Shape Rule
digit i repeats exactly i times
Row rows prints rows repeated rows times, then each next row repeats its digit fewer times down to a single 1.
Inverted Repeating Number Triangle in JavaScript
Overview
What You’ll Learn
The inverted repeating number triangle widens first then shrinks: digit i repeats exactly i times — nested loops, line += i vs console.log(line), and a clear visual result. This tutorial covers the shape rule, loop structure, a live preview, algorithm steps, worked JavaScript examples, edge cases, and complexity.
Outer Loop
Rows
for (let i = rows; i >= 1; i--) walks each line from the longest down to one digit.
Inner Loop
Digits
for (let j = 1; j <= i; j++) prints digit i exactly i times on that row.
line += i vs console.log(line)
Same line / next line
Repeated digits use line += i; end each row with console.log(line).
Live Preview
1–20 rows
Pick a row count and draw the inverted repeating number triangle instantly in the browser.
O(n²)
Complexity
Total digit prints still = n(n+1)/2; extra memory stays O(1).
Introduction
An inverted repeating number triangle pattern starts with the widest row and loses one repeat each line as the digit decreases. Each row prints the same digit multiple times; the repeat count shrinks while the digit value shrinks.
In JavaScript you usually solve it with two nested for loops: the outer loop picks the digit, the inner loop repeats it with line += i, then console.log(line) moves to the next line.
Why it matters?
It pairs naturally with Program 10 in JavaScript courses. Once nested loops and line += i/console.log(line) click, pyramids, diamonds, and hollow shapes become much easier.
Key Highlights
Row = Repeat Count
On row i, print digit i exactly i times.
Two Nested Loops
Outer picks the digit; inner controls how many times it repeats.
Print Then Break
line += i in the inner loop; console.log(line) after.
Series Foundation
Gateway to Program 10 (5, 44, 333, …) and Program 12.
In short: for each row i from rows down to 1, repeat line += i exactly i times, then call console.log(line).
Foundation
📝 Problem & Approach
Given a positive integer rows, print an inverted repeating number triangle: row i repeats digit i exactly i times, with the outer loop counting from rows down to 1.
JavaScript
# First 5 rows (conceptual shape)
# 55555
# 4444
# 333
# 22
# 1
for (let i = rows; i >= 1; i--)
for (let j = 1; j <= i; j++)
line += i # digit i, repeated i times
console.log(line) # next rowInputs & Outputs
| Item | Type | Description |
|---|---|---|
rows | int | Number of triangle lines to print (typically ≥ 1). |
| Printed output | text | Each row repeats one digit; the top row has rows copies of rows, the bottom row has a single 1. |
Minimal workflow
Pseudocode
for i from rows down to 1:
repeat i times:
append i to line
console.log(line)Approach comparison
| Approach | Idea | Best for |
|---|---|---|
| Nested loops | Outer digit + inner repeats | Learning and interviews |
String(i).repeat(i) in inner loop | Print a full repeated row with String.repeat | Leaner JavaScript style once loops are understood |
Cheat Sheet
⚡ Quick Reference
| Goal | Pattern |
|---|---|
| Walk each row | for (let i = rows; i >= 1; i--) |
Repeat digit i | for (let j = 1; j <= i; j++) line += i |
| End the row | console.log(line) |
| One-line row shortcut | String(i).repeat(i) repeated per row |
| Program 10 variant | for (let j = rows; j >= i; j--) line += i (descending repeats) |
Compare
📋 line += i vs console.log(line) vs String.repeat
Same triangle — different ways to emit characters.
line += i
same linePrints a digit without moving to the next line
console.log(line)
new lineEnds the current row after all digits are printed
String(i).repeat(i)
one digitWrites digit i when i is 1..9 — no format string
Learning tip
loops firstMaster nested loops before the String.repeat shortcut
Context
When This Pattern Shows Up
Reach for this triangle when teaching or testing nested-loop basics.
