How do you structure your comparison functions? - c++

Question

I frequently encounter situations, especially with sorting in C++, where I am comparing a series of fields in order to compare a larger structure. A simplified example:
struct Car{
Manufacturer make;
ModelName model;
Year year;
};
bool carLessThanComparator( const Car & car1, const Car & car2 ){
if( car1.make < car2.make ){
return true;
}else if( car1.make == car2.make ){
if( car1.model < car2.model ){
return true;
}else if( car1.model == car2.model ){
if( car1.year < car2.year ){
return true;
}
}
}
return false;
}
My instinctive approach seems cumbersome, especially for more than 3 fields. How would you structure this series of comparisons in C++? Do other languages provide a more succinct or elegant syntax?

Answer 1

Well, if your function hits a return in the if clause, there's no need for an explicit else, since it would have already bailed out. That can save on the "indent valley":
bool carLessThanComparator( const Car & car1, const Car & car2 ) {
if( car1.make < car2.make )
return true;
if ( car1.make != car2.make )
return false;
if( car1.model < car2.model )
return true;
if( car1.model != car2.model )
return false;
if( car1.year < car2.year )
return true;
return false;
}
I like MarkusQ's LISPish short-circuiting approach as well.

Answer 2

If this happens a lot you could put a template like this into a common header:
template<typename T, typename A1, typename A2, typename A3>
bool
do_less_than(
const typename T& t1,
const typename T& t2,
const typename A1 typename T::* a1,
const typename A2 typename T::* a2,
const typename A3 typename T::* a3)
{
if ((t1.*a1) < (t2.*a1)) return true;
if ((t1.*a1) != (t2.*a1)) return false;
if ((t1.*a2) < (t2.*a2)) return true;
if ((t1.*a2) != (t2.*a2)) return false;
return (t1.*a3) < (t2.*a3);
}
Add other templates for different numbers of arguments as required. For each less than function, you can then do something like this:
bool carLessThanComparator(const Car& car1, const Car& car2)
{
return do_less_than(car1, car2, &Car::make, &Car::model, &Car::year);
}

Answer 3

Personally I'd suggest NOT using the != or == operators like we seem to recommend here - this requires the arguments/members to have both less then and equal operators just to do a less then check on a class containing them - using just the less then operator is enought and will save you redundancy and potential defects in the future.
I suggest you write:
bool operator<(const Car &car1, const Car &car2)
{
if(car1.make < car2.make)
return true;
if(car2.make < car1.make)
return false;
if(car1.model < car2.model)
return true;
if(car2.model < car1.model)
return false;
return car1.year < car2.year;
}

Answer 4

I know it's an old question, but for future visitors: the modern C++11 solution is to use std::tie
struct Car{
Manufacturer make;
ModelName model;
Year year;
};
bool operator<(Car const& lhs, Car const& rhs)
{
return std::tie(lhs.make, lhs.model, lhs.year) < std::tie(rhs.make, rhs.model, rhs.year);
}
std::tie converts the struct into a std::tuple so that the above comparison operator delegates to std::tuple::operator<. This in turn does a lexicographical compare with respect to the order in which the members are marshalled into std::tie.
The lexicographic comparison is short-circuited in the same way as in the other solutions to this question. But it is even succinct enough to define on the fly inside a C++ lambda expression. For classes with private data members, it's best defined inside the class as friend function.

Answer 5

bool carLessThanComparator( const Car & car1, const Car & car2 ){
return (
( car1.make < car2.make ) or (( car1.make == car2.make ) and
( car1.model < car2.model ) or (( car1.model == car2.model ) and
( car1.year < car2.year )
)));
-- MarkusQ

Answer 6

Personally, I'd override the ==, <, >, and any other operators needed. That would clean up the code, not in the comparison, but when you need to make the comparison.
For the actual comparison itself, I would write it similarly to what Crashworks said.
bool operator<(const Car &car1, const Car &car2) {
if(car1.make < car2.make)
return true;
if(car1.make != car2.make)
return false;
if(car1.model < car2.model)
return true;
if(car1.model != car2.model)
return false;
return car1.year < car2.year;
}

Answer 7

I was wondering the same thing as the OP and stumbled upon this question. After reading the answers I have been inspired by janm and RnR to write a lexicographicalMemberCompare template function that only uses only operator< on the compared members. It also uses boost::tuple so that you can specify as many members as you want. Here it is:
#include <iostream>
#include <string>
#include <boost/tuple/tuple.hpp>
template <class T, class Cons>
struct LessThan
{
static bool compare(const T& lhs, const T& rhs, const Cons& cons)
{
typedef LessThan<T, typename Cons::tail_type> NextLessThan;
typename Cons::head_type memberPtr = cons.get_head();
return lhs.*memberPtr < rhs.*memberPtr ?
true :
(rhs.*memberPtr < lhs.*memberPtr ?
false :
NextLessThan::compare(lhs, rhs, cons.get_tail()));
}
};
template <class T>
struct LessThan<T, class boost::tuples::null_type>
{
static bool compare(const T& lhs, const T& rhs,
const boost::tuples::null_type& cons)
{
return false;
}
};
template <class T, class Tuple>
bool lexicographicalMemberCompare(const T& lhs, const T& rhs,
const Tuple& tuple)
{
return LessThan<T, typename Tuple::inherited>::compare(lhs, rhs, tuple);
}
struct Car
{
std::string make;
std::string model;
int year;
};
bool carLessThanCompare(const Car& lhs, const Car& rhs)
{
return lexicographicalMemberCompare(lhs, rhs,
boost::tuples::make_tuple(&Car::make, &Car::model, &Car::year));
}
int main()
{
Car car1 = {"Ford", "F150", 2009};
Car car2 = {"Ford", "Escort", 2009};
std::cout << carLessThanCompare(car1, car2) << std::endl;
std::cout << carLessThanCompare(car2, car1) << std::endl;
return 0;
}
Hope this is useful to someone.