Alternating 1-0 Triangle in PHP

Beginner
⏱️ 5 min read
📚 Updated: Aug 2025
🎯 2 Code Examples
Modulo Operator
Alternating 1-0 triangle output in PHP (1, 01, 101, 0101, 10101)

What You’ll Learn

How to print an alternating 1-0 triangle in PHP using nested loops and the modulo operator %. Each row prints a sequence of 0 and 1 based on whether the loop counter is even or odd.

This is a great mini-exercise for mastering nested loops and understanding how modulo can generate repeating patterns.

⭐ Pattern Output

For $rows = 5, the pattern looks like this:

Output
1
01
101
0101
10101
1

Complete PHP Program

We print each digit using $j % 2, which yields 0 for even and 1 for odd values of $j.

PHP
<?php
$rows = 5;

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

🧠 How It Works

1

Set the number of rows

$rows = 5; controls the triangle height.

Setup
2

Outer loop (rows)

for ($i = 1; $i <= $rows; $i++) increments the row number from 1 up to $rows.

Row control
3

Inner loop (count down)

for ($j = $i; $j >= 1; $j--) prints exactly $i digits on row $i.

Column control
4

Print 0/1 with modulo

echo $j % 2; prints 1 for odd $j and 0 for even $j.

Alternation
=

Alternating 1-0 triangle

Because the inner loop counts down, each row begins with the parity of $i (even rows start with 0, odd rows start with 1).

2

Variation — User Input (CLI) Version

Let the user decide the number of rows at runtime using standard input (run from terminal with php):

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

if ($rows < 1) {
    echo "Please enter a positive number." . PHP_EOL;
    exit(1);
}

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

💡 Tips for Enhancement

Try These

  • Flip the direction by looping $j upward from 1 to $i
  • Print spaces between digits to make larger triangles readable
  • Start with 0 every row by using ($j + 1) % 2 instead
  • Build a full binary grid by printing fixed-length rows
  • Validate input (reject $rows < 1) in CLI mode

Avoid

  • Forgetting the newline after each row
  • Assuming user input is always valid (especially empty input)
  • Mixing row/column logic (outer loop = rows, inner loop = columns)
  • Hardcoding output strings instead of generating via loops

Key Takeaways

1

Use nested loops to control rows and columns.

2

$j % 2 generates alternating 0/1 values automatically.

3

With the countdown inner loop, even rows start with 0 and odd rows start with 1.

4

The same logic can generate other repeating patterns (like 0123 cycles) using % k.

❓ Frequently Asked Questions

$j % 2 is 1 for odd numbers and 0 for even numbers, so it naturally alternates as $j changes.
Because the inner loop begins at $j = $i. When $i is even, $i % 2 is 0, so the row starts with 0.
Print 1 - ($j % 2) instead of $j % 2. That inverts 0/1 for every position.
O(n²) for n rows, because the total printed digits are \(1+2+\dots+n\).

Explore More PHP Number Patterns!

Once you master modulo-based alternation, try patterns with symmetry, spacing, and right-alignment.

All Number Patterns →
Did you know?

The modulo operator is a common way to create repeating cycles. For example, $x % 3 cycles through 0, 1, 2, which is handy for repeating colors, labels, or pattern symbols.

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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