# Inbuilt function for calculating LCM in C++

Many times while we do programming, we need to calculate the Least Common Multiple (LCM) between two numbers. We have already discussed how to find LCM in this post.
In place of defining and then using a function for calculating lcm , we can simply use an inbuilt function of boost library of C++ , boost::math::lcm ().

For using this function , we have to declare a header file <boost/math/common_factor.hpp>.

Syntax:

```boost::math::lcm (m,n)
Parameters: m, n
Return Value: 0 if either m or n are zero,
else lcm of mod(m) and mod(n).
```
`// CPP program to illustrate`
`// boost::math::lcm function of C++ `
`#include <iostream>`
`#include <boost/math/common_factor.hpp>`
`using` `namespace` `std;`
`int` `main()`
`{`
`    ``cout << ``"LCM(10,20) = "` `<< boost::math::lcm(10,20) `
`         ``<< endl;`
`    ``return` `0;`
`}`

Output:

```LCM(10,20) = 20
```

Important points:

1. The function will calculate the lcm after taking the modulus of both the numbers, so in case if any of the number being negative, it will be converted to its modulus and then LCM is calculated.
2. In case if any of the number being a non-integer data type , then this function will throw an error.
`// CPP program to illustrate illegal`
`// behaviour of boost::math::lcm function of C++ `
`#include <iostream>`
`#include <boost/math/common_factor.hpp>`
`using` `namespace` `std;`
`int` `main()`
`{`
`    ``cout << ``"LCM(1.0,20) = "` `<< boost::math::lcm(1.0,20) `
`         ``<< endl;`
`    ``return` `0;`
`}`

This code will throw an error, as one of the argument of the function is a double type, so this code will not work.

3. In C++17, a new STL function for calculating LCM of two numbers, std::lcm(), has been introduced, which can be used on any compiler supporting C++17 features.

Disclaimer: This does not belong to TechCodeBit, its an article taken from the below
source and credits.
source and credits: http://www.geeksforgeeks.org
We have built the accelerating growth-oriented website for budding engineers and aspiring job holders of technology companies such as Google, Facebook, and Amazon
If you would like to study our free courses you can join us at

http://www.techcodebit.com. #techcodebit #google #microsoft #facebook #interview portal #jobplacements
#technicalguide

rakesh