Comparison tricks in C++

Question

A class:

class foo{
public:
    int data;
};

Now I want to add a method to this class, to do some comparison, to see if its data is equal to on

Answer 1

set is a good option, but if you really want to roll your own, initializer_list is convienient:

bool is_in( int val, initializer_list<int> lst )
{
    for( auto i : lst )
        if( i == val ) return true;
    return false;
}

use is trivial:

is_in( x, { 3, 5, 7 } ) ;

it's O(n) thou, set / unordered is faster

Answer 2

The easy way

The simplest approach is to write a member function wrapper called in() around std::find with a pair of iterators to look for the data in question. I wrote a simple template<class It> in(It first, It last) member function for that

template<class It>
bool in(It first, It last) const
{
    return std::find(first, last, data) != last;
}

If you have no access to the source of foo, you can write a non-member functions of signature template<class T> bool in(foo const&, std::initializer_list<T>) etc., and call it like

in(f, {1, 2, 3 });

The hard way

But let's go completely overboard with that: just add two more public overloads:

• one taking a std::initializer_list parameter that calls the previous one with the begin() and end() iterators of the corresponding initializer list argument.
• one for an arbitrary container as input that will do a little tag dispatching to two more private overloads of a detail_in() helper:
  • one overload doing a SFINAE trick with trailing return type decltype(c.find(data), bool()) that will be removed from the overload set if the container c in question does not have a member function find(), and that returns bool otherwise (this is achieved by abusing the comma operator inside decltype)
  • one fallback overload that simply takes the begin() and end() iterators and delegates to the original in() taking two iterators

Because the tags for the detail_in() helper form an inheritance hierarchy (much like the standard iterator tags), the first overload will match for the associative containers std::set and std::unordered_set and their multi-cousins. All other containers, including C-arrays, std::array, std::vector and std::list, will match the second overload.

#include <algorithm>
#include <array>
#include <initializer_list>
#include <type_traits>
#include <iostream>
#include <set>
#include <unordered_set>
#include <vector>

class foo
{
public:
    int data;

    template<class It>
    bool in(It first, It last) const
    {
        std::cout << "iterator overload: ";
        return std::find(first, last, data) != last;
    }

    template<class T>
    bool in(std::initializer_list<T> il) const
    {
        std::cout << "initializer_list overload: ";
        return in(begin(il), end(il));
    }

    template<class Container>
    bool in(Container const& c) const 
    {
        std::cout << "container overload: ";
        return detail_in(c, associative_container_tag{});    
    }

private:
    struct sequence_container_tag {};
    struct associative_container_tag: sequence_container_tag {};

    template<class AssociativeContainer>
    auto detail_in(AssociativeContainer const& c, associative_container_tag) const 
    -> decltype(c.find(data), bool())
    {
        std::cout << "associative overload: ";
        return c.find(data) != end(c);    
    }

    template<class SequenceContainer> 
    bool detail_in(SequenceContainer const& c, sequence_container_tag) const
    {
        std::cout << "sequence overload: ";
        using std::begin; using std::end;
        return in(begin(c), end(c));    
    }
};

int main()
{
    foo f{1};
    int a1[] = { 1, 2, 3};
    int a2[] = { 2, 3, 4};

    std::cout << f.in({1, 2, 3}) << "\n";
    std::cout << f.in({2, 3, 4}) << "\n";

    std::cout << f.in(std::begin(a1), std::end(a1)) << "\n";
    std::cout << f.in(std::begin(a2), std::end(a2)) << "\n";

    std::cout << f.in(a1) << "\n";
    std::cout << f.in(a2) << "\n";

    std::cout << f.in(std::array<int, 3>{ 1, 2, 3 }) << "\n";
    std::cout << f.in(std::array<int, 3>{ 2, 3, 4 }) << "\n";

    std::cout << f.in(std::vector<int>{ 1, 2, 3 }) << "\n";
    std::cout << f.in(std::vector<int>{ 2, 3, 4 }) << "\n";

    std::cout << f.in(std::set<int>{ 1, 2, 3 }) << "\n";
    std::cout << f.in(std::set<int>{ 2, 3, 4 }) << "\n";

    std::cout << f.in(std::unordered_set<int>{ 1, 2, 3 }) << "\n";
    std::cout << f.in(std::unordered_set<int>{ 2, 3, 4 }) << "\n";    
}

Live Example that -for all possible containers- prints 1 and 0 for both number sets.

