Right-Aligned Decreasing Number Triangle in PHP

Q: What pattern does this program print?

A right-aligned decreasing triangle: row 1 prints 5, row 2 prints 5 4, row 3 prints 5 4 3, and so on until 5 4 3 2 1.

Q: How is the triangle right-aligned?

An indentation loop prints two spaces while $j < $i before the number loop runs, pushing shorter rows to the right.

Q: Why use %2d in printf?

The format specifier reserves 2 columns per number (right-aligned), keeping columns stable in the console output.

Q: Why does each row start from 5?

The number loop runs for ($j = $rows; $j >= $i; $j--), so every row begins at $rows and counts down to the current $i.

Beginner

⏱️ 9 min read

📚 Updated: Aug 2026

🎯 3 Code Examples

🚀 Live Preview

Nested Loops + Alignment

Right-aligned decreasing number triangle output in PHP (5, 5 4, 5 4 3, ...)

Overview

What You’ll Learn

The right-aligned decreasing triangle prints 5, then 5 4, then 5 4 3, … — a natural follow-up after Program 35’s continuous counter triangle. This tutorial covers indentation loops, decreasing sequences, fixed-width formatting, nested loops, a live preview, worked PHP examples, edge cases, and complexity.

Shape Rule

Right-aligned triangle

Row $i prints $rows down to $i, with indentation spaces before the numbers.

Outer Loop

$i = $rows..1

for ($i = $rows; $i >= 1; $i--) — descending outer loop, one row per iteration.

Indent Loop

1..$i-1

for ($j = 1; $j < $i; $j++) — prints " " for right alignment.

Number Loop

%2d format

for ($j = $rows; $j >= $i; $j--) then printf("%2d", $j).

Live Preview

3–7 rows

Pick a row count and draw the right-aligned decreasing triangle in the browser.

O(n²)

Complexity

Total prints = n(n+1)/2 — work scales as n².

Introduction

A right-aligned decreasing number triangle prints a sequence that starts at $rows on every row and counts down to $i: 5, then 5 4, then 5 4 3, and so on. With $rows = 5, shorter rows shift right thanks to an indentation loop.

In PHP you use three nested loops: print " " while $j < $i, then print printf("%2d", $j) for $j = $rows..$i, then echo PHP_EOL.

Why it matters?

It combines two inner loops — one for spaces, one for numbers — a step after Program 35’s continuous counter pattern.

Key Highlights

$rows..$i

Decreasing sequence.

Indent loop

$j < $i spaces.

%2d

Fixed-width columns.

Series Foundation

Follow Program 35; continue to Program 37 next.

In short: outer $i = $rows..1, indent $j = 1..$i-1 with " ", numbers $j = $rows..$i with %2d, then echo PHP_EOL.

Foundation

📝 Problem & Approach

Given $rows = 5, print a right-aligned decreasing triangle: indentation spaces while $j < $i, then numbers from $rows down to $i with fixed-width formatting.

PHP

// $rows = 5
//        5
//      5 4
//    5 4 3
//  5 4 3 2
//5 4 3 2 1

Inputs & Outputs

Item	Type	Description
`$rows`	int	Triangle height and starting number for each row.
`$i`	int	Outer loop — current row limit (rows down to 1).
`$j`	int	Indent loop (`1..$i-1`) or number loop (`$rows..$i`).

Minimal workflow

Pseudocode

for i from rows down to 1:
    for j from 1 to i-1: print 2 spaces
    for j from rows down to i: print j in width 2
    print newline

Approach comparison

Approach	Idea	Best for
Fixed rows	`5`, `5 4`, …	Learning and interviews
User-input rows	`(int) trim(fgets(STDIN));`	Configurable triangle size
Compact trace	`$rows = 3` on paper first	Debugging loop bounds

Cheat Sheet

⚡ Quick Reference

Goal	Pattern
Outer loop	`for ($i = $rows; $i >= 1; $i--)`
Indent loop	`for ($j = 1; $j < $i; $j++) echo " ";`
Number loop	`for ($j = $rows; $j >= $i; $j--)`
Print number	`printf("%2d", $j);`
End the row	`echo PHP_EOL;`
User input	`(int) trim(fgets(STDIN))`

Compare

📋 Fixed vs User Input vs Compact Demo

Same right-aligned decreasing triangle — different ways to control the row count.

Outer loop

$i = $rows..1

Descending row limit

Indent

$j < $i

Two spaces per step

Numbers

$j = $rows..$i

Decreasing sequence

Learning tip

%2d

Fixed-width columns

Context

When This Pattern Shows Up

Reach for this pattern when teaching dual inner loops, decreasing sequences, and right-aligned console output.

