Growing Reverse Number Pattern in C#

Beginner

⏱️ 10 min read

📚 Updated: Aug 2026

🎯 3 Code Examples

🚀 Live Preview

While Loop + Modulo

Growing reverse number pattern output (3, 32, 325, 3256, 32568) in C# programming

Overview

What You’ll Learn

Program 61 prints a growing reverse-number pattern: starting from an integer like 86523, build the reverse one digit at a time and print each partial result — 3, 32, 325, and so on. This tutorial covers modulo, reverse building, a live preview, worked C# examples, edge cases, and O(d) complexity.

Shape Rule

Growing reverse

Each line adds one digit on the right of the partial reverse — 3 → 32 → 325 → 3256 → 32568.

While Loop

num != 0

while (num != 0) repeats once per digit until the source number is fully consumed.

Modulo

num % 10

num % 10 extracts the last digit — 3 from 86523, then 2 from 8652, and so on.

Build Reverse

reverse * 10 + digit

reverse = reverse * 10 + (num % 10) shifts left and appends the new digit on the right.

Live Preview

Any integer

Enter a starting number and watch the growing reverse pattern in the browser.

O(d)

Complexity

One iteration per digit — 86523 produces five lines; total steps equal digit count.

Introduction

A growing reverse number pattern builds the reversed form of an integer one digit at a time and prints each partial result. With num = 86523, you get 3, 32, 325, 3256, 32568.

In C# use while (num != 0), extract digits with num % 10, update reverse = reverse * 10 + digit, print, then num /= 10.

Why it matters?

Modulo plus reverse building is the standard technique for reversing numbers, checking palindromes, and digit-sum problems.

Key Highlights

Extract digit

num % 10 — last digit each step.

Build reverse

reverse * 10 + digit appends on the right.

vs Program 60

Program 60 shrinks the original; Program 61 grows the partial reverse.

Series Foundation

Follow Program 60; continue to Program 62 next.

In short: while (num != 0), reverse = reverse * 10 + num % 10, WriteLine(reverse), then num /= 10.

Foundation

📝 Problem & Approach

Given starting integer num = 86523, build its reverse one digit at a time and print each partial reverse until all digits are processed.

// num = 86523
//3
//32
//325
//3256
//32568

Inputs & Outputs

Item	Type	Description
`num`	int	Source number — shrinks each iteration via `/ 10`.
`reverse`	int	Partial reverse — starts at `0`, grows each step.
Extract digit	int	`num % 10` — last digit of current `num`.
Append digit	int	`reverse = reverse * 10 + digit`.
Print step	void	`Console.WriteLine(reverse)` after each append.
Line count	int	Equals digit count of the starting number (86523 → 5 lines).

Minimal workflow

Pseudocode

reverse = 0
while num is not 0:
    digit = num % 10
    reverse = reverse * 10 + digit
    print reverse
    num = num / 10

Approach comparison

Approach	Idea	Best for
Two-step append	`reverse *= 10;` then `reverse += digit`	Teaching each operation separately
Combined append	`reverse = reverse * 10 + (num % 10)`	Compact production code
long + Math.Abs	Wider range and negative handling	User-input programs
Compact trace	`num = 123` on paper first	Quick dry-runs
BigInteger	Arbitrary precision	Very large starting values

Cheat Sheet

⚡ Quick Reference

Goal	Pattern
Loop	`while (num != 0) { ... }`
Extract digit	`int digit = num % 10;`
Append to reverse	`reverse = reverse * 10 + digit;`
Print & shrink	`Console.WriteLine(reverse); num /= 10;`

Compare

📋 Fixed Value vs User Input vs Compact Trace

Same growing-reverse pattern — three ways to set the starting number and trace the logic.

Fixed value

num = 86523

Hard-coded start for demos

User input

long TryParse

Read starting number from console

Compact trace

num = 123

Quick dry-run on paper

Extract

num % 10

Last digit each step

Build

rev * 10 + d

Append digit on the right

Context

When This Pattern Shows Up

Reach for this pattern when teaching modulo, reverse building, and while loops together in C#.

Post Program 60 exercise
Natural follow-up — same while loop, but build a growing reverse instead of shrinking the original.
Number reversal drills
Standard technique for reversing integers and checking palindromes.
Interview warm-ups
Classic modulo + division question — explain extract-append-shrink before coding.
Gateway to Program 62
Compare single-number loops with 2D spiral grid filling.
Overflow awareness
Use long or BigInteger when reversed values exceed int range.

