C++ Program to calculate the LCM

In this C++ program, we are discussing how to calculate the LCM of two integers.

What is LCM?

LCM stands for Least Common Multiple. A multiple is a number you get when you multiply a number by a whole number greater than zero. LCM is a method to find the smallest common multiple between any two or more numbers.

How do you find LCM? 

LCM of numbers can be calculated using various methods. There are 3 methods to find the least common multiple of two numbers.
•    LCM by Listing Method
•    LCM using Prime Factorization
•    LCM using Division Method

 LCM by listing multiples
1.    List the multiples of each number until at least one of the multiples appears on all lists.
2.    Find the smallest number that is on all of the lists.
3.    This number is the LCM.
Example: let us find the LCM of 4 and 5.
Multiples of 4 are : 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, ... and the multiples of 5 are: 5, 10, 15, 20, 25, 30, 35, 40, ...
Hence, by the definition of LCM, the LCM of 4 and 5 is 20.

C++ program to find LCM.

Here we are going to find the LCM of the given two integers a and b to find the smallest positive integer that is divisible by both a and b.
The user is asked to enter two numbers. Read the value to the variables num1 and num2. Find the maximum value between num1 and num2 and store the maximum value in the variable max. here we are using the ternary or conditional operator ( ?: ) in C++. 
variable = Expression1 ? Expression2: Expression3
Here, Expression1 is the condition to be evaluated. If the condition(Expression1) is True then Expression2 will be executed and the result will be returned. Otherwise, if the condition(Expression1) is false then Expression3 will be executed and the result will be returned.
So, the comparison can be performed like 
max = ( num1 > num2 ) ? num1 : num2
If num1 is greater than num2. Then max = num1 else max = num2;
Now the variable max contains the largest among the two numbers.
Divide max with both variables. If max is evenly divisible by both num1 and num2 then LCM = max; else increase the value max by 1. Repeat the process of the division until we get a number that is evenly divisible by both the integers num1 and num2. Print the result. This can be performed by using an if-else statement and do------ while loop.
Syntax of do-----while:
{ body of loop; }
while ( condition );  

Syntax of if------else:
if ( condition )
{ code block if condition is true }
{ code block if condition is false }

our program condition will be like 

if ( max % num1 = 0 & max % num2 = 0)
{ LCM = max }
Increment max by 1
while ( condition is true )
Return 0 }


Step 1:  Call the header file iostream.

Step 2: Use the namespace std.

Step 3: Open the integer type main function; int main().

Step 4: Declare integer variable num1, num2, max;

Step 5: Print a message to enter two numbers.

Step 6: Read the number into the variables num1 and num2.

Step 7: compare num1 and num2. Load the greater value to the variable max;

Step 8: check whether if max is evenly divisible by both num1 and num2. Then print LCM = max

Step 9: else increment max by 1 and go to step 8.

Step 10: Exit;

C++ Source Code

                                          #include <iostream>
using namespace std;

int main()
    int num1, num2, max;

    cout << "Enter two numbers: ";
    cin >> num1 >> num2;
    // maximum value between num1 and num2 is stored in max
    max = (num1 > num2) ? num1 : num2;

        if (max % num1 == 0 && max % num2 == 0)
            cout << "LCM = " << max;
    } while (true);
    return 0;


Enter two numbers: 4
LCM = 12