Shrinking Repeating Number Pattern in C

Q: What is the time complexity?

O(n²) where n is the number of rows. Total printf digit calls equal n+(n-1)+…+1 = n(n+1)/2.

Q: Can I use putchar instead of an inner loop?

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.

Q: How should I read the row count safely?

Check scanf's return value: if (scanf("%d", &rows) != 1) handle bad input. Unchecked scanf leaves rows uninitialized on failure.

Q: What happens if rows is 0 or negative?

The outer loop never runs, so nothing is printed. Validate and prompt again if you want a clear user message.

Beginner

⏱️ 8 min read

📚 Updated: Aug 2026

🎯 3 Code Examples

🚀 Live Preview

Nested Loops

Shrinking repeating number pattern output (11111, 2222, 333, 44, 5) in C programming

Overview

What You’ll Learn

The shrinking repeating number pattern teaches how a changing inner-loop start value controls row width. 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 rows − i + 1 times

Row 1 prints 11111, row 2 prints 2222, and so on until the last row prints a single digit.

Outer Loop

Rows

for (i = 1; i <= rows; i++) walks each line as the digit increases from 1 to rows.

Inner Loop

Digits

for (j = i; j <= rows; j++) prints digit i exactly rows − i + 1 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 shrinking repeating number pattern instantly in the browser.

O(n²)

Complexity

Total digit prints still = n(n+1)/2; extra memory stays O(1).

Introduction

A shrinking repeating number pattern repeats the row digit on each line, but the row becomes shorter as i increases. With rows = 5, the output is 11111, 2222, 333, 44, 5.

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

Why it matters?

It is a great follow-up after growing and inverted repeating patterns. Once you see how the inner-loop start controls width, many number triangles become predictable.

Key Highlights

Row = Repeat Count

On row i, print digit i exactly rows − i + 1 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

Compare Program 9 (growing repeats) and Program 11 (inverted triangle).

In short: for each row i from 1 to rows, repeat printf("%d", i) for j from i to rows, then call printf("\n").

Foundation

📝 Problem & Approach

Given a positive integer rows, print a shrinking repeating number pattern: row i repeats digit i exactly rows − i + 1 times, with the outer loop counting from 1 up to rows.

// First 5 rows (conceptual shape)
// 11111
// 2222
// 333
// 44
// 5

Inputs & Outputs

Item	Type	Description
`rows`	int	Number of triangle lines to print (typically ≥ 1).
Printed output	text	Each row repeats one digit; row `i` has `rows − i + 1` copies of `i`, so the first row is widest.

Minimal workflow

Pseudocode

for i from 1 to rows:
    for j from i to rows:
        print i (no newline)
    print newline

Approach comparison

Approach	Idea	Best for
Nested loops	Outer digit + inner repeats	Learning and interviews
`putchar('0' + i)` in inner loop	Write each repeated digit without printf	Leaner C style once loops are understood

Cheat Sheet

⚡ Quick Reference

Goal	Pattern
Walk each row	`for (i = 1; i <= rows; i++)`
Repeat digit `i`	`for (j = i; j <= rows; j++) printf("%d", i);`
End the row	`printf("\n");`
One-line row shortcut	`putchar('0' + i)` repeated per row
Program 11 variant	`for (j = 1; j <= i; j++) printf("%d", i)` (inverted triangle)

Compare

📋 printf vs putchar vs newline

Same pattern — 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 pattern when teaching how inner-loop bounds control row width.

First lab exercise
Most C 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 scanf for a flexible row count.
Gateway to variants
Compare Program 9 (1, 22, 333, …) and Program 11 (55555, 4444, …) 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 shrinking repeating number pattern in the browser.

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 shrinking repeating number pattern with nested loops.

Example 1 — Fixed `rows = 5`

Hard-coded height — outer picks digit, inner runs from i to rows.

#include <stdio.h>

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

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

    return 0;
}

How It Works

When i = 1, the inner loop runs from 1 to 5 and prints 11111. When i = 2, it prints 2222, and so on until i = rows prints a single 5. 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).

#include <stdio.h>

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

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

    for (i = 1; i <= rows; ++i) {
        for (j = i; j <= rows; ++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').

#include <stdio.h>

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

    for (i = 1; i <= rows; ++i) {
        for (j = i; j <= rows; ++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

Set up

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

Setup

Outer loop (rows)

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

Row

Inner loop (repeats)

for (j = i; j <= rows; j++) repeats digit i with printf("%d", i) as the row shrinks.

Repeats

New line

printf("\n") ends the row so the next outer iteration starts fresh.

Break

Pattern 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
`1`	`1..4`	4	`1111`
`2`	`2..4`	3	`222`
`3`	`3..4`	2	`33`
`4`	`4..4`	1	`4`

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: change inner bound to j <= i for growing repeats (Program 9).

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. 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 putchar('0' + i) in inner loop as a polish shortcut afterward.

Usage Tips

Small habits that keep number-pattern code clean.

1. Name Bounds Clearly
Use rows (or n) and keep i/j for row/column — or rename to row/col.
2. Prefer scanf
Avoid crashes when the user types letters instead of a number.
3. Keep printf("\n") Outside
Only call printf("\n") after the inner loop finishes the row.
4. Start at i, Not 1
for (j = i; j <= rows; j++) with printf("%d", i) is the key to shrinking width.
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 shrinking repeating number patterns.

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. Wrong Inner Bound
j <= i grows repeats; j = 1 with wrong bound prints a rectangle.
→ For this shape, keep for (j = i; j <= rows; j++).
3. Forgetting the Row Break
Omitting printf("\n") glues every repeat onto one endless line.
→ Always end the row after the inner loop.
4. Unchecked scanf
Letters or empty input throw undefined rows.
→ Prefer scanf and re-prompt on failure.
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 digit

Output is just 1 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 = 1 by mistake

Use j = i so row width shrinks as i grows.

