Alternating Zigzag Number Triangle Pattern in PHP

Q: What is the time complexity?

O(n²) where n is the number of rows. Total echo calls equal n+(n-1)+…+1 = n(n+1)/2.

Q: Can I flip the parity rule?

Yes. Swap the if and else branches so odd rows descend and even rows ascend — the zigzag still works, just mirrored.

Q: How should I read the row count safely?

Use trim(fgets(STDIN)) and check is_numeric($input) before casting to int so bad input does not produce unexpected results.

Q: What happens if rows is 0 or negative?

The outer loop never runs, so nothing is printed. Validate and prompt again if you want a clear user message.

Beginner

⏱️ 8 min read

📚 Updated: Aug 2026

🎯 3 Code Examples

🚀 Live Preview

If / Else + Loops

Alternating zigzag number triangle output (12345, 4321, 123, 21, 1) in PHP programming

Overview

What You’ll Learn

The alternating zigzag number triangle combines nested loops with an if/else parity check to flip print direction each row. This tutorial covers the shape rule, loop structure, a live preview, algorithm steps, worked PHP examples, edge cases, and complexity.

Shape Rule

odd i: 1..i, even i: i..1

Row length 5 prints 12345; length 4 prints 4321; length 3 prints 123, and so on.

Outer Loop

Rows

for ($i = $rows; $i >= 1; $i--) walks each line from the longest down to a single digit.

Parity Branch

if / else

if ($i % 2 === 0) prints descending; else prints ascending with echo $j.

echo vs PHP_EOL

Same line / next line

Digits use echo $j; end each row with echo PHP_EOL;.

Live Preview

1–20 rows

Pick a row count and draw the zigzag number triangle instantly in the browser.

O(n²)

Complexity

Total digit prints still = n(n+1)/2; extra memory stays O(1).

Introduction

An alternating zigzag number triangle shrinks each row while flipping print direction based on row length parity. With rows = 5, the output is 12345, 4321, 123, 21, 1.

In PHP you solve it with a descending outer loop plus an if/else: odd $i uses for ($j = 1; $j <= $i; $j++), even $i uses for ($j = $i; $j >= 1; $j--), then echo PHP_EOL; ends each row.

Why it matters?

It introduces conditions inside loops — a stepping stone from pure nested loops to logic-heavy pattern problems.

Key Highlights

Parity Switches Direction

Odd i prints 1..i; even i prints i..1.

If / Else Branch

i % 2 picks ascending vs descending inner loop.

Print Then Break

echo $j in the inner loop; echo PHP_EOL; after.

Series Foundation

Follow Program 12; continue to Program 14 (odd-length rows).

In short: for each row length i from rows down to 1, use i % 2 to print 1..i or i..1, then call echo PHP_EOL;.

Foundation

📝 Problem & Approach

Given a positive integer rows, print an alternating zigzag number triangle: row length i prints digits ascending when i is odd and descending when i is even, with the outer loop counting from rows down to 1.

PHP

// First 5 rows (conceptual shape)
// 12345
// 4321
// 123
// 21
// 1

Inputs & Outputs

Item	Type	Description
`rows`	int	Number of triangle lines to print (typically ≥ 1).
Printed output	text	Each row prints sequential digits; odd-length rows ascend, even-length rows descend.

Minimal workflow

Pseudocode

for i from rows down to 1:
    if i is even:
        for j from i down to 1: print j
    else:
        for j from 1 to i: print j
    print newline

Approach comparison

Approach	Idea	Best for
if/else + nested loops	Parity branch + inner direction	Learning and interviews
Direction variables	Compute start, end, step from parity	Shorter production-style demos

Cheat Sheet

⚡ Quick Reference

Goal	Pattern
Walk each row	`for ($i = $rows; $i >= 1; $i--)`
Odd row: ascending	`for ($j = 1; $j <= $i; $j++) echo $j;`
Even row: descending	`for ($j = $i; $j >= 1; $j--) echo $j;`
Parity check	`if ($i % 2 === 0) { ... } else { ... }`
End the row	`echo PHP_EOL;`
Program 12 variant	`for ($j = $i; $j <= $rows; $j++) echo $i;` (repeating digits)

Compare

📋 Ascending vs Descending vs Parity

Same zigzag triangle — different ways to pick inner-loop direction.

