How do I sort efficiently a quadruple structs in C++?

Question

I have a struct with members x,y,z and w. How do I sort efficiently first by x, then by y, by z and finally by w in C++?

Answer 1

If you want to implement a lexicographical ordering, then the simplest way is to use std::tie to implement a less-than or greater-than comparison operator or functor, and then use std::sort on a collection of your structs.

struct Foo
{
  T x, y, z, w;
};

....    
#include <tuple> // for std::tie

bool operator<(const Foo& lhs, const Foo& rhs)
{
  // assumes there is a bool operator< for T
  return std::tie(lhs.x, lhs.y, lhs.z, lhs.w) < std::tie(rhs.x, rhs.y, rhs.z, rhs.w);
}

....

#include <algorithm> // for std::sort

std::vector<Foo> v = ....;
std::sort(v.begin(), v.end());

If there is not a natural ordering for Foo, it might be better to define comparison functors instead of implementing comparison operators. You can then pass these to sort:

bool cmp_1(const Foo& lhs, const Foo& rhs)
{
  return std::tie(lhs.x, lhs.y, lhs.z, lhs.w) < std::tie(rhs.x, rhs.y, rhs.z, rhs.w);
}

std::sort(v.begin(), v.end(), cmp_1);

If you do not have C++11 tuple support, you can implement this using std::tr1::tie (use header <tr1/tuple>) or using boost::tie from the boost.tuple library.

Answer 2

You can turn a struct into a std::tuple using std::tie, and use the lexicographic comparison std::tuple::operator<. Here's an example using a lambda to std::sort

#include <algorithm>
#include <tuple> 
#include <vector>

struct S 
{
   // x, y, z, w can be 4 different types!
   int x, y, z, w;
};

std::vector<S> v;

std::sort(std::begin(v), std::end(v), [](S const& L, S const& R) {
    return std::tie(L.x, L.y, L.z, L.w) < std::tie(R.x, R.y, R.z, R.w);
});

This example supplies std:sort with a comparison operator on-the-fly. If you always want to use lexicographic comparison, you could write a non-member bool operator<(S const&, S const&) that would automatically be selected by std::sort, or by ordered associative containers like std::set and std::map.

Regarding efficiency, from an online reference:

All comparison operators are short-circuited; they do not access tuple elements beyond what is necessary to determine the result of the comparison.

If you have a C++11 environment, prefer std::tie over hand-written solutions given here. They are more error-prone and less readable.

Answer 3

If you roll your own comparison operator, then you can freely throw objects into std::maps or invoke std::sort. This implementation's designed to be simple so you can easily verify and modify it if needed. By only using operator< to compare x, y, z and w, it minimises the number of operators you may need to implement if those variables are not already comparible (e.g. if they're your own structs rather than ints, double, std::strings etc.).

bool operator<(const Q& lhs, const Q& rhs)
{
    if (lhs.x < rhs.x) return true;
    if (rhs.x < lhs.x) return false;
    if (lhs.y < rhs.y) return true;
    if (rhs.y < lhs.y) return false;
    if (lhs.z < rhs.z) return true;
    if (rhs.z < lhs.z) return false;
    if (lhs.w < rhs.w) return true;
    return false;
}

Sometimes types will define a comparison function that returns -1, 0 or 1 to indicate less-than, equal or greater-than, both as a way to support the implementation of <, <=, ==, !=, >= and > and also because sometimes doing a < then a != or > would repeat a lot of work (consider comparing long textual strings where only the last character differs). If x, y, z and w happen to be of such types and have a higher-performance compare function, you can possibly improve your overall performance with:

bool operator<(const Q& lhs, const Q& rhs)
{
    int c;
    return (c = lhs.x.compare(rhs.x) ? c :
           (c = lhs.y.compare(rhs.y) ? c :
           (c = lhs.z.compare(rhs.z) ? c :
           lhs.w < rhs.w;
}

Answer 4

This solution has at most 4 comparisons per element-compare and does not require construction of other objects:

// using function as comp
std::sort (quadrupleVec.begin(), quadrupleVec.end(), [](const Quadruple& a, const Quadruple& b)
{
    if (a.x != b.x) 
        return a.x < b.x;

    if (a.y != b.y)
        return a.y < b.y;

    if (a.z != b.z)
        return a.z < b.z;

    return a.w < b.w;
});