Inverted Repeating Number Triangle in C

Beginner
⏱️ 8 min read
📚 Updated: Aug 2026
🎯 3 Code Examples
🚀 Live Preview
Nested Loops
Inverted repeating number triangle output (55555, 4444, 333, 22, 1) in C programming
Overview

What You’ll Learn

The inverted repeating number triangle widens first then shrinks: digit i repeats exactly i times — nested loops, printf vs newline, and a clear visual result. This tutorial covers the shape rule, loop structure, a live preview, algorithm steps, worked C examples, edge cases, and complexity.

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.

Outer Loop

Rows

for (i = rows; i >= 1; i--) walks each line from the longest down to one digit.

Inner Loop

Digits

for (j = 1; j <= i; j++) prints digit i exactly i times on that row.

printf vs newline

Same line / next line

Repeated digits use printf("%d", i); end each row with printf("\n").

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 C you usually solve it with two nested for loops: the outer loop picks the digit, the inner loop repeats it with printf("%d", i), then printf("\n") moves to the next line.

Why it matters?

It pairs naturally with Program 10 in C courses. Once nested loops and printf 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

printf("%d", i) in the inner loop; printf("\n") 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 printf("%d", i) exactly i times, then call printf("\n").

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.

c
// First 5 rows (conceptual shape)
// 55555
// 4444
// 333
// 22
// 1

Inputs & Outputs

ItemTypeDescription
rowsintNumber of triangle lines to print (typically ≥ 1).
Printed outputtextEach 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:
        print i (no newline)
    print newline

Approach comparison

ApproachIdeaBest for
Nested loopsOuter digit + inner repeatsLearning and interviews
putchar('0' + i) in inner loopWrite each repeated digit without printfLeaner C style once loops are understood
Cheat Sheet

⚡ Quick Reference

GoalPattern
Walk each rowfor (i = rows; i >= 1; i--)
Repeat digit ifor (j = 1; j <= i; j++) printf("%d", i);
End the rowprintf("\n");
One-line row shortcutputchar('0' + i) repeated per row
Program 10 variantfor (j = rows; j >= i; j--) printf("%d", i) (descending repeats)
Compare

📋 printf vs putchar vs newline

Same triangle — different ways to emit characters.

printf("%d", i)
same line

Prints a digit without moving to the next line

printf("\n")
new line

Ends the current row after all digits are printed

putchar('0' + i)
one digit

Writes digit i when i is 1..9 — no format string

Learning tip
loops first

Master nested loops before the putchar shortcut

Context

When This Pattern Shows Up

Reach for this triangle when teaching or testing nested-loop basics.

  1. First lab exercise

    Most C pattern series start here before pyramids and diamonds.

  2. Nested-loop warm-up

    Outer/inner bound practice with an immediate visual check.

  3. Console I/O practice

    Combine loops with scanf for a flexible row count.

  4. Gateway to variants

    Compare Program 10 (5, 44, 333, …) and Program 12 next.

  5. 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.

Try 5, 7, or 10. Larger values still work up to 20.

Live result
Press "Draw pattern".
Hands-On

Examples Gallery

Three complete C programs — fixed row count, scanf input, and a putchar shortcut. Click View Output to reveal sample console results.

📚 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.

c
#include <stdio.h>

int main() {
    int rows = 5;
    int i, j;

    for (i = rows; i >= 1; --i) {
        for (j = 1; j <= i; ++j) {
            printf("%d", i);
        }
        printf("\n");
    }

    return 0;
}

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. printf("\n") 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 scanf("%d", &rows) (check the return value in real apps).

c
#include <stdio.h>

int main() {
    int rows;
    int i, j;

    printf("Enter the number of rows: ");
    scanf("%d", &rows);

    for (i = rows; i >= 1; --i) {
        for (j = 1; j <= i; ++j) {
            printf("%d", i);
        }
        printf("\n");
    }

    return 0;
}

How It Works

Same nested-loop core as Example 1; only the source of rows changes. Non-numeric input leaves rows unset if you ignore scanf’s return value — always check it in safer labs.

⚡ Shortcut Style

Same shape using putchar instead of printf for each digit.

Example 3 — putchar('0' + i)

Repeat each digit with putchar, then end the row with putchar('\n').

c
#include <stdio.h>

int main() {
    int rows = 5;
    int i, j;

    for (i = rows; i >= 1; --i) {
        for (j = 1; j <= i; ++j) {
            putchar('0' + i);
        }
        putchar('\n');
    }

    return 0;
}

How It Works

putchar('0' + i) writes one digit when i is 1..9 without a format string. Same nested-loop structure as Example 1; a leaner alternative to printf("%d", i).

🧠 How the Algorithm Prints Rows

1

Set up

#include <stdio.h> brings in printf / scanf. Set rows (fixed or from input).

Setup
2

Outer loop (rows)

for (i = rows; i >= 1; i--) picks the digit; the inner loop prints it i times.

Row
3

Inner loop (repeats)

for (j = 1; j <= i; j++) repeats digit i exactly i times with printf("%d", i).

Repeats
4

New line

printf("\n") 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.

iInner j rangeRepeatsPrinted row
41..444444
31..33333
21..2222
11..111

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 (j = rows; j >= i; j--) so repeats grow as the digit shrinks.

3. Console Formatting Drills

Practice printf vs row newline without complex math.