The use cases for the std::initializer_list overload are for member-ship testing for small sets of numbers that you write out explicitly in calling code. It has O(N) complexity but avoids any heap allocations.

For anything heavy-duty like membership testing of large sets, you could store the numbers in an associative container like std::set, or its multi_set or unordered_set cousins. This will go to the heap when storing these numbers, but has O(log N) or even O(1) lookup complexity.

But if you happen to have just a sequence container full of numbers around, you can also throw that to the class and it will happily compute membership for you in O(N) time.

Answer 3

There are many ways of doing this with the STL.

If you have an incredibly large number of items and you want to test if your given item is a member of this set, use set or unordered_set. They allow you to check membership in log n and constant time respectively.

If you keep the elements in a sorted array, then binary_search will also test membership in log n time.

For small arrays, a linear search might however preform significantly faster (as there is no branching). A linear search might even do 3-8 comparisons in the time it takes the binary search to 'jump around'. This blog post suggests there to be a break-even point at proximately 64 items, below which a linear search might be faster, though this obviously depends on the STL implementation, compiler optimizations and your architecture's branch prediction.

Answer 4

I would recommend to use standard container like std::vector, but that would still imply a linear complexity with worst-case runtime of O(N).

class Foo{
public:
    int data;
    bool is_equal_to_one_of_these(const std::vector<int>& arguments){
        bool matched = false;
        for(int arg : arguments){ //if you are not using C++11: for(int i = 0; i < arguments.size(); i++){
            if( arg == data ){ //if you are not using C++11: if(arguments[i] == data){
                matched = true;
            }
        }
        return matched;
    }
};

std::vector<int> exampleRange{ {1,2,3,4,5} };
Foo f;
f.data = 3;
std::cout << f.is_equal_to_one_of_these(exampleRange); // prints "true"

Answer 5

It isn't pretty, but this should work:

class foo {
    bool equals(int a) { return a == data; }
    bool equals(int a, int b) { return (a == data) || (b == data); }
    bool equals(int a, int b, int c) {...}     
    bool equals(int a, int b, int c, int d) {...} 
private:
    int data; 
}

And so on. That'll give you the exact syntax you were after. But if you are after the completely variable number of arguments then either the vector, or std::initalizer list might be the way to go:

See: http://en.cppreference.com/w/cpp/utility/initializer_list

This example shows it in action:

#include <assert.h>
#include <initializer_list>

class foo {
public:
        foo(int d) : data(d) {}
        bool equals_one_of(std::initializer_list<int> options) {
                for (auto o: options) {
                        if (o == data) return true;
                }
                return false;
        }
private:
        int data;
};

int main() {
        foo f(10);
        assert(f.equals_one_of({1,3,5,7,8,10,14}));
        assert(!f.equals_one_of({3,6,14}));
        return 0;
}

Answer 6

Does anyone have better idea ? thanks for sharing !

There's a standard algitrithm for that:

using std::vector; // & std::begin && std::end

// if(data is equal to one of these(1,2,3,4,5,6))
/* maybe static const */vector<int> criteria{ 1, 2, 3, 4, 5, 6 };
return end(criteria) != std::find(begin(criteria), end(criteria), data);

Edit: (all in one place):

bool is_equal_to_one_of_these(int data, const std::vector<int>& criteria)
{
    using std::end; using std::begin; using std::find;
    return end(criteria) != find(begin(criteria), end(criteria), data);
}

auto data_matches = is_equal_to_one_of_these(data, {1, 2, 3, 4, 5, 6});

Edit:

I prefer the interface in terms of a vector, instead of an initializer list, because it is more powerful:

std:vector<int> v = make_candidate_values_elsewhere();
auto data_matches = is_equal_to_one_of_these(data, v);

The interface (by using a vector), doesn't restrict you to define the values, where you call is_equal_to_one_of_these.