After Program 35
Natural follow-up — replaces the continuous counter with a per-row decreasing sequence and a separate indent loop.
Dual inner-loop drills
Practice separating space printing from number printing before tackling more complex shapes.
Console I/O practice
Combine loops with fgets(STDIN) and is_numeric() checks for flexible row counts.
Gateway to variants
Compare Program 35 (incremental counter) and Program 37 (next in series) next.
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 dual inner loops, formatted output, and O(n²) thinking.

Interactive

🔮 Live Preview

Choose a row count between 3 and 7 and draw the right-aligned decreasing triangle in the browser.

Hands-On

Examples Gallery

Three complete PHP programs — fixed rows, user input, and a smaller trace demo. Click View Output to reveal sample console results.

📚 Getting Started

Print five rows of the right-aligned decreasing triangle with indent and number loops.

Example 1 — Fixed `$rows = 5`

Hard-coded row count — ideal for first demos and screenshots.

PHP

<?php
$rows = 5;

for ($i = $rows; $i >= 1; $i--) {
    for ($j = 1; $j < $i; $j++) {
        echo "  ";
    }

    for ($j = $rows; $j >= $i; $j--) {
        printf("%2d", $j);
    }

    echo PHP_EOL;
}

How It Works

When $i = 5, the indent loop prints four space pairs, then 5. When $i = 1, no indentation — output 5 4 3 2 1 with fixed-width columns.

📈 User Input

Read the row count with fgets(STDIN) instead of hard-coding 5.

Example 2 — User input rows

Read $rows with (int) trim(fgets(STDIN)) after checking is_numeric().

PHP

<?php
echo "Enter rows: ";
$input = trim(fgets(STDIN));
if (!is_numeric($input)) return;
$rows = (int) $input;
if ($rows < 1) return;

for ($i = $rows; $i >= 1; $i--) {
    for ($j = 1; $j < $i; $j++) {
        echo "  ";
    }

    for ($j = $rows; $j >= $i; $j--) {
        printf("%2d", $j);
    }

    echo PHP_EOL;
}

How It Works

Same dual inner-loop core as Example 1; only $rows comes from user input instead of being hard-coded as 5. Non-numeric input leaves $rows unset if you skip is_numeric() checks — always check it in safer labs.

⚡ Smaller Demo

Run with $rows = 3 to trace every row on paper before scaling up.

Example 3 — Compact `$rows = 3`

Same indent and number loops with a smaller row count for quick tracing.

PHP

<?php
$rows = 3;

for ($i = $rows; $i >= 1; $i--) {
    for ($j = 1; $j < $i; $j++) {
        echo "  ";
    }

    for ($j = $rows; $j >= $i; $j--) {
        printf("%2d", $j);
    }

    echo PHP_EOL;
}

How It Works

Only $rows changes from 5 to 3 — the indent and number loops stay identical. Trace $i = 3, 2, 1 on paper to see how indentation shrinks each row.

🧠 How the Algorithm Prints Rows

Set up

Set $rows = 5; and use fgets(STDIN) when reading input. Set loop variables $i, $j.

Setup

Outer loop walks rows

for ($i = $rows; $i >= 1; $i--) — descending outer loop; row limit shrinks each iteration.

Row

Indent loop

for ($j = 1; $j < $i; $j++) — prints " " for right alignment.

Align

Number loop

for ($j = $rows; $j >= $i; $j--) then printf("%2d", $j) — decreasing sequence.

New line

echo PHP_EOL ends the row after both inner loops finish.

Break

Right-aligned triangle complete

Total numbers = n(n+1)/2 — O(n²) time, O(1) extra memory.

Trace

🔎 Worked Walkthrough — `$rows = 5`

Trace each outer-loop value of $i, indentation steps, numbers printed, and full row output.

`$i`	Indent steps	Numbers printed	Row output
`5`	4	`5`	`5`
`4`	3	`5, 4`	`5 4`
`3`	2	`5, 4, 3`	`5 4 3`
`2`	1	`5, 4, 3, 2`	`5 4 3 2`
`1`	0	`5, 4, 3, 2, 1`	`5 4 3 2 1`

Indent steps per row = $i - 1 — zero when $i = 1. Numbers per row = $rows - $i + 1.

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: remove the indent loop and watch the triangle snap left.

2. Pattern Series Base

Foundation for right-aligned variants with separate space and number loops.

Example: compare with Program 35 (incremental counter) and Program 37 next.

3. Console Formatting Drills

Practice %2d formatting and fixed-width columns.

Example: change %2d to {0,3} for wider spacing on large row counts.

