Alternating 1s and 0s Pattern in PHP

Beginner
⏱️ 5 min read
📚 Updated: Aug 2025
🎯 2 Code Examples
Nested Loops
PHP Number Pattern 40 output (11111, 0000, 111, 00, 1)

What You’ll Learn

How to print Number Pattern 40 in PHP: a decreasing-width triangle where each row prints either all 1s or all 0s.

Odd-numbered rows print 1 and even-numbered rows print 0, controlled by the modulus check $i % 2.

⭐ Pattern Output

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

Output
11111
0000
111
00
1
1

Complete PHP Program

The inner loop prints $rows - $i + 1 characters. The printed digit depends on whether $i is odd or even.

PHP
<?php
$rows = 5;

for ($i = 1; $i <= $rows; $i++) {
    for ($j = $i; $j <= $rows; $j++) {
        echo ($i % 2 === 0) ? "0" : "1";
    }
    echo PHP_EOL;
}

🧠 How It Works

1

Set the row count

$rows = 5; controls both the height and the first row width.

Setup
2

Outer loop (rows)

for ($i = 1; $i <= $rows; $i++) moves from row 1 to row $rows.

Row control
3

Inner loop (decreasing width)

for ($j = $i; $j <= $rows; $j++) prints fewer characters each row.

Width control
4

Choose 1 or 0 using parity

($i % 2 === 0) decides the digit for the entire row.

Alternation
=

Alternating rows

Total printed characters are n(n+1)/2, so runtime is O(n²).

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) {
    exit("Rows must be at least 1." . PHP_EOL);
}

for ($i = 1; $i <= $rows; $i++) {
    for ($j = $i; $j <= $rows; $j++) {
        echo ($i % 2 === 0) ? "0" : "1";
    }
    echo PHP_EOL;
}

💡 Tips for Enhancement

Try These

  • Start with 0 on the first row by swapping the parity output
  • Print spaces between digits for readability
  • Replace 1/0 with any two characters (like * and -)
  • Center-align the triangle by printing leading spaces
  • Make it a growing triangle by changing the inner loop bounds

Avoid

  • Forgetting the newline after each row
  • Using invalid row counts without validation
  • Printing different digits within a single row (this pattern is row-based)
  • Assuming browser output matches CLI output without changing line breaks

Key Takeaways

1

Odd rows print 1; even rows print 0.

2

Each row gets shorter because the inner loop starts at $i.

3

Total printed characters are n(n+1)/2.

4

Runtime is O(n²) for n rows.

❓ Frequently Asked Questions

Row 2 is even, so the program prints 0. It prints 4 characters because the inner loop runs from $j=2 to $j=5.
Yes, but you’d switch based on $j (column) instead of $i (row).
Replace PHP_EOL with <br> so the output breaks lines in HTML.
O(n²) because you print a triangular number of characters: n(n+1)/2.

Explore More PHP Number Patterns!

Try more bit-style patterns to strengthen your loop fundamentals.

All Number Patterns →
Did you know?

The parity check $i % 2 is a common trick for alternating outputs (like 0/1, true/false, or even zebra-striping UI rows).

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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