can I do this binary search using the STL?

问题

I am trying to refactor some code that doesn't use the STL to use the generic algorithms it provide. I have a struct like this :

struct A {
int i;
//other stuff...
};
// ...
A* array; // array of A objects, sorted by A::i member
int n = ...; // array size

there is then a function that was coded that takes A, n and an integer k, whose purpose is to give me pointers to to the first and the last element of the array that have their i member equal to k.

This is implemented by hand in terms of binary search. I was thinking about using std::equal_range. The problem is that it requires an object of type A to work, and it forces me to introduce a "dummy" A object with it's i member equal to k.

Is there a way to do this using the STL, without having to introduce a "dummy" object? thanks

回答1:

Provided your range is sorted according to the value of A::i, this is trivially done with a custom comparator, but note that the comparator must be able compare both ways:

struct AComp
{
    bool operator()(int n, A const & a) const { return n < a.i; }
    bool operator()(A const & a, int n) const { return a.i < n; }
};

auto p = std::equal_range(array, array + n, 5, AComp());

Now the range [p.first, p.second) contains the elements with A::i equal to 5.

The linked page contains more or less exactly this example.

回答2:

You may also use std::binary_search() defined in <algorithm> for binary search generally. The prorotype is:

template <class ForwardIterator, class T>
  bool binary_search (ForwardIterator first, ForwardIterator last,
                      const T& val);

or:

template <class ForwardIterator, class T, class Compare>
  bool binary_search (ForwardIterator first, ForwardIterator last,
                      const T& val, Compare comp);

回答3:

You can define a conversion operator(This is more like a hack, I guess)

class A
{
    private:
        int i;
    public:
        A(int x) : i(x){}
        operator int(){
            return i;
        }
};

If this is done you dont have to define operator< in your struct.

来源：https://stackoverflow.com/questions/18749844/can-i-do-this-binary-search-using-the-stl

标签

c++

stl