Ascending (odd i)

1..i

Prints 123 when row length is 3

Descending (even i)

i..1

Prints 4321 when row length is 4

i % 2

parity

Even i reverses; odd i goes forward

Learning tip

if/else first

Master explicit branches before direction variables

Context

When This Pattern Shows Up

Reach for this pattern when teaching conditions inside nested loops.

First lab exercise
Most PHP pattern series start here before pyramids and diamonds.
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 row count.
Gateway to variants
Compare Program 12 (11111, 2222, …) and Program 14 (odd-length rows) 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 nested loops, output sequencing, and O(n²) thinking.

Interactive

🔮 Live Preview

Choose a row count between 1 and 20 and draw the alternating zigzag number triangle in the browser.

Hands-On

Examples Gallery

Three complete PHP programs — fixed row count, CLI input, and a direction-variables refactor. Click View Output to reveal sample console results.

📚 Getting Started

Print five rows of the alternating zigzag number triangle with if/else branches.

Example 1 — Fixed `rows = 5`

Hard-coded height — odd rows ascend, even rows descend.

PHP

<?php
$rows = 5;

for ($i = $rows; $i >= 1; $i--) {
    if ($i % 2 === 0) {
        for ($j = $i; $j >= 1; $j--) {
            echo $j;
        }
    } else {
        for ($j = 1; $j <= $i; $j++) {
            echo $j;
        }
    }
    echo PHP_EOL;
}

How It Works

When i = 5 (odd), the inner loop prints 12345. When i = 4 (even), it prints 4321, and so on until i = 1 prints 1. echo PHP_EOL; after the inner loop starts the next row.

📈 Practical Variant

Let the user choose the height at runtime.

Example 2 — User Input Version

Read the row count with (int) trim(fgets(STDIN)) (check is_numeric() in real apps).

PHP

<?php
echo "Enter the number of rows: ";
$rows = (int) trim(fgets(STDIN));

for ($i = $rows; $i >= 1; $i--) {
    if ($i % 2 === 0) {
        for ($j = $i; $j >= 1; $j--) {
            echo $j;
        }
    } else {
        for ($j = 1; $j <= $i; $j++) {
            echo $j;
        }
    }
    echo PHP_EOL;
}

How It Works

Same nested-loop core as Example 1; only the source of rows changes. Non-numeric input casts to 0 without validation — check is_numeric(trim($line)) in safer labs.

⚡ Direction Variables

Same zigzag shape with one inner loop and computed start/end/step.

Example 3 — Direction Variables

Replace the if/else blocks with start, end, and step values derived from parity.

PHP

<?php
$rows = 5;

for ($i = $rows; $i >= 1; $i--) {
    $start = ($i % 2 === 0) ? $i : 1;
    $end = ($i % 2 === 0) ? 1 : $i;
    $step = ($i % 2 === 0) ? -1 : 1;

    for ($j = $start; $step > 0 ? $j <= $end : $j >= $end; $j += $step) {
        echo $j;
    }
    echo PHP_EOL;
}

How It Works

start, end, and step encode the same ascending/descending logic as the if/else version in one inner loop. Great once you understand parity branching; keep the explicit if/else for exams that ask you to show both inner loops.

🧠 How the Algorithm Prints Rows

Set up

Set $rows = 5; and use fgets(STDIN) when reading input from the terminal.

Setup

Outer loop (rows)

for ($i = $rows; $i >= 1; $i--) picks the row length; parity decides print direction.

Row

Parity branch

if ($i % 2 === 0) runs descending j; else runs ascending j with echo $j.

Direction

New line

echo PHP_EOL; ends the row so the next outer iteration starts fresh.

Break

Zigzag complete

Total digit prints: 1+2+…+n = n(n+1)/2 — O(n²) time, O(1) extra memory.

Trace

🔎 Worked Walkthrough — `rows = 4`

Trace each outer-loop value of i and note whether the row prints ascending or descending.

`i`	Parity	Inner `j` range	Printed row
`4`	even	`4..1`	`4321`
`3`	odd	`1..3`	`123`
`2`	even	`2..1`	`21`
`1`	odd	`1..1`	`1`

Total digit prints: 1 + 2 + 3 + 4 = 10 = 4×5/2.

