对结构向量进行二进制搜索

Question

I have a vector of structs that contains struct with this architecture

struct Main{

   int mainID;
   string mainDIV;
   string mainNAME;

}

is it possible to use binary search on struct? I know its easy to use on value using

binary_search( vector.begin() , vector.end() , 5 )

But is there a way how to pass callback or something to actually find attribute of struct? I fail to find anything related to thi topic.

Answer 1

Yes, it's possible. The value that std::binary_search takes is only meaningful when compared to the elements of the container. In the simple case (if Main supports operator< somewhere), you would provide an element of type Main as the value:

// check if this specific Main exists
bool yes = std::binary_search(v.begin(), v.end(), Main{0, "some", "strings"});

// does exactly the same thing as above
bool yes = std::binary_search(v.begin(), v.end(), Main{0, "some", "strings"}
    , std::less<Main>{});

If it doesn't support operator< (or your container is ordered by something else, eg mainID ), then you will have to provide a comparator yourself that the algorithm will use:

// check if there is a Main with mainID 5
bool yes = std::binary_search(v.begin(), v.end(), 5,
    [](const Main& element, const int value) {
        return element.mainID < value;
    });

Answer 2

You have to provide information to binary_search() to tell it how to compare your objects. The two most common ways, are to either add an operator<() to the struct if that is possible, or provide a helper function that can compare two struct s.

The first form would look something like this:

struct Main {
  int mainID ;
  string mainDIV ;
  string mainNAME ;
  bool operator<(const Main & other) const
  {
    return mainID < other.mainID ;
  }
}

This will only compare on on mainID , but you can expand it from there.

Also, this only teaches the compiler how to compare two struct Main , while @Barry's answer above will match an int and a struct Main . But lets keep going with this answer.

Now to find the record for 5 , we have to make it into a struct Main :

struct Main search_key = { 5 } ;
bool yes = std::binary_search( v.begin(), v.end(), search_key ) ;

Now, this isn't very elegant, and besides if you have a constructor for struct Main ( and haven't put it in your example ), this won't even work. So we add another constructor just for int .

struct Main
{
    Main(int id, const string & a_div, const string & a_name ) : id(id), div(a_div), name(a_name) { }
    Main(int id) : id(id) { }
    int id ;
    string div, name ;

    bool operator<(const Main &o) const { return id < o.id ; }
} ;

Now we can do a slightly shorter form:

bool has_3 = std::binary_search( v.begin(), v.end(), Main( 3) ) ;

Historical note: Bjarne has been trying for quite some time to get default comparison operators into the standard, but not everyone was excited about it at the standards meetings. I though there was some progress on it at the last meeting and so it may eventually appear when C++17 is a thing.