First lab exercise
Most JavaScript pattern series start here before pyramids and diamonds.
Nested-loop warm-up
Outer/inner bound practice with an immediate visual check.
Console I/O practice
Combine loops with
prompt()for a flexible row count.Gateway to variants
Compare Program 10 (5, 44, 333, …) and Program 12 next.
Not a UI layout tool
This is a console teaching pattern — not how you build modern app screens.
Key benefit: one small program that locks in nested loops, output sequencing, and O(n²) thinking.
Interactive
🔮 Live Preview
Choose a row count between 1 and 20 and draw the inverted repeating number triangle in the browser.
Hands-On
Examples Gallery
Three complete JavaScript programs — fixed row count, prompt(), and a String.repeat shortcut. Click View Output to reveal sample results, or Try it Yourself to run the code live.
📚 Getting Started
Print five rows of the inverted repeating number triangle with nested loops.
Example 1 — Fixed rows = 5
Hard-coded height — outer picks digit, inner repeats it i times.
JavaScript
const rows = 5;
for (let i = rows; i >= 1; i--) {
let line = "";
for (let j = 1; j <= i; j++) {
line += i;
}
console.log(line);
}How It Works
When i = 5, the inner loop runs five times and prints 55555. When i = 4, it prints 4444, and so on until i = 1 prints 1. console.log(line) after the inner loop starts the next row.
📈 Practical Variant
Let the user choose the height at runtime.
Example 2 — User Input Version
Read the row count with prompt() and parseInt() (check Number.isFinite in real apps).
JavaScript
const rowsInput = prompt("Enter the number of rows:");
const rows = parseInt(rowsInput, 10);
if (!Number.isFinite(rows) || rows <= 0) {
console.log("Please enter a positive integer.");
} else {
for (let i = rows; i >= 1; i--) {
let line = "";
for (let j = 1; j <= i; j++) {
line += i;
}
console.log(line);
}
}How It Works
Same nested-loop core as Example 1; only the source of rows changes. Non-numeric input yields NaN with bare parseInt(prompt()) — validate with Number.isFinite for safer labs.
⚡ Shortcut Style
Same shape without an explicit inner repeat loop.
Example 3 — String(i).repeat(i)
Build each row with String(i).repeat(i), then log it.
JavaScript
const rows = 5;
for (let i = rows; i >= 1; i--) {
console.log(String(i).repeat(i));
}How It Works
String(i).repeat(i) creates a string of repeated digit i. Great once you understand the nested-loop idea; keep the two-loop version for exams that ask you to show both bounds.
🧠 How the Algorithm Prints Rows
1
Set up
console.log is built in; use prompt() when reading input. Set rows (fixed or from prompt).
Setup
2
Outer loop (rows)
for (let i = rows; i >= 1; i--) picks the digit; the inner loop prints it i times.
Row
3
Inner loop (repeats)
for (let j = 1; j <= i; j++) repeats digit i exactly i times with line += i.
Repeats
4
New line
console.log(line) ends the row so the next outer iteration starts fresh.
Break
=
Triangle complete
Total digit prints: 1+2+…+n = n(n+1)/2 — O(n²) time, O(1) extra memory.
Trace
🔎 Worked Walkthrough — rows = 4
Trace each outer-loop value of i and count how many times the inner loop prints digit i.
i | Inner j range | Repeats | Printed row |
|---|---|---|---|
4 | 1..4 | 4 | 4444 |
3 | 1..3 | 3 | 333 |
2 | 1..2 | 2 | 22 |
1 | 1..1 | 1 | 1 |
Total digit prints: 1 + 2 + 3 + 4 = 10 = 4×5/2.
Use Cases
Where this tiny pattern (and its loop structure) shows up beyond the homework prompt.
1. Teaching Nested Loops
Clearest visual proof that outer and inner bounds interact.
Example: change j <= i and watch the shape change.
2. Pattern Series Base
Foundation for inverted, pyramid, diamond, and hollow variants.
