formatting Drills

4. Star Diagonals

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 Variables Clearly

2. Prefer fgets(STDIN)

3. Row Break After Inner Loop

4. Use print for Diagonal Cells

5. Dry-Run One Small n

Common Pitfalls

1. echo PHP_EOL Inside an Inner Loop

2. Bottom Half Starts at n

3. Forgetting the Row Break

4. Mirror Loop Starts at n

5. Unchecked fgets(STDIN) Input

6. Hard-coding 5 Everywhere

Edge Cases

Single line

Empty pattern

n < 0

Large n

Non-numeric fgets(STDIN) input

Single-line form

🎯 Practice Problems

1. Change n

2. Star diagonals

3. Bottom half bounds

Example 1 — Fixed `$n = 5`

🔎 Worked Walkthrough — `n = 3` full diamond

2. Prefer `fgets(STDIN)`

`n < 0`