PHP Incremental Number Triangle Pattern (Right-Aligned)

Beginner
5 min read
Updated: Sep 2026
3 programs
Live preview
Right-aligned continuous counter triangle 1, 2 3, 4 5 6 through 15 in PHP programming
Definition

What Is This Pattern?

A right-aligned continuous counter triangle prints a growing sequence with counter $k that never resets — leading pads while $j > $i, then printf("%3d", $k++).

Remember
Rule: k = 1
      for i from 1 to rows
        for j from rows down to 1
          if j > i: print 3 spaces
          else: print k (width 3); k++

              1
           2  3
        4  5  6
     7  8  9 10
 11 12 13 14 15     ← rows = 5

Follows the zero-based triangle in Program 34; next is the right-aligned decreasing triangle in Program 36.

Approach

How to Solve It

Outer loop grows $i. One fixed-width inner loop runs $rows..1 — print a 3-space pad or the next $k with printf("%3d", …).

MethodIdeaBest for
Fixed width + $k++Spaces while $j > $i; else print and increment $kLearning, interviews, exams
%3d columnsKeeps two-digit values aligned with single digitsReadable demos past 9

Pseudocode

Pseudocode
k = 1
for i from 1 to rows:
    for j from rows down to 1:
        if j > i: print 3 spaces
        else: print k in width 3; k = k + 1
    print newline

Cheat sheet

GoalPattern
Grow each rowfor ($i = 1; $i <= $rows; $i++)
Fixed widthfor ($j = $rows; $j >= 1; $j--)
Leading padsif ($j > $i) echo " ";
Next numberprintf("%3d", $k++);
End of rowecho PHP_EOL;

Printing Numbers vs Starting a New Line

APIEffectUse for
echo " " / printf("%3d", $k++)Stays on the same lineEach pad or number on the row
echo PHP_EOLEnds the current lineAfter the fixed-width loop finishes

Print characters without a newline, then end the row once.

Try it

Live Preview

Change the row count and the right-aligned counter updates instantly — capped at 7 for readable demos.

Whole numbers from 1 to 7. Tap a chip or type a value — the preview redraws as you go.

Live result rows = 5 · 15 numbers
              1
           2  3
        4  5  6
     7  8  9 10
 11 12 13 14 15
Trace

Worked Walkthrough — $rows = 5

Trace how leading pads shrink and counter $k keeps climbing.

$iPadsNumbersPrints
1411
232 32 3
324 5 64 5 6
417..107 8 9 10
5011..1511 12 13 14 15

Pads = rows − i. Numbers printed = n(n+1)/2 → O(n²).

Code

PHP Programs

Three complete programs: fixed $rows = 5, CLI input, and a compact $rows = 3 demo. Use View Output to reveal sample results.

Example 1 — Fixed $rows = 5

Hard-coded height — continuous $k with printf("%3d", …) and leading pads.

PHP
<?php
$k = 1;

for ($i = 1; $i <= 5; $i++) {
    for ($j = 5; $j >= 1; $j--) {
        if ($j > $i) {
            echo "   ";
        } else {
            printf("%3d", $k++);
        }
    }
    echo PHP_EOL;
}
?>

How It Works

1. Counter lives outside. $k starts at 1 and only advances when a number prints.

2. Fixed width. Inner loop always runs 5 times — unused left slots become " ".

3. Width 3 columns. printf("%3d", …) keeps 10 and 11 aligned with single digits.

Example 2 — User Input Rows

Read $rows from the CLI, validate, then use the same counter and pad logic.

PHP
<?php
echo "Enter rows: ";
$input = trim(fgets(STDIN));

if (!is_numeric($input) || (int) $input <= 0) {
    echo "Please enter a positive integer." . PHP_EOL;
    exit(1);
}

$rows = (int) $input;
$k = 1;

for ($i = 1; $i <= $rows; $i++) {
    for ($j = $rows; $j >= 1; $j--) {
        if ($j > $i) {
            echo "   ";
        } else {
            printf("%3d", $k++);
        }
    }
    echo PHP_EOL;
}
?>

How It Works

1. Prompt and validate. Reject non-numeric and non-positive values before printing.

2. Same core. Pads + printf("%3d", $k++) match Example 1 — only $rows comes from the user.

3. Safer input tip. Cap demos for readable output:

Safer input
if (!is_numeric($input) || (int) $input < 1 || (int) $input > 7) {
    echo "Enter a whole number from 1 to 7." . PHP_EOL;
    exit(1);
}

Example 3 — Compact $rows = 3

Same structure with only three rows — easy to confirm pads shrink and $k never resets.

