It’s a very useful function to find elements in map.

iterator lower_bound (const key_type& k);
Returns an iterator pointing to the first element in the container whose key is not considered to go before k (i.e., either it is equivalent or goes after).

iterator upper_bound (const key_type& k);
Returns an iterator pointing to the first element in the container whose key is considered to go after k.

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#include <stdio.h>
#include <stdlib.h>
#include <map>
#include <iostream>
using namespace std;

int main(){
	std::map<char,int> mymap;  
	std::map<char,int>::iterator itlow,itup;   
	  
	mymap['a']=0;
	mymap['b']=0;  
	mymap['d']=0;  
	mymap['t']=0;
	mymap['x']=0;

	itlow=mymap.lower_bound ('c');  
	// find the first key element that is at least greater(>=) than 'c'
	// in this case, it's 'd'
  	
	itlow=mymap.lower_bound ('b');  
	// find the first key element that is at least greater(>=) than 'c'
	// in this case, it's 'd'

	itup=mymap.upper_bound ('d');   
	// find the first element that is definite greater than 'd'
	// point to something > 'd' and not including 'd', so it's 't'

	cout << itlow->first << " " << itlow->second<<endl;
	cout << itup->first << " " << itup->second<<endl;
}

reference:
https://goo.gl/BkfLdA
https://goo.gl/TXon4t