Example: Program 10 uses for (let j = rows; j >= i; j--) so repeats grow as the digit shrinks.
3. Console Formatting Drills
Practice print vs row newline without complex math.
Example: put console.log(line) inside the inner loop by mistake.
4. Character Substitution
Swap digits for letters, stars, or spaced output once the loop works.
Example: print i + " " for spaced repeated digits on each row.
5. Complexity Intuition
Triangular totals make O(n²) concrete for beginners.
Example: count printed digits for n = 10 still → 55.
6. Input Validation Labs
Pair the pattern with prompt() and positive-row checks.
Example: reject rows <= 0 and re-prompt.
Pro Tip: when an interviewer asks for patterns, explain the outer/inner roles first — then write the loops. The story matters as much as the code.
Advantages
Why this pattern earns a permanent spot in beginner C courses.
1. Instant Visual Feedback
Wrong bounds show up immediately as a broken staircase.
2. Minimal Concepts
Only loops and console output — no arrays or math libraries.
3. Easy to Extend
Invert, center, hollow, or change the fill character with small edits.
4. O(1) Extra Memory
Streaming output needs no storage beyond loop counters.
Pro Tip: learn the nested-loop version first; treat String(i).repeat(i) as a polish shortcut afterward.
Usage Tips
Small habits that keep number-pattern code clean.
1. Name Bounds Clearly
Use
rows(orn) and keepi/jfor row/column — or rename torow/col.2. Validate with
Number.isFiniteAvoid crashes when the user types letters instead of a number.
3. Keep console.log(line) Outside
Only call
console.log(line)after the inner loop finishes the row.4. Use j <= i for Repeat Count
for (let j = 1; j <= i; j++)withline += irepeats digitiexactlyitimes.5. Dry-Run One Small n
Trace
rows = 3on paper before coding larger demos.
Pro Tip: if the output is a vertical list of single digits per line, you almost certainly put console.log(line) inside the inner loop.
Common Pitfalls
Mistakes that commonly break inverted repeating number patterns.
1. console.log(line) Inside the Inner Loop
Each repeated digit lands on its own line — you get a column, not a triangle.
→ Use
line += ifor repeats;console.log(line)only after the inner loop.2. Wrong Inner Bound
Using
for (let j = rows; j >= i; j--)builds Program 10 (5, 44, 333…). Usingfor (let j = 1; j <= rows; j++)on every row prints a rectangle.→ For this shape, keep
for (let j = 1; j <= i; j++)so digitirepeats exactlyitimes.3. Forgetting the Row Break
Omitting
console.log(line)glues every repeat onto one endless line.→ Always end the row after the inner loop.
4. Bare parseInt(prompt())
Letters or empty input return
NaNwith bareparseInt(prompt(), 10).→ Check
Number.isFinite(rows)and re-prompt on failure.5. Off-by-One on 0-Based Loops
Switching to
i = 0without adjusting the digit and repeat count prints an empty first row or wrong counts.→ Prefer 1-based
for (let i = rows; i >= 1; i--)withfor (let j = 1; j <= i; j++), or carefully map both values if you go 0-based.
Edge Cases
Check these inputs before calling the solution done.
rows = 1
Single repeated digit
Output is just one copy of rows on one line.
rows = 0
Empty pattern
Outer loop never runs — print nothing or show a message.
Negative
rows < 0
Treat as invalid; re-prompt instead of silent empty output.
Large n
Many rows
Output grows as n²/2 characters — fine for labs, noisy for huge n.
Bad input
Non-numeric prompt()
parseInt(prompt(), 10) returns NaN on bad input — validate first.
Fill char
Inner bound j <= rows by mistake
Use j <= i so row i repeats exactly i times.
Drill
🎯 Practice Problems
Try these variations to lock in the pattern.
