Returning the greatest key strictly less than the given key in a C++ Map

Question

Is there a way the C++ STL Maps support this, since lower_bound and upper_bound on maps strictly return the value greater than the passed value.

Lower key

U

Answer 1

I always find to compute floor(x) and ceil(x) to be patricularly useful

floor(x) -> greatest element <=x 
ceil(x) -> smallest element >= x

floor_it = m.lower_bound(x); 
if (floor_it != m.end() && floor_it->first != x)
      --floor_it;

ceil_it = m.upper_bound(x);
if (ceil_it != m.end() && (ceil_it-1)->first == x)
    --ceil_it;