Key benefit: one small program that locks in modulo, reverse building, and O(d) thinking.

Interactive

🔮 Live Preview

Enter a positive integer between 10 and 99999999 and draw the growing reverse pattern in the browser.

Hands-On

Examples Gallery

Three complete C# programs — fixed starting value, user input with long, and a compact num = 123 trace. Click View Output to reveal sample console results.

📚 Getting Started

Print the growing reverse pattern for a hard-coded starting integer with a while loop.

Example 1 — Fixed `num = 86523`

Hard-coded start — build reverse one digit at a time and print after each append.

using System;

namespace MyApp
{
    class Program
    {
        static void Main(string[] args)
        {
            int num = 86523;
            int reverse = 0;

            while (num != 0)
            {
                reverse = reverse * 10;
                reverse = reverse + (num % 10);
                Console.WriteLine(reverse);
                num = num / 10;
            }
        }
    }
}

How It Works

Extract 3 from 86523 → reverse becomes 3; then 2 → 32; then 5 → 325 — each line shows the partial reverse so far.

📈 Practical Variant

Read the starting number from the user with long and handle negatives with Math.Abs.

Example 2 — User Input Version

Read num from the console with safe parsing and combined append in one expression.

using System;

namespace MyApp
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Enter a number: ");
            if (!long.TryParse(Console.ReadLine(), out long num))
            {
                Console.WriteLine("Please enter a valid integer.");
                return;
            }

            num = Math.Abs(num);
            long reverse = 0;

            while (num != 0)
            {
                reverse = reverse * 10 + (num % 10);
                Console.WriteLine(reverse);
                num /= 10;
            }
        }
    }
}

How It Works

Same loop core as Example 1; uses long for wider range and combines multiply-and-add into one line.

⚡ Compact Trace

Use num = 123 for a quick paper trace before larger demos.

Example 3 — Compact `num = 123`

Same while loop with a smaller starting value — easy to dry-run on paper.

using System;

namespace MyApp
{
    class Program
    {
        static void Main(string[] args)
        {
            int num = 123;
            int reverse = 0;

            while (num != 0)
            {
                reverse = reverse * 10 + (num % 10);
                Console.WriteLine(reverse);
                num /= 10;
            }
        }
    }
}

How It Works

Three iterations: extract 3 → reverse 3; extract 2 → reverse 32; extract 1 → reverse 321.

🧠 How the Algorithm Builds the Reverse

Set up

num is the source number; reverse starts at 0.

Setup

Extract last digit

num % 10 gives the current last digit (3, then 2, then 5, …).

Digit

Append to reverse

reverse = reverse * 10 + digit shifts existing digits left and appends the new one.

Build

Print and shrink num

Console.WriteLine(reverse) then num /= 10 moves to the next digit.

Step

Growing reverse pattern complete

One line per digit processed — O(d) time for d digits, O(1) extra memory.

Trace

🔎 Worked Walkthrough — `num = 86523`

Trace each loop iteration: extract the digit, update reverse, print, then shrink num.

Step	`num`	Digit `num % 10`	`reverse` after append	Printed
1	`86523`	`3`	`3`	`3`
2	`8652`	`2`	`32`	`32`
3	`865`	`5`	`325`	`325`
4	`86`	`6`	`3256`	`3256`
5	`8`	`8`	`32568`	`32568`

Total lines printed: 5 = digit count of the starting number. Final reverse equals the full reversed number.

Use Cases

Where this tiny pattern (and its loop structure) shows up beyond the homework prompt.

1. Teaching Modulo

num % 10 extracts digits — concrete introduction to the remainder operator.

Example: trace num = 123 and watch reverse grow 3, 32, 321.

2. Number Reversal

Standard building block for reversing integers and palindrome checks.

Example: stop after the loop and compare final reverse to the original.

3. Pair with Program 60

Program 60 shrinks the original; Program 61 grows the partial reverse — same loop, different output.

Example: print both patterns side by side for 86523.

4. Gateway to Program 62

Move from single-number loops to 2D spiral grid filling next.

Example: compare while-loop digit work with nested boundary loops.

5. Complexity Intuition

One iteration per digit — makes O(d) concrete for beginners.

Example: num = 1000000 produces seven lines — seven digits.

6. Input Validation Labs

Pair the pattern with TryParse and overflow checks.