Example: put printf("\n") 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 scanf 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. 1. Instant Visual Feedback

    Wrong bounds show up immediately as a broken staircase.

  2. 2. Minimal Concepts

    Only loops and console output — no arrays or math libraries.

  3. 3. Easy to Extend

    Invert, center, hollow, or change the fill character with small edits.

  4. 4. O(1) Extra Memory

    Streaming output needs no storage beyond loop counters.

Pro Tip: learn the nested-loop version first; treat putchar('0' + i) in inner loop as a polish shortcut afterward.

Usage Tips

Small habits that keep number-pattern code clean.

  1. 1. Name Bounds Clearly

    Use rows (or n) and keep i/j for row/column — or rename to row/col.

  2. 2. Prefer scanf

    Avoid crashes when the user types letters instead of a number.

  3. 3. Keep printf("\n") Outside

    Only call printf("\n") after the inner loop finishes the row.

  4. 4. Use j <= i for Repeat Count

    for (j = 1; j <= i; j++) with printf("%d", i) repeats digit i exactly i times.

  5. 5. Dry-Run One Small n

    Trace rows = 3 on paper before coding larger demos.

Pro Tip: if the output is a vertical list of single digits per line, you almost certainly put printf("\n") inside the inner loop.

Common Pitfalls

Mistakes that commonly break inverted repeating number patterns.

  1. 1. printf("\n") Inside the Inner Loop

    Each repeated digit lands on its own line — you get a column, not a triangle.

    → Use printf("%d", i) for repeats; printf("\n") only after the inner loop.

  2. 2. Wrong Inner Bound

    j <= rows prints a rectangle; wrong outer bounds flatten or invert the shape.

    → For this shape, keep j <= i.

  3. 3. Forgetting the Row Break

    Omitting printf("\n") glues every repeat onto one endless line.

    → Always end the row after the inner loop.

  4. 4. Unchecked scanf

    Letters or empty input throw undefined rows.

    → Prefer scanf and re-prompt on failure.

  5. 5. Off-by-One on 0-Based Loops

    Switching to i = 0 without adjusting the inner bound prints an empty first row or wrong counts.

    → If 0-based, print i with wrong inner bound (e.g. j <= i + 1).

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 scanf input

Unchecked scanf leaves rows unset — check the return value.

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 (i = rows; i >= 1; i--) as outer loop
  • Shows the loop structure is reusable

3. Safe input loop

  • Use scanf until rows >= 1
  • Then draw the triangle

4. Same digit, different count

  • Use printf("%d ", i) between 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.
  • printf stays on the line; printf("\n") advances — mix them carefully.
  • Validate rows > 0 for interactive programs; rows = 1 should print rows copies 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

ProgramTimeExtra space
Nested loops (Examples 1–2)O(rows²)O(1)
putchar('0' + i) in inner loop (Example 3)O(rows²)O(rows) per row string (temporary)
Wrap Up

🎉 Conclusion

The inverted repeating number triangle pattern is a small nested-loop exercise with lasting payoff: row/column thinking, printf vs row newline, and O(n²) intuition. Master the classic two-loop version, then optionally shorten rows with putchar('0' + i) in inner loop.

Practice the three examples above, then continue to Program 12 for the next pattern in the series.

Row i repeats digit i — keep printf("%d", i) for each copy and printf("\n") for the break, and validate row counts when reading input.

View Examples Gallery Continue to Program 12
Pro Tips

💡 Best Practices

✅ Do

  • Explain outer picks digit, inner counts repeats before coding
  • Use printf("%d", i) for repeats and printf("\n") after each row
  • Validate rows ≥ 1 for interactive programs
  • Check scanf return value before using rows
  • State O(n²) time when asked about complexity

❌ Don’t

  • Call printf("\n") inside the inner repeat loop
  • Use j >= i when you meant Program 10 instead
  • Skip the newline after each row
  • Ignore bad console input in user-facing demos
  • Skip the rows = 1 edge 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

Definition
↻ 02

Outer loop

Picks the digit each row

Code
1 03

Inner loop

Repeats digit with printf("%d", i)

Code
↵ 04

printf("\n")

Ends each row

I/O
O 05

Complexity

O(n²) time

Analysis
🎯 You’re ready to print an inverted repeating number triangle in C
Review Quick Reference →

❓ 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 (j = 1; j <= i; j++) printf("%d", i) runs the inner body i times. When i is 5 you get 55555; when i is 1 you get a single 1.
printf("%d", i) stays on the same line. printf("\n") ends the current line. Repeated digits use printf; the row break uses printf("\n") after the inner loop.
Program 10 grows repeat count as the digit shrinks (5, 44, 333, 2222, 11111) using for (j = rows; j >= i; j--). Program 11 repeats digit i exactly i times (55555, 4444, 333, 22, 1) using for (j = 1; j <= i; j++).
O(n²) where n is the number of rows. Total printf digit calls equal n+(n-1)+…+1 = n(n+1)/2.
Yes. A loop with putchar('0' + i) writes each repeated digit without a format string. Nested loops with printf are better for learning; putchar is a leaner shortcut once the bounds make sense.
Check scanf's return value: if (scanf("%d", &rows) != 1) handle bad input. Unchecked scanf leaves rows uninitialized on failure.
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.

Program 12 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
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I write practical tutorials so students and working developers can learn by doing—from databases and APIs to deployment on AWS.

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