Odd-Length Descending Number Triangle Pattern in PHP

What You’ll Learn

The odd-length descending number triangle teaches how a custom outer-loop step (i -= 2) skips even row widths. This tutorial covers the shape rule, loop structure, a live preview, algorithm steps, worked PHP examples, edge cases, and complexity.

📝 Problem & Approach

Given a positive odd integer max (or adjusted to odd), print an odd-length descending number triangle: row length i prints digits 1 through i, with the outer loop stepping i -= 2 from max down to 1.

// max = 7 (conceptual shape)
// 1234567
// 12345
// 123
// 1

Inputs & Outputs

Minimal workflow

for i from max down to 1 step -2:
    for j from 1 to i:
        print j (no newline)
    print newline

Approach comparison

🔮 Live Preview

Enter an odd maximum between 1 and 20 and draw the odd-length descending triangle in the browser.

Press "Draw pattern".

Examples Gallery

Three complete PHP programs — fixed maximum, configurable user input, and a step-by-1 comparison. Click View Output to reveal sample console results.

<?php
for ($i = 7; $i >= 1; $i -= 2) {
    for ($j = 1; $j <= $i; $j++) {
        echo $j;
    }
    echo PHP_EOL;
}

1234567
12345
123
1

<?php
echo "Enter an odd maximum (e.g., 9): ";
$max = (int) trim(fgets(STDIN));

if ($max % 2 === 0) $max -= 1;

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

Enter an odd maximum (e.g., 9): 6
12345
123
1

<?php
$max = 7;

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

1234567
123456
12345
1234
123
12
1

🔎 Worked Walkthrough — max = 7

Trace each outer-loop value of i and note the ascending digits printed on each odd-length row.

Skipped even lengths: 6, 4, 2. Total digit prints: 7+5+3+1 = 16.

🎯 Practice Problems

Try these variations to lock in the pattern.

Notes

⏱️ Time and Space Complexity

💡 Best Practices