Example: use long when reversed values may exceed int range.

Pro Tip: when an interviewer asks for reverse building, explain extract-append-shrink before writing the loop.

Advantages

Why this pattern earns a permanent spot in beginner C# courses.

1. Instant Visual Feedback
Wrong append order or missing shrink shows up immediately as broken output.
2. Teaches Two Operators
Modulo and integer division together — essential digit-manipulation toolkit.
3. Easy to Extend
Print both num and reverse, collect steps in a list, or check palindromes with small edits.
4. O(1) Extra Memory
Only num and reverse needed — no arrays required.

Pro Tip: trace num and reverse on paper for 123 before coding — watch reverse grow 3, 32, 321.

Usage Tips

Small habits that keep growing-reverse code clean.

1. Initialize reverse to Zero
reverse = 0 before the loop — first digit becomes the first printed value.
2. Prefer TryParse
Use long.TryParse for user input when values may be large.
3. Multiply Before Adding
reverse * 10 + digit — multiply shifts left, then append the new digit.
4. Shrink num Each Step
num /= 10 after printing — without it the loop never advances.
5. Dry-Run One Small Number
Trace num = 123 on paper before coding larger demos.

Pro Tip: if reverse stays at single digits, you probably forgot to multiply by 10 before adding.

Common Pitfalls

Mistakes that commonly break growing-reverse patterns.

1. Forgetting to Multiply by 10
Adding digits without shifting leaves reverse as single digits — 3, 2, 5 instead of 3, 32, 325.
→ Always do reverse = reverse * 10 + digit.
2. Not Shrinking num
Without num /= 10, the same digit is extracted forever — infinite loop.
→ Divide by 10 after each append and print.
3. Integer Overflow
Large inputs can overflow int when building reverse — silent wrong results.
→ Use long or BigInteger for large numbers.
4. Blind Convert.ToInt32
Letters or empty input throw FormatException.
→ Prefer TryParse and re-prompt on failure.
5. Using Floating-Point Math
Float division on digits introduces precision errors — stick to integer operations.
→ Keep num and reverse as integer types.

Edge Cases

Check these inputs before calling the solution done.

num = 0

Zero input

while (num != 0) never runs — print nothing or show a message.

num = 7

Single digit

One line prints 7 — simplest case.

120

Trailing zero

Builds 0, 2, 21 — zero is extracted first from the right.

Negative

Negative input

Apply Math.Abs before the loop — see Example 2.

Bad input

Non-numeric ReadLine

Convert.ToInt32 throws — use TryParse.

Large n

Overflow risk

Use long or BigInteger when reverse exceeds int range.

Drill

🎯 Practice Problems

Try these variations to lock in the pattern.

1. Compare with Program 60

Program 60 prints shrinking original
Program 61 prints growing partial reverse

2. Palindrome check

Build full reverse, then compare to original
Do not print intermediate steps

3. Next in series

Continue with Program 62
Perfect square spiral number pattern

4. Paper trace

Dry-run num = 123 before coding
Fill the walkthrough table by hand

Important

Notes

Line count. Total lines equal the digit count of the starting number — hence O(d) time.
reverse * 10 shifts existing digits left before the new digit is appended on the right.
Use long when reversed values may exceed int.MaxValue.
Final reverse after the loop equals the fully reversed number — 32568 for input 86523.

Quick Takeaway: while (num != 0), reverse = reverse * 10 + num % 10, print, then num /= 10.

Analysis

⏱️ Time and Space Complexity

Program	Time	Extra space
While loop (Examples 1–2)	`O(d)` — d = digit count	`O(1)`
Compact trace (Example 3)	`O(d)`	`O(1)`

Wrap Up

🎉 Conclusion

The growing reverse number pattern is a small while-loop exercise with lasting payoff: modulo, reverse building, and O(d) intuition. Master the fixed-value version, then try user input with long and the compact num = 123 trace.

Practice the three examples above, then continue to Program 62 for the next pattern in the series.

Extract with modulo, append with multiply-and-add, print reverse, then shrink num — validate input when reading from the console.

