Search code examples
c++multimap

float value used as a key in multimap

If compare between float, I think cannot just use equal ==, need to check if abs(a-b) < epsilon. So when float type value is used as a key, can we use equal_range function?

such as:

std::multimap<float, string>  ds;
ds.insert(make_pair(2.0, string("a")));
ds.insert(make_pair(2.0, string("b")));
ds.insert(make_pair(3.0, string("d")));
ds.equal_range(2.0)
Solution

  • std::multimap::equal_range is not actually calculated using operator== at all. It is calculated using < AND > only. It is actually two iterators, the first being the std::multimap::lower_bound (first element not less than the given key) and the second being the std::multimap::upper_bound (first element greater than the given key).

    So it is quite safe to use with floats and doubles.