CSCI-1200 Computer Science II — Fall 2008
Lecture 16 – Trees, Part I
Review from Lectures 15
•Maps containing more complicated values.
Example: index mapping words to the text line numbers on which they appear.
•Maps whose keys are class objects.
Example: maintaining student records.
•Summary discussion of when to use maps.
Today’s Lecture
•STL set container class (like STL map without the pairs)
•Binary Trees and Binary Search Trees
•Definition & basic operations
•Implementation of cs2set class using binary search trees
•Note: Lots more tree stuff in CSCI 2300 Data Structures & Algorithms (DSA)!
16.1 Standard Library Sets
•STL sets are ordered containers storing unique “keys”. An ordering relation on the keys, which defaults to
operator<, is necessary. Because STL sets are ordered, they are technically not traditional mathematical sets.
•Sets are like maps except they have only keys, there are no associated values. Like maps, the keys are constant.
This means you can’t change a key while it is in the set. You must remove it, change it, and then reinsert it.
•Access to items in sets is extremely fast! O(log n), just like maps.
•Like other containers, sets have the usual constructors as well as the size member function.
16.2 Set iterators
•Set iterators, similar to map iterators, are bidirectional: they allow you to step forward (++) and backward
(--) through the set. Sets provide begin() and end() iterators to delimit the bounds of the set.
•Set iterators refer to const keys (as opposed to the pairs referred to by map iterators). For example, the
following code outputs all strings in the set words:
for (set<string>::iterator p = words.begin(); p!= words.end(); ++p)
cout << *p << endl;
16.3 Set insert,erase, and find
•There are two different versions of the insert member function. The first version inserts the entry into the
set and returns a pair. The first component of the returned pair refers to the location in the set containing the
entry. The second component is true if the entry wasn’t already in the set and therefore was inserted. It is
false otherwise. The second version also inserts the key if it is not already there. The iterator pos is a “hint”
as to where to put it. This makes the insert faster if the hint is good.
pair<iterator,bool> set<Key>::insert(const Key& entry);
iterator set<Key>::insert(iterator pos, const Key& entry);
•There are three versions of erase. The first erase returns the number of entries removed (either 0 or 1). The
second and third erase functions are just like the corresponding erase functions for maps. Note that the erase
functions do not return iterators. This is different from the vector and list erase functions.
size_type set<Key>::erase(const Key& x);
void set<Key>::erase(iterator p);
void set<Key>::erase(iterator first, iterator last);
•The find function returns the end iterator if the key is not in the set:
const_iterator set<Key>::find(const Key& x) const;