View Examples Gallery Continue to Program 62

Pro Tips

💡 Best Practices

✅ Do

Explain extract-append-shrink before coding
Initialize reverse = 0 before the loop
Use reverse = reverse * 10 + (num % 10)
Print reverse after each append
Use long.TryParse for large user input
State O(d) time when asked about complexity

❌ Don’t

Forget to multiply reverse by 10 before adding
Skip num /= 10 inside the loop
Use floating-point math for digit extraction
Ignore integer overflow on large inputs
Skip the num = 123 dry-run before larger demos

Summary

Key Takeaways

Knowledge Unlocked

Five things to remember about this growing reverse pattern

Print the growing reverse pattern the beginner-friendly way.

Core concepts

△ 01

Rule

Build reverse digit by digit

Definition

% 02

Modulo

num % 10 extracts digit

Math

10 03

Append

reverse * 10 + digit

Code

↵ 04

Shrink

num /= 10 each step

Loop

O 05

Complexity

O(d) time

Analysis

❓ Frequently Asked Questions

What does Number Pattern 61 print?

It prints a growing reverse-number pattern like 3, 32, 325, 3256, 32568 when starting from 86523.

How does the program generate each next line?

It takes the last digit using num % 10 and appends it to reverse using reverse = reverse * 10 + digit, then prints reverse.

Why do we divide num by 10 each step?

num = num / 10 removes the last digit so the loop can move to the next digit from the right.

How is this different from Program 60?

Program 60 prints the shrinking original number. Program 61 builds and prints a growing partial reverse using modulo and multiplication.

How is this different from Program 62?

Program 61 uses a single while loop on one number. Program 62 fills an n×n spiral grid with a 2D array.

Why multiply reverse by 10 before adding the digit?

Multiplying shifts existing digits left — reverse * 10 + digit appends the new digit on the right.

Can trailing zeros appear in the output?

If the source number ends in 0, that digit is extracted last — e.g. 120 gives 0, 2, 21 (building from the right).

Does this work for negative numbers?

Apply Math.Abs(num) before the loop — see Example 2 which uses long and absolute value.

What is the time complexity?

O(d) where d is the number of digits — the loop runs once per digit.

How should I read the starting number safely?

Prefer int.TryParse or long.TryParse so bad input does not throw FormatException.

Did you Know? 🔊

Each iteration takes the last digit with num % 10, appends it to reverse via reverse = reverse * 10 + digit, prints the partial reverse, then shrinks num — runtime is O(d) for d digits.

Continue to Program 62

Move on to the next pattern in the C# number-pattern series.

Program 62 tutorial →

About the author

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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C# Number Pattern 60 C# Number Pattern 62

Growing Reverse Number Pattern in C#

What You’ll Learn

Shape Rule

While Loop

Modulo

Build Reverse

Live Preview

O(d)

Introduction

Why it matters?

Extract digit

Build reverse

vs Program 60

Series Foundation

📝 Problem & Approach

Inputs & Outputs

Minimal workflow

Approach comparison

⚡ Quick Reference

📋 Fixed Value vs User Input vs Compact Trace

When This Pattern Shows Up

Post Program 60 exercise

Number reversal drills

Interview warm-ups

Gateway to Program 62

Overflow awareness

🔮 Live Preview

Examples Gallery

📚 Getting Started

Example 1 — Fixed num = 86523

How It Works

📈 Practical Variant

Example 2 — User Input Version

How It Works

⚡ Compact Trace

Example 3 — Compact num = 123

How It Works

🧠 How the Algorithm Builds the Reverse

Set up

Extract last digit

Append to reverse

Print and shrink num

Growing reverse pattern complete

🔎 Worked Walkthrough — num = 86523

Use Cases

1. Teaching Modulo

2. Number Reversal

3. Pair with Program 60

4. Gateway to Program 62

5. Complexity Intuition

6. Input Validation Labs

Advantages

1. Instant Visual Feedback

2. Teaches Two Operators

3. Easy to Extend

4. O(1) Extra Memory

Usage Tips

1. Initialize reverse to Zero

2. Prefer TryParse

3. Multiply Before Adding

4. Shrink num Each Step

5. Dry-Run One Small Number

Common Pitfalls

1. Forgetting to Multiply by 10

2. Not Shrinking num

3. Integer Overflow

4. Blind Convert.ToInt32

5. Using Floating-Point Math

Edge Cases

Zero input

Single digit

Trailing zero

Negative input

Non-numeric ReadLine

Overflow risk

🎯 Practice Problems

1. Compare with Program 60

2. Palindrome check

3. Next in series

4. Paper trace

Example 1 — Fixed `num = 86523`

Example 3 — Compact `num = 123`

🔎 Worked Walkthrough — `num = 86523`