4. Padding character

Add two-space indent groups once the three-loop structure works.

Example: use a single space instead of " " and watch columns drift.

5. Complexity Intuition

Triangular totals make O(n²) concrete for beginners.

Example: count printed numbers for $rows = 5 — total is 1+2+3+4+5 = 15.

6. Input Validation Labs

Pair the pattern with is_numeric($input) checks and positive-row validation.

Example: reject $rows <= 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 PHP courses.

1. Instant Visual Feedback
Wrong bounds show up immediately as a broken staircase.
2. Minimal Concepts
Only loops and console output — no arrays or math libraries.
3. Easy to Extend
Invert, center, hollow, or change the fill character with small edits.
4. O(1) Extra Memory
Streaming output needs no storage beyond loop counters.

Pro Tip: trace $i and both $j loops on paper for $rows = 3 before coding — watch how indent steps shrink each row.

Usage Tips

Small habits that keep number-pattern code clean.

1. Two Inner Loops
Indent loop ($j < $i) and number loop ($j = $rows..$i) must run in order before echo PHP_EOL.
2. Call is_numeric($input)
Avoid undefined behavior when the user types letters instead of a number.
3. Keep newline outside inner loops
Only call echo PHP_EOL after both inner loops finish the row.
4. Trace $rows..$i on Paper
Write the decreasing sequence for each $i before coding the number loop.
5. Dry-Run $rows = 3
Trace $i = 3..1 on paper before coding the full $rows = 5 demo.

Pro Tip: if the output is a vertical list of single digits per line, you almost certainly put echo PHP_EOL inside an inner loop.

Common Pitfalls

Mistakes that commonly break right-aligned decreasing triangles.

1. Newline Inside an Inner Loop
Each digit lands on its own line — you get a column, not a triangle.
→ Use printf("%2d", $j); echo PHP_EOL only after both inner loops.
2. Skipping the Indent Loop
Without for ($j = 1; $j < $i; $j++), every row starts at the left margin.
→ Run the indent loop before the number loop on every row.
3. Wrong Space Width
Single spaces instead of " " break column alignment with %2d.
→ Print two spaces per indent step to match the fixed-width number columns.
4. Skipping %2d
Plain echo $j makes multi-digit values crowd earlier columns.
→ Use printf("%2d", $j) for consistent column width.
5. Unchecked CLI input
Letters or empty input leave $rows uninitialized or unchanged.
→ Call is_numeric($input) and re-prompt on failure.

Edge Cases

Check these inputs before calling the solution done.

$rows = 1

Single row

Output is just 1 with leading spaces — one value, one row.

$rows = 0

Empty pattern

Outer loop never runs when $rows < 1 — print nothing or show a message.

Negative

`$rows < 1`

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

$rows = 2

Smallest triangle

Two rows: 1 and 2 3.

Bad input

Non-numeric CLI input

Unchecked CLI input leaves $rows unset — call is_numeric($input) first.

Large rows

Large row count

Total numbers = $rows($rows+1)/2 — grows quadratically with rows.

Drill

🎯 Practice Problems

Try these variations to lock in the pattern.

1. Incremental counter

Review Program 35
Right-aligned triangle with continuous counter k

2. Next in series

Continue with Program 37
Next pattern in the number-pattern series

3. Sequence trace

Prove on paper: row $i prints $rows down to $i
Indent steps = $i - 1

4. Safe input loop

Call is_numeric($input) until $rows >= 1
Then draw the triangle

Important

Notes

Loop rule. Outer $i = $rows..1. Indent loop $j = 1..$i-1 prints " ". Number loop $j = $rows..$i prints %2d.
printf("%2d", $j) stays on the line; echo PHP_EOL advances — mix them carefully.
Validate $rows >= 1 for interactive programs; $rows = 1 prints a single 1.
Indent steps = $i - 1 — compare with Program 35 where a continuous counter replaces the per-row sequence.

Quick Takeaway: outer $i = $rows..1, indent $j = 1..$i-1 with " ", numbers $j = $rows..$i with %2d, then echo PHP_EOL.

Analysis

⏱️ Time and Space Complexity

Program	Time	Extra space
Nested loops (Examples 1–3)	`O(n²)`	`O(1)`
Smaller demo (Example 3)	`O(n²)`	`O(1)`

Wrap Up

🎉 Conclusion

The right-aligned decreasing number triangle is a compact lesson in dual inner loops and formatted output: indent with " ", print $rows down to $i with printf("%2d", $j), and end each row with echo PHP_EOL. Master the fixed-$rows version, then try user input and a smaller trace demo.

Practice the three examples above, then continue to Program 37 for the next pattern in the series.