PHP
<?php
$rows = 3;
$k = 1;

for ($i = 1; $i <= $rows; $i++) {
    for ($j = $rows; $j >= 1; $j--) {
        if ($j > $i) {
            echo "   ";
        } else {
            printf("%3d", $k++);
        }
    }
    echo PHP_EOL;
}
?>

How It Works

1. Three rows. Ends at 6 — if $k reset each row you would see 1, 1 2, 1 2 3.

2. Trace on paper. If the inner loop stops at $i instead of $rows, right alignment is lost.

3. Scale up next. Once the small demo is clear, use Examples 1–2 for five rows or user input.

Edge Cases & Pitfalls

Check these before calling the solution done.

k = 1

Resetting the counter

Putting $k = 1 inside the outer loop restarts every row. Declare $k once before both loops.

no %3d

Broken columns

Without printf("%3d", …), two-digit values misalign with single digits. Keep fixed width.

width ≠ 3

Pad mismatch

Leading pads must match number width — use " " (3 spaces) with %3d.

PHP_EOL inside

Broken rows

If echo PHP_EOL sits inside the inner loop, you get one cell per line. Call it only after the loop.

rows = 1

Single value

Output is just 1 with no leading pads. A good sanity check for input validation.

fgets

Check is_numeric

Bare (int) casts turn non-numeric strings into 0 — prefer is_numeric before casting.

Analysis

Time and Space Complexity

ProgramTimeExtra space
Fixed / input (Examples 1–2)O(n²)O(1)
Compact rows = 3 (Example 3)O(n²)O(1)

Each of n rows visits n columns (pads or numbers). Numbers printed = n(n+1)/2 → O(n²) time. Only a few loop variables are needed.

Remember

Key Takeaways

  • Continuous $k: declare once outside; use $k++ only when printing a number.
  • Fixed width: $j = $rows..1 with pads when $j > $i keeps the triangle right-aligned.
  • %3d: match pad width to number width so two-digit values stay aligned.
  • Complexity: O(n²) time; O(1) extra space.

One line: for each row $i, walk $j = $rows..1 and print a pad or printf("%3d", $k++), then echo PHP_EOL.

Frequently Asked Questions

A right-aligned continuous counter triangle: for rows=5 you get indented 1 / 2 3 / 4 5 6 / 7 8 9 10 / 11 12 13 14 15 — $k never resets between rows.
Numbers keep increasing across rows without resetting — row 1 prints 1, row 2 prints 2 3, row 3 prints 4 5 6, and so on.
Before printing numbers on each row, the program prints three spaces while $j > $i. This indents the left side so numbers shift right.
The format specifier reserves 3 columns per number (right-aligned), keeping columns aligned when values become two digits.
$k is declared outside the loops and increments with $k++ each time a number prints, so the sequence continues across rows.
Program 30 prints descending digits per row. Program 35 uses a continuous counter $k with fixed-width formatting.
Use trim(fgets(STDIN)) and check is_numeric($input) before casting to int — see Example 2.
O(n²) for n rows because total prints are 1 + 2 + … + n = n(n+1)/2.

Did you know?

A counter $k starts at 1 and increments every time a number is printed. Leading spaces appear while $j > $i, and printf("%3d", $k++) keeps columns aligned as values grow past single digits.

Next: Right-Aligned Decreasing Triangle

Continue with a right-aligned pattern that counts downward.

Program 36 tutorial →

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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PHP Number Pattern 34 — Increasing Triangle from 0 PHP Number Pattern 36 — Right-Aligned Decreasing Triangle