Summary: In this example, we will learn different ways to reverse a number in the C++ programming language.
Example:
1 2 3 4 5 | Input: 456 Output: 654 Input: 132 Output: 231 |
Method 1: Using While Loop
The idea is to extract the digits from the input number in reverse order and append them to form its reverse using the while loop.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 | #include <iostream> using namespace std; int main() { int num; //Input a number cout << "Enter a Number: "; cin >> num; //Initialize rev with 0 int rev=0; //Append digits in reverse order while(num>0){ rev = rev*10 + num%10; num = num/10; } cout << "Reverse: " << rev; return 0; } |
Output:
Enter a Number: 243Reverse: 342
To extract and append the digits from the original number to its reverse we use the following: rev =rev*10 + num%10.
After the operation on each digit, we remove it from the input number using the following: num=num/10.
Method 2: Using Recursion
What we have implemented in the above method can also be implemented using recursion.
To append the digits in reverse order in the recursive approach:
- Extract the last digit i.e.
num%10 - Multiply the digit with
pow(10, l-1), wherelis the length of the number passed as an argument - Add the result to the next recursive call.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 | #include <iostream> #include <cmath> using namespace std; //recursive_function(number, length_of_the_number) int reverse(int num, int l){ //base contition if(num==0) return 0; //extract the last digit int digit = num%10; /* *Append them in reverse order *For e.g., to reverse 859 *return 9*100 + 58 = 958 */ return digit*pow(10, l-1) + reverse(num/10, l-1); } int main() { int num; //Input a number cout << "Enter a Number: "; cin >> num; //call the recursive funtion to reverse the number int length = to_string(num).length(); int rev = reverse(num, length); //output the number cout << "Reverse: " << rev; return 0; } |
Output:
Enter a Number: 859Reverse: 958
For each recursive call, we are eliminating the last digit from the number using num/10 and also decreasing the length of the number by 1 i.e. l-1.
Method 3: Converting int To string
We can transform the integer into a string using the inbuilt to_string() method and reverse it by using any string reversal methods in C++.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 | #include <iostream> #include<algorithm> using namespace std; int main() { int num; //Input a number cout << "Enter a Number: "; cin >> num; //convert the integer to string string str = to_string(num); /* *reverse the string using the reverse() method *defined in the algorithm header file */ reverse(str.begin(), str.end()); //output the string cout << "Reverse: " << str; return 0; } |
Output:
Enter a Number: 700Reverse: 007
These were the three methods using which we can reverse any number in C++ programming language.