Right-Aligned Increasing Number Triangle Pattern in PHP

What You’ll Learn

The right-aligned increasing number triangle prints 1, then 1 2, then 1 2 3, and so on — with leading spaces so each row aligns on the right. This tutorial covers the space loop, number loop, live preview, algorithm steps, worked PHP examples, edge cases, and complexity.

📝 Problem & Approach

Given a positive integer $rows (e.g. 5), print a right-aligned triangle: row i shows numbers 1..$i with leading spaces.

// $rows = 5 (conceptual shape)
//         1
//       1 2
//     1 2 3
//   1 2 3 4
// 1 2 3 4 5

Inputs & Outputs

Minimal workflow

for $i from 1 to $rows:
    print ($rows - $i) spaces
    for $k from 1 to $i: print k with %2d
    print newline

Approach comparison

🔮 Live Preview

Choose a row count between 1 and 20 and draw the right-aligned increasing number triangle in the browser.

Press "Draw pattern".

Examples Gallery

Three complete PHP programs — fixed row count, fgets(STDIN) input, and a left-aligned variant. Click View Output to reveal sample console results.

<?php
$rows = 5;

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

        1
      1 2
    1 2 3
  1 2 3 4
1 2 3 4 5

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

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

Enter the number of rows: 5
        1
      1 2
    1 2 3
  1 2 3 4
1 2 3 4 5

<?php
$rows = 5;

for ($i = 1; $i <= $rows; $i++) {
    for ($k = 1; $k <= $i; $k++) {
        printf("%2d", $k);
    }
    echo PHP_EOL;
}

 1
 1 2
 1 2 3
 1 2 3 4
 1 2 3 4 5

🔎 Worked Walkthrough — $rows = 5, row $i = 3

Trace one middle row to see spaces and numbers combine.

Row output: 1 2 3 — total numbers for full triangle: 1+2+3+4+5 = 15 = n(n+1)/2.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices