C ++中10 ^ 9中的数字计算

Question

I was trying to find the lcm of two numbers and for one of the input cases (28851539 and 1183019) my program returns a negative value . Apparently it is not able to compute (28851529*1183019)/9 .

#include <iostream>
long long gcd(int a, int b) {
long long int temp;
if(a%b==0)
{
  return b;
}
else
{
 temp=a%b;
 return gcd(b,temp);
}
}
long long lcm(int a, int b , int g) {
//std::cout<<g;
long long int f=(a*b)/g;
return f;
}

int main() {
long long int a, b;
std::cin >> a >> b;
long long int g = gcd(a,b);
long long int q=lcm(a, b, g);
std::cout << q << std::endl;
return 0;
}

How do i compute that accurately ?

Answer 1

You problem is

long long int f=(a*b)/g;

since all of the types in (a*b)/g are int then this will be calculated as an int and if an int is 16 or 32 bits then it will overflow . Do note that since you have signed types this is actually undefined behavior . To get around this you either need to make a , b or g a long long int or you can change the parameters of the function to make them all long long int s.

long long int lcm(long long int a, long long int b , long long int g)

I would also suggest you use an unsigned long long int if you are not dealing with negative numbers. If you are not then you can use the type uint64_t from <cstdint> otherwise int64_t to make the type names shorter.