1. Descending repeating triangle
- Inner loop j from rows down to i (see Program 10)
- Continue with Program 10
2. Program 12 follow-up
- Use
for (let i = rows; i >= 1; i--)as outer loop - Shows the loop structure is reusable
3. Safe input loop
- Validate with
Number.isFiniteafterparseInt(prompt(), 10) - Then draw the triangle
4. Same digit, different count
- Use
line += ibetween repeated digits - Harder follow-up after this page
Important
Notes
- Triangular count. Total digit prints for n rows is
n(n+1)/2— hence O(n²) time. line += istays on the line;console.log(line)advances — mix them carefully.- Validate
rows > 0for interactive programs;rows = 1should printrowscopies of the top digit. - This page is left-aligned. Centered pyramids need leading spaces — covered later in the series.
Quick Takeaway: outer loop picks the row, inner loop repeats digit i, then break the line — that is the whole pattern.
Analysis
⏱️ Time and Space Complexity
| Program | Time | Extra space |
|---|---|---|
| Nested loops (Examples 1–2) | O(rows²) | O(1) |
String(i).repeat(i) (Example 3) | O(rows²) | O(rows) per row string (temporary) |
🎉 Conclusion
The inverted repeating number triangle pattern is a small nested-loop exercise with lasting payoff: row/column thinking, line += i vs console.log(line), and O(n²) intuition. Master the classic two-loop version, then optionally shorten rows with String(i).repeat(i).
Practice the three examples above, then continue to Program 12 for the next pattern in the series.
Row i repeats digit i — keep line += i for each copy and console.log(line) for the break, and validate row counts when reading input.
Pro Tips
💡 Best Practices
✅ Do
- Explain outer picks digit, inner counts repeats before coding
- Use
line += ifor repeats andconsole.log(line)after each row - Validate
rows ≥ 1for interactive programs - Check
Number.isFinite(rows)afterparseInt(prompt(), 10)before usingrows - State O(n²) time when asked about complexity
❌ Don’t
- Call
console.log(line)inside the inner repeat loop - Use
for (let j = rows; j >= i; j--)when you meant Program 10 instead - Skip the newline after each row
- Ignore bad console input in user-facing demos
- Skip the
rows = 1edge case
Summary
Key Takeaways
Knowledge Unlocked
Five things to remember about this repeating pattern
Print the triangle the beginner-friendly way.
5
Core concepts 01
Rule
Row i repeats digit i
02
Outer loop
Picks the digit each row
Code 03
Inner loop
Repeats digit with line += i
04
console.log(line)
Ends each row
I/O 05
Complexity
O(n²) time
Analysis❓ Frequently Asked Questions
The outer loop decreases i from rows down to 1, and the inner loop runs j from 1 to i. Row i repeats digit i exactly i times, so the widest row is first and each next row is shorter.
for (let j = 1; j <= i; j++) line += i runs the inner body i times. When i is 5 you get 55555; when i is 1 you get a single 1.
line += i stays on the same row while building digits. console.log(line) prints the completed row and adds a newline. Repeated digits use += inside the inner loop; the row break uses console.log after the inner loop.
Program 10 grows repeat count as the digit shrinks (5, 44, 333, 2222, 11111) using for (let j = rows; j >= i; j--). Program 11 repeats digit i exactly i times (55555, 4444, 333, 22, 1) using for (let j = 1; j <= i; j++).
O(n²) where n is the number of rows. Total digit appends equal n+(n-1)+…+1 = n(n+1)/2.
Yes. console.log(String(i).repeat(i)) prints a full row in one call. Nested loops are better for learning; String.repeat is a handy shortcut once the bounds make sense.
Use parseInt with Number.isFinite and a rows > 0 check after prompt(), or validate the raw string before converting so bad input does not produce NaN.
The outer loop never runs, so nothing is printed. Validate and prompt again if you want a clear user message.
Did you Know? 🔊
Row digit i repeats exactly i times. The outer loop counts down from rows, so the first row is widest (rows copies of rows) and each line shortens — still O(n²) total prints.
Continue to Program 12
Next up — ascending repeating shrinking pattern in the series.
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