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: change j <= i and watch the shape change.

2. Pattern Series Base

Foundation for inverted, pyramid, diamond, and hollow variants.

Example: swap if/else branches to flip odd/even direction.

3. Console Formatting Drills

Practice echo vs PHP_EOL without complex math.

Example: put echo PHP_EOL; inside the inner loop by mistake.

4. Character Substitution

Swap digits for letters, stars, or spaced output once the loop works.

Example: use echo $j . " "; for spaced digits on each row.

5. Complexity Intuition

Triangular totals make O(n²) concrete for beginners.

Example: count printed digits for n = 10 still → 55.

6. Input Validation Labs

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

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 C 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: learn the if/else version first; treat direction variables as a polish refactor afterward.

Usage Tips

Small habits that keep number-pattern code clean.

1. Name Bounds Clearly
Use rows (or n) and keep i/j for row/column — or rename to row/col.
2. Validate CLI input
Avoid crashes when the user types letters instead of a number.
3. Keep echo PHP_EOL; Outside
Only call echo PHP_EOL; after the inner loop finishes the row.
4. Use i % 2 for Direction
if ($i % 2 === 0) is the standard parity check for zigzag rows.
5. Dry-Run One Small n
Trace rows = 3 on paper before coding larger demos.

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

Common Pitfalls

Mistakes that commonly break alternating zigzag number patterns.

1. echo PHP_EOL; Inside the Inner Loop
Each digit lands on its own line — you get a column, not a triangle.
→ Use echo $j for digits; echo PHP_EOL; only after the inner loop.
2. Swapped Inner Bounds
Using ascending on even rows or descending on odd rows reverses the zigzag.
→ Even i: j = i; j >= 1; j--. Odd i: j = 1; j <= i; j++.
3. Forgetting the Row Break
Omitting echo PHP_EOL; glues every digit onto one endless line.
→ Always end the row after the inner loop.
4. Unchecked CLI input
Letters or empty input throw undefined rows.
→ Check is_numeric(trim($line)) and re-prompt on failure.
5. Off-by-One on 0-Based Loops
Switching to i = 0 without adjusting the inner bound prints an empty first row or wrong counts.
→ If 0-based, print i with wrong inner bound (e.g. j <= i + 1).

Edge Cases

Check these inputs before calling the solution done.

rows = 1

Single digit

Output is just 1 on one line.

rows = 0

Empty pattern

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

Negative

`rows < 0`

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

Large n

Many rows

Output grows as n²/2 characters — fine for labs, noisy for huge n.

Bad input

Non-numeric CLI input

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

Fill char

Flipped parity rule

Swapping if/else still works — output direction mirrors, not broken.

Drill

🎯 Practice Problems

Try these variations to lock in the pattern.

1. Shrinking repeating pattern

Repeat digit $i with for ($j = $i; $j <= $rows; $j++)
Continue with Program 12

2. Odd-length descending triangle

Outer loop steps by 2 (7, 5, 3, 1)
Continue with Program 14

3. Flip parity rule

Swap if/else so odd rows descend
Compare output with the default zigzag

4. Spaced digits

Use echo $j between digits
Harder follow-up after this page

Important

Notes

Triangular count. Total digit prints for n rows is n(n+1)/2 — hence O(n²) time.
echo $j; stays on the line; echo PHP_EOL; advances — mix them carefully.
Validate rows > 0 for interactive programs; rows = 1 should print a single 1.
This page is left-aligned. Centered pyramids need leading spaces — covered later in the series.

Quick Takeaway: outer loop sets row length, i % 2 picks ascending or descending inner loop, then break the line.

Analysis

⏱️ Time and Space Complexity

Program	Time	Extra space
Nested loops (Examples 1–2)	`O(rows²)`	`O(1)`
Direction variables (Example 3)	`O(rows²)`	`O(1)`

Wrap Up

🎉 Conclusion

The alternating zigzag number triangle combines nested loops with parity branching — a natural step after repeating-digit patterns. Master the if/else version, then optionally refactor rows with start/end/step direction variables.

Practice the three examples above, then continue to Program 14 for odd-length descending rows.

Odd i prints 1..i; even i prints i..1 — keep echo $j for digits and echo PHP_EOL; for the break.

