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STL newbie question:
Regarding functions std::map::upper_bound and std::map::lower_bound is it valid to specify a key that is not actually present in the map?
Example
std::map<int,int> intmap; std::map<int,int>::iterator it1, it2; intmap[1] = 10; intmap[2] = 20; intmap[4] = 40; intmap[5] = 50; it1 = intmap.lower_bound (3); // Is this valid? it2 = intmap.upper_bound (3); // Is this valid? 
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map::upper_bound() function is an inbuilt function in C++ STL, which is defined in header file. upper_bound() returns an iterator to the upper bound of the map container. This function returns an iterator which points to the last element which is considered to go after the key k.
Yes, they are both valid. map::lower_bound returns an iterator pointing to the first element that is not less than key. map::upper_bound returns an iterator pointing to the first element that is greater than key.
map lower_bound() function in C++ STL Parameters: This function accepts a single mandatory parameter key which specifies the element whose lower_bound is to be returned. Return Value: The function returns an iterator pointing to the key in the map container which is equivalent to k passed in the parameter.
Yes, they are both valid.
map::lower_bound returns an iterator pointing to the first element that is not less than key.
map::upper_bound returns an iterator pointing to the first element that is greater than key.
intmap[1]=10; intmap[2]=20; intmap[4]=40; // <<---both lower_bound(3)/upper_bound(3) will points to here intmap[5]=50; lower_bound/upper_bound return the position where value would get inserted.
Note, If you want to check the value key is map or not, you could use std::map::find
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