Reverse a Number in C++

How to Reverse a Number in C++ (6 Programs With Output).

BeginnerAdvanced Number ProgramsExample 4 of 13

reverse-a-number.cpp

1#include <iostream>
2using namespace std;
3
4int main() {
5    int num, reversed = 0, remainder;
6    
7    cout << "Enter a number: ";
8    cin >> num;
9    
10    int original = num;
11    
12    while (num != 0) {
13        remainder = num % 10;
14        reversed = reversed * 10 + remainder;
15        num /= 10;
16    }
17    
18    cout << "Reverse of " << original << " is: " << reversed << endl;
19    
20    return 0;
21}

Output

Enter a number: 1234
Reverse of 1234 is: 4321

What's going on

This C++ program reverses the digits of an integer. 1234 becomes 4321. 100 becomes 1 — integers don't keep leading zeros.

int num, reversed = 0, remainder;

reversed starts at 0. That's the empty number you're about to build.

cin >> num;

int original = num;

Save it. The loop destroys num. Without original, the cout at the end has nothing honest to print.

while (num != 0)

Peel one digit per pass.

remainder = num % 10;

Last digit. 1234 % 10 is 4.

reversed = reversed * 10 + remainder;

Shift the digits you already have one place left, glue the new one on the right.

num /= 10;

Throw the last digit away. 1234 / 10 is 123.

Walk 1234:

rem 4, reversed 4, num 123

rem 3, reversed 43, num 12

rem 2, reversed 432, num 1

rem 1, reversed 4321, num 0

loop stops

100: rem 0, reversed 0, num 10 → rem 0, reversed 0, num 1 → rem 1, reversed 1. Output 1, not 001.

Negatives: % on a negative dividend is messy. This program is written for positives.

Palindrome check is this reverse, then original == reversed.

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