View Examples Gallery Continue to Program 14

Pro Tips

💡 Best Practices

✅ Do

Explain parity rule before coding: odd ascends, even descends
Use echo $j for digits and echo PHP_EOL; after each row
Validate rows ≥ 1 for interactive programs
Check is_numeric() before using $rows
State O(n²) time when asked about complexity

❌ Don’t

Call echo PHP_EOL; inside the inner digit loop
Swap ascending/descending branches between odd and even rows
Skip the newline after each row
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 zigzag pattern

Print the pattern the beginner-friendly way.

Core concepts

△ 01

Rule

Odd i: 1..i; even i: i..1

Definition

↻ 02

Outer loop

Controls row length each line

Code

% 03

Parity

i % 2 picks direction

Logic

↵ 04

echo PHP_EOL;

Ends each row

I/O

O 05

Complexity

O(n²) time

Analysis

❓ Frequently Asked Questions

Why does the direction change on each row?

The program checks whether i is even or odd. For odd i it prints 1..i ascending; for even i it prints i..1 descending, creating a zigzag direction.

Why does the second row print 4321?

The second row from the top has length i = 4, which is even. The even branch runs for ($j = i; $j >= 1; $j--) and prints 4, 3, 2, 1.

What is the difference between echo and PHP_EOL?

echo stays on the same line. echo PHP_EOL ends the current line. Digits use echo $j; the row break uses echo PHP_EOL after the inner loop.

How can I make every row print in the same direction?

Remove the if/else and keep only one inner loop — either 1..i for ascending or i..1 for descending on every row.

How is this different from Program 12?

Program 12 repeats the row digit with for ($j = $i; $j <= $rows; $j++) (11111, 2222, 333, 44, 5). Program 13 prints sequential digits 1..i or i..1 and alternates direction using i % 2.

What is the time complexity?

O(n²) where n is the number of rows. Total echo calls equal n+(n-1)+…+1 = n(n+1)/2.

Can I flip the parity rule?

Yes. Swap the if and else branches so odd rows descend and even rows ascend — the zigzag still works, just mirrored.

How should I read the row count safely?

Use trim(fgets(STDIN)) and check is_numeric($input) before casting to int so bad input does not produce unexpected results.

What happens if rows is 0 or negative?

The outer loop never runs, so nothing is printed. Validate and prompt again if you want a clear user message.

Did you Know? 🔊

Odd row length i prints 1..i; even row length prints i..1. The outer loop shrinks from rows to 1, and i % 2 flips direction — still O(n²) total digit prints.

Continue to Program 14

Move on to the odd-length descending number triangle in the PHP number-pattern series.

Program 14 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 12 — Shrinking Repeating Number Pattern PHP Number Pattern 14 — Odd-Length Descending Pyramid

Alternating Zigzag Number Triangle Pattern in PHP

What You’ll Learn

Shape Rule

Outer Loop

Parity Branch

echo vs PHP_EOL

Live Preview

O(n²)

Introduction

Why it matters?

Parity Switches Direction

If / Else Branch

Print Then Break

Series Foundation

📝 Problem & Approach

Inputs & Outputs

Minimal workflow

Approach comparison

⚡ Quick Reference

📋 Ascending vs Descending vs Parity

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 rows = 5

How It Works

📈 Practical Variant

Example 2 — User Input Version

How It Works

⚡ Direction Variables

Example 3 — Direction Variables

How It Works

🧠 How the Algorithm Prints Rows

Set up

Outer loop (rows)

Parity branch

New line

Zigzag complete

🔎 Worked Walkthrough — rows = 4

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. Validate CLI input

3. Keep echo PHP_EOL; Outside

4. Use i % 2 for Direction

5. Dry-Run One Small n

Common Pitfalls

1. echo PHP_EOL; Inside the Inner Loop

2. Swapped Inner Bounds

3. Forgetting the Row Break

4. Unchecked CLI input

5. Off-by-One on 0-Based Loops

Edge Cases

Single digit

Empty pattern

rows < 0

Many rows

Non-numeric CLI input

Flipped parity rule

🎯 Practice Problems

1. Shrinking repeating pattern

2. Odd-length descending triangle

3. Flip parity rule

4. Spaced digits

Example 1 — Fixed `rows = 5`

🔎 Worked Walkthrough — `rows = 4`

`rows < 0`