Run the indent loop before the number loop — validate $rows when reading from the console.

View Examples Gallery Continue to Program 37

Pro Tips

💡 Best Practices

✅ Do

Use for ($i = $rows; $i >= 1; $i--) in the outer loop
Indent: for ($j = 1; $j < $i; $j++) echo " ";
Numbers: printf("%2d", $j) for $j = $rows..$i
Call is_numeric($input) instead of ignoring bad input
State O(n²) time when asked about complexity

❌ Don’t

Call echo PHP_EOL inside an inner loop
Skip the indent loop (breaks right alignment)
Use single spaces instead of " " for indentation
Ignore bad console input in user-facing demos
Skip the $rows = 1 edge case

Summary

Key Takeaways

Knowledge Unlocked

Five things to remember about this decreasing triangle

Print the pattern the beginner-friendly way.

Core concepts

△ 01

$rows..$i

Decreasing seq

Definition

↻ 02

Indent loop

$j < $i spaces

Code

0 03

%2d

Fixed width

Code

↓ 04

Row break

echo PHP_EOL after both inner loops

Shape

O 05

Complexity

O(n²) time

Analysis

❓ Frequently Asked Questions

What pattern does this program print?

A right-aligned decreasing triangle: row 1 prints 5, row 2 prints 5 4, row 3 prints 5 4 3, and so on until 5 4 3 2 1.

How is the triangle right-aligned?

An indentation loop prints two spaces while $j < $i before the number loop runs, pushing shorter rows to the right.

Why use %2d in printf?

The format specifier reserves 2 columns per number (right-aligned), keeping columns stable in the console output.

Why does each row start from 5?

The number loop runs for ($j = $rows; $j >= $i; $j--), so every row begins at $rows and counts down to the current $i.

How is this different from Program 35?

Program 35 uses a continuous counter $k. Program 36 restarts from $rows on each row with a separate indentation loop.

How do I change the number of rows?

Replace 5 with $rows in the outer loop bound — see Example 2.

What is the time complexity?

O(n²) for n rows because total prints are 1 + 2 + ... + n = n(n+1)/2.

How should I read rows safely?

Use trim(fgets(STDIN)) and check is_numeric($input) before casting to int — see Example 2 notes.

What happens when rows is 1?

Only one row prints — a single number with no leading spaces.

Did you Know? 🔊

Each row starts from $rows and counts down to $i. An indentation loop prints two spaces per step ($j = 1..$i-1), then printf("%2d", $j) keeps number columns aligned.

Continue to Program 37

Move on to the next pattern in the PHP number-pattern series.

Program 37 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 35 — Right-Aligned Incremental Triangle PHP Number Pattern 37 — Palindrome Number Triangle

Right-Aligned Decreasing Number Triangle in PHP

What You’ll Learn

Shape Rule

Outer Loop

Indent Loop

Number Loop

Live Preview

O(n²)

Introduction

Why it matters?

$rows..$i

Indent loop

%2d

Series Foundation

📝 Problem & Approach

Inputs & Outputs

Minimal workflow

Approach comparison

⚡ Quick Reference

📋 Fixed vs User Input vs Compact Demo

When This Pattern Shows Up

After Program 35

Dual inner-loop drills

Console I/O practice

Gateway to variants

Not a UI layout tool

🔮 Live Preview

Examples Gallery

📚 Getting Started

Example 1 — Fixed $rows = 5

How It Works

📈 User Input

Example 2 — User input rows

How It Works

⚡ Smaller Demo

Example 3 — Compact $rows = 3

How It Works

🧠 How the Algorithm Prints Rows

Set up

Outer loop walks rows

Indent loop

Number loop

New line

Right-aligned triangle complete

🔎 Worked Walkthrough — $rows = 5

Use Cases

1. Teaching Nested Loops

2. Pattern Series Base

3. Console Formatting Drills

4. Padding character

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. Two Inner Loops

2. Call is_numeric($input)

3. Keep newline outside inner loops

4. Trace $rows..$i on Paper

5. Dry-Run $rows = 3

Common Pitfalls

1. Newline Inside an Inner Loop

2. Skipping the Indent Loop

3. Wrong Space Width

4. Skipping %2d

5. Unchecked CLI input

Edge Cases

Single row

Empty pattern

$rows < 1

Smallest triangle

Non-numeric CLI input

Large row count

🎯 Practice Problems

1. Incremental counter

2. Next in series

3. Sequence trace

Example 1 — Fixed `$rows = 5`

Example 3 — Compact `$rows = 3`

🔎 Worked Walkthrough — `$rows = 5`

2. Call `is_numeric($input)`

`$rows < 1`