Drill

🎯 Practice Problems

Try these variations to lock in the pattern.

1. Growing repeating pattern

Inner loop j from 1 to i (see Program 9)
Prints 1, 22, 333, …

2. Inverted repeating triangle

Outer loop from rows down to 1
Continue with Program 11

3. Safe input loop

Use scanf until rows >= 1
Then draw the pattern

4. Spaced output

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 a single 1.
This page is left-aligned. Centered pyramids need leading spaces — covered later in the series.

Quick Takeaway: outer loop picks the digit, inner loop runs from i to rows, 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)`
`putchar('0' + i) in inner loop` (Example 3)	`O(rows²)`	`O(rows)` per row string (temporary)

Wrap Up

🎉 Conclusion

The shrinking repeating number pattern is a compact nested-loop exercise: digit increases each row while repeat count shrinks. 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 13 for the alternating zigzag number triangle.

Row i repeats digit i exactly rows - i + 1 times — keep printf("%d", i) for each copy and printf("\n") for the break.

View Examples Gallery Continue to Program 13

Pro Tips

💡 Best Practices

✅ Do

Explain outer picks digit, inner start at i 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 = 1 when you meant Program 9 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 shrinking pattern

Print the pattern the beginner-friendly way.

Core concepts

△ 01

Rule

Row i repeats digit i (rows-i+1) times

Definition

↻ 02

Outer loop

Picks the digit each row

Code

1 03

Inner loop

Runs from i to rows

Code

↵ 04

printf("\n")

Ends each row

I/O

O 05

Complexity

O(n²) time

Analysis

❓ Frequently Asked Questions

Why does the row length shrink each time?

The inner loop runs from j = i to rows. As i increases (1, 2, 3, …), the inner loop runs fewer times, so each row prints fewer copies of the row digit.

Why does the first row have five 1s?

On the first row, i = 1 and the inner loop runs from 1 to rows, so it prints 1 exactly rows times — five 1s when rows = 5.

What is the difference between printf("%d", i) and printf("\n")?

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.

How do I print 1, 22, 333, … instead?

Use the inner loop as 1..i instead of i..rows so the repeat count grows (see Program 9).

How is this different from Program 11?

Program 11 counts the outer loop down and repeats digit i exactly i times (55555, 4444, 333, 22, 1). Program 12 counts up and uses for (j = i; j <= rows; j++) so digit i repeats rows - i + 1 times (11111, 2222, 333, 44, 5).

What is the time complexity?

O(n²) where n is the number of rows. Total printf digit calls equal n+(n-1)+…+1 = n(n+1)/2.

Can I use putchar instead of an inner loop?

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.

How should I read the row count safely?

Check scanf's return value: if (scanf("%d", &rows) != 1) handle bad input. Unchecked scanf leaves rows uninitialized on failure.

What happens if rows is 0 or negative?

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 rows − i + 1 times. The outer loop counts up from 1, while the inner loop runs from i to rows, so each row shrinks — still O(n²) total prints.

Continue to Program 13

Next up — alternating ascending/descending rows in Program 13.

Program 13 tutorial →

About the author

Mari Selvan M P 🔗

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Experience 12 years building web and cloud systems
Focus Full Stack Development, AWS, and Developer Education

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C Number Pattern 11 C Number Pattern 13 — Alternating Rows

Shrinking Repeating Number Pattern in C

What You’ll Learn

Shape Rule

Outer Loop

Inner Loop

printf vs newline

Live Preview

O(n²)

Introduction

Why it matters?

Row = Repeat Count

Two Nested Loops

Print Then Break

Series Foundation

📝 Problem & Approach

Inputs & Outputs

Minimal workflow

Approach comparison

⚡ Quick Reference

📋 printf vs putchar vs newline

When This Pattern Shows Up

First lab exercise

Nested-loop warm-up

Console I/O practice

Gateway to variants

Not a UI layout tool

🔮 Live Preview

Examples Gallery

📚 Getting Started

Example 1 — Fixed rows = 5

How It Works

📈 Practical Variant

Example 2 — User Input Version

How It Works

⚡ Shortcut Style

Example 3 — putchar('0' + i)

How It Works

🧠 How the Algorithm Prints Rows

Set up

Outer loop (rows)

Inner loop (repeats)

New line

Pattern complete

🔎 Worked Walkthrough — rows = 4

Use Cases

1. Teaching Nested Loops

2. Pattern Series Base

3. Console Formatting Drills

4. Character Substitution

5. Complexity Intuition

6. Input Validation Labs

Advantages

1. Instant Visual Feedback

2. Minimal Concepts

3. Easy to Extend

4. O(1) Extra Memory

Usage Tips

1. Name Bounds Clearly

2. Prefer scanf

3. Keep printf("\n") Outside

4. Start at i, Not 1

5. Dry-Run One Small n

Common Pitfalls

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

2. Wrong Inner Bound

3. Forgetting the Row Break

4. Unchecked scanf

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

Edge Cases

Single digit

Empty pattern

rows < 0

Many rows

Non-numeric scanf input

Inner bound j = 1 by mistake

🎯 Practice Problems

1. Growing repeating pattern

2. Inverted repeating triangle

3. Safe input loop

4. Spaced output

Example 1 — Fixed `rows = 5`

Example 3 — `putchar('0' + i)`

🔎 Worked Walkthrough — `rows = 4`

2. Prefer `scanf`

`rows < 0`