iterator lower_bound ( const key_type& x );
const_iterator lower_bound ( const key_type& x ) const;

Return iterator to lower bound

Returns an iterator pointing to the first element in the container whose key does not compare less than x (using the container's comparison object), i.e. it is either equal or greater.

Unlike upper_bound, this member function returns an iterator to the element also if it compares equal to x and not only if it compares greater.

Notice that, internally, all the elements in a map container are always ordered by their keys following the criterion defined by its comparison object, therefore all the elements that follow the one returned by this function will have a key that compares greater than x.

Parameters

x

Key value to be compared.
key_type is a member type defined in map containers as an alias of Key, which is the first template parameter and the type of the keys for the elements stored in the container.

Return value

Both iterator and const_iterator are member types. In the map class template, these are bidirectional iterators.
Dereferencing this iterator accesses the element's value, which is of type pair<const Key,T>.

Example

// map::lower_bound/upper_bound
#include <iostream>
#include <map>
using namespace std;

int main ()
{
  map<char,int> mymap;
  map<char,int>::iterator it,itlow,itup;

  mymap['a']=20;
  mymap['b']=40;
  mymap['c']=60;
  mymap['d']=80;
  mymap['e']=100;

  itlow=mymap.lower_bound ('b');  // itlow points to b
  itup=mymap.upper_bound ('d');   // itup points to e (not d!)

  mymap.erase(itlow,itup);        // erases [itlow,itup)

  // print content:
  for ( it=mymap.begin() ; it != mymap.end(); it++ )
    cout << (*it).first << " => " << (*it).second << endl;

  return 0;
}

a => 20
e => 100

Complexity

See also