C++: Counting number of equality comparison in hash map

Question

I have an unordered map with user-defined hash and equality functions.

I would like to count the number of times equality comparison function is called after adding all the elements to the map. Is there an easy way to do this?

BigBoss · Accepted Answer · 2012-10-29 00:42:16Z

2

In your custom equality function count them:

struct equality_comparer : std::binary_function<MyType, MyType, bool> {
    static int counter_;

    bool operator()( MyType const& lhs, MyType const& rhs ) {
        ++counter_;
        return lhs == rhs;
    }
};
int equality_comparer::counter_ = 0;

And then after insertion to the map complete: equality_comparer::counter_ = 0.

As mentioned by @PiotrNycz you can use this:

struct equality_comparer : std::binary_function<MyType, MyType, bool> {
    mutable int counter_;
  //^^^^^^^
    equality_comparer() : counter_(0) {}
    bool operator()( MyType const& lhs, MyType const& rhs ) {
        ++counter_;
        return lhs == rhs;
    }
    void reset_counter() {counter_ = 0;}
};

Then you can have myMap.key_eq().reset_counter() instead of equality_comparer::counter_ = 0 in previous code and myMap.key_eq().counter_ to access counter value.

edited Oct 29, 2012 at 0:42

answered Oct 28, 2012 at 19:04

BigBoss

6,9243 gold badges26 silver badges39 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

PiotrNycz Over a year ago

static is not needed. And it gives wrong result if OP uses more than one map. Results from non static counter can be get by: someMap.key_eq().counter

BigBoss Over a year ago

@PiotrNycz +1 You are certainly right about this, that was first thing that I think about it, and then I want to edit my answer but I think its already work for a test.

PiotrNycz Over a year ago

@BigBoos you can always add this as an alternative (or just better proposition) for next readers. The answers are read not only by OPs.

user1781302 Over a year ago

Thanks, but now I'm getting an error because bool operator() is no longer const. Is there a way around this?

Collectives™ on Stack Overflow

C++: Counting number of equality comparison in hash map

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related