Reputation: 91
The program below computes a LCM of the 2 numbers, the expected output is 216 with a input of 54 and 24, but I get 57.
Can someone help with the same, and let me know whats wrong with the code snippet below.
/* *********** /
*** LCM ******/
/**************/
template<bool cond, int V1, int V2>
struct IfCond
{
enum
{
value = V1
};
};
template<int V1, int V2>
struct IfCond<0, V1, V2>
{
enum
{
value = V2
};
};
template<int V1, int V2>
struct findMax
{
enum
{
result = V1 > V2,
value = IfCond<result, V1, V2>::value
};
};
template<int V1, int V2, int max>
struct findLCM
{
enum
{
result = findLCM<max % V1, max % V2, max+1>::result
};
};
template<int V2, int max>
struct findLCM<0, V2, max>
{
enum
{
result = findLCM<0, max % V2, max+1>::result
};
};
template<int V1, int max>
struct findLCM<V1, 0, max>
{
enum
{
result = findLCM<max % V1, 0, max+1>::result
};
};
template<int max>
struct findLCM<0, 0, max>
{
enum
{
result = max
};
};
int main()
{
std::cout<< findLCM<54, 24, findMax<54, 24>::value>::result << std::endl;
}
Upvotes: 0
Views: 396
Reputation: 29017
What you need is:
template<int V1, int V2>
struct findGCD
{
enum { result = findGCD<V2, V1%V2>::result };
};
template<int V1>
struct findGCD<V1,0>
{
enum { result = V1 };
};
template<int V1, int V2>
struct findLCM
{
enum { result = (V1/findGCD<V1,V2>::result) * V2 };
};
int main()
{
std::cout<< findGCD<54, 24>::result << std::endl; // 6
std::cout<< findLCM<54, 24>::result << std::endl; // 216
}
If you want to do it by linear search, you would need something like:
template <int V1, int V2, bool finished, int target>
struct findLCMHelper
{
enum { result = findLCMHelper<V1, V2,
target%V1 == 0 && target%V2==0,
target+1>::result };
};
template<int V1, int V2, int target>
struct findLCM<V1, V2, true, target>
{
enum { result = target-1 }; // Correct overshoot
};
template<int V1, int V2>
struct findLCM
{
enum { target = findMax<V1,V2>::value,
result = findLCMHelper<V1, V2,
target%V1 == 0 && target%V2==0,
target+1>::result };
};
I don't like that double test though. There has to be a way to refactor that. I would expect this to break the compiler - it will cause 150+ odd template instantiations.
Amazingly, cpp.sh refuses to break - even with 5400,24
Upvotes: 3
Reputation: 1579
With C++11 you can use constexpr
functions:
constexpr unsigned abs(int const op) {
return op >= 0 ? op : -op;
}
constexpr unsigned gcd(unsigned const a, unsigned const b) {
return b ? gcd(b, a % b) : a;
}
constexpr unsigned lcm(int const a, int const b) {
return abs(a * b) / gcd(abs(a), abs(b));
}
If you want to use templates:
template <int Op>
struct abs {
static constexpr unsigned const value{Op >= 0 ? Op : -Op};
};
template <unsigned A, unsigned B>
static gcd : gcd<B, A % B> {};
template <unsigned A>
static gcd<A, 0> {
static constexpr unsigned const value{A};
};
template <int A, int B>
static lcm {
static constexpr unsigned const value{
abs<A * B>::value / gcd<abs<A>::value, abs<B>::value>::value
};
};
Upvotes: 0
Reputation: 66230
I think you coul'd do something like this
#include <iostream>
template <int X, int Y>
struct findGDMH
{ static int const result = findGDMH<Y, X%Y>::result; };
template <int X>
struct findGDMH<X, 0>
{ static int const result = X; };
template <int X, int Y>
struct findGDM
{
static bool const largerIsX = (X > Y);
static int const A = (largerIsX ? X : Y);
static int const B = (largerIsX ? Y : X);
static int const result = findGDMH<A, B>::result;
};
template <int X>
struct findABS
{ static int const result = (X > 0 ? X : -X); };
template <int X, int Y>
struct findLCM
{ static int const result = findABS<X*Y>::result
/ findGDM<findABS<X>::result, findABS<Y>::result>::result; };
template <int X>
struct findLCM<X, 0>
{ static int const result = 0; };
template <int X>
struct findLCM<0, X>
{ static int const result = 0; };
template <>
struct findLCM<0, 0>
{ static int const result = 0; };
int main()
{
std::cout<< findLCM<54, 24>::result << std::endl;
return 0;
}
Upvotes: 0