1. A Sorted, Unique Key Container
STL Sets
A Sorted, Unique Key Container
Copyright © Curt Hill

2. General characteristics
Library is <set> or <set.h>
Sorted, thus type must suitable for > or < or provide a compare object
Maintains a total ordering, so you cannot change a key
You must reinsert, that is delete and add
Copyright © Curt Hill

3. Declaration The template has three parameters
Last two have default values
set<T,COMPARE<T>,ALLOCATOR<T> >
Compare is a binary function object that returns a boolean
Allocator will do memory management
Copyright © Curt Hill

4. Constructors The default constructor creates the empty set
A two iterator constructor creates a set with everything between the two iterators
These iterators only have to be from a container that allows forward iteration
A copy constructor also exists
Copyright © Curt Hill

5. Operations Assignment Comparisons Not subset at all
Makes as a deep a copy as the contained class supports
Comparisons
Typical unusual STL compare
s1 < s2 only if corresponding members are less
Thus: {0, 1} <={0,2} but not {0,3} <={0,2,3}
Not subset at all
Copyright © Curt Hill

6. Member functions void clear() int size() int max_size()
Deletes all elements
int size()
Set cardinality
int max_size()
Maximum capacity
This is a function of available memory
empty() is true if the empty set
Copyright © Curt Hill

7. Other member functions
iterator find(const TYPE&x)
Returns the iterator or end()
if(st.find(x)!=st.end()) is the same as xst
iterator erase(interator N)
Deletes an element, corresponds to insert
Copyright © Curt Hill

8. Insert insert(const TYPE & n) If successful Otherwise
Inserts n into the set
It returns a pair of items bundled together
If successful
The first is an iterator to the item
The second is Boolean true
Otherwise
The first is the end iterator and false
Pair is a template struct
Used for bundling two things that need to be a function result
Used here and other places (like maps)
Copyright © Curt Hill

9. Other Inserts iterator insert(iterator N, TYPE t)
Insert value t before position N
Notice that N is an iterator and not an integer
The N iterator is a hint
If the hint is accurate the insertion occurs in O(C) time otherwise in O(n) time
Returned iterator points at the new t
iterator insert(iterator first, iterator second)
Inserts a sequence of items
From another or same container class
Copyright © Curt Hill

10. Bounds Two methods that set an iterator Used for finding the closest
iterator lower_bound(const TYPE & x)
Returns an iterator to first element that is lesser or equal than x
Returns the end if none exists
iterator upper_bound(const TYPE & x)
Returns an iterator to first element that is greater or equal than x
Copyright © Curt Hill

11. Algorithms Most of the set algorithms are not unique to the set
These include:
Union
Intersection
Difference
These may require an additional include
<algorithm>
They may be applied to any sorted sequential containers other than set
Copyright © Curt Hill

12. Union
OutputIterator set_union (InputIterator,InputIterator, InputIterator,InputIterator, OutputIterator)
The first two iterators are the beginning and end of the first set, the second pair the second set and the OutputIterator is where they go
The output iterator needs to be in a container suitable for output iterators
Set does not allow outputiterators
Copyright © Curt Hill

13. Example // s1 = s1  s2 // All are vectors vector<int> s3;
insert_iterator<set<int> > sout(s3,s3.begin());
set_union(s1.begin(),s1.end(), s2.begin(),s2.end(), sout);
s1 = s3;
Copyright © Curt Hill

14. Insert Iterators An insert iterator is a form of output iterator
Most iterators we have looked at are for looking at the container class
An output iterator may change the class
However, sets do not allow insert iterators
Copyright © Curt Hill

15. So? The set only allows const_iterators
We store the output in any other STL container class
In our example a vector
Then put into set
This shows the flexibility STL has in dealing with containers of differing organization
Lets look at this one more time
Copyright © Curt Hill

16. Example vector<float> temp(set1.size() + set2.size());
vector<float>::iterator iter;
iter = set_union(set1.begin(), set1.end(),
set2.begin(), set2.end(),
temp.begin());
temp.resize(iter-temp.begin());
res_set = set<float>(temp.begin(), temp.end());
Copyright © Curt Hill

17. Other set operations Intersection Difference (regular)
Same form as the union but name is set_intersection
Difference (regular)
Same form as the union but name is set_difference
The result has all those in A but not B
Difference (irregular)
set_symmetric_difference
Produces the union of both set differences
Copyright © Curt Hill

18. Subsets There is no method for subset
Construct them with union and equality
A  B == (A  B == B && A != B)
A  B == (A  B == B)
This is not pretty using the STL operations
You may be better off iterating both set simulaneously
Copyright © Curt Hill

19. STL Bags or Multisets
Pretty much the same as sets with a few exceptions
Still use the <set> include
Class name is multiset
The insert no longer returns a pair since insertion should always work
Copyright © Curt Hill

20. Bitsets in the STL An unusual STL container class
It is a template class but the template is not a type but the size of the bitstring
Each bitstring is fixed size
Copyright © Curt Hill

21. Constructors
bitset<20> b; Creates a set that can handle zero to 19
Usually the N value should be divisible by 8
bitset<128> b(i); Converts i to a bitstring and initializes b with it
Copyright © Curt Hill

22. Operators
The usual operators are allowed =, &=, |=, ^=, >>=, <<= ==, != >>, <<, |, &, !
Copyright © Curt Hill

23. Methods any() – true if not empty none() – true if empty
count() – cardinality
flip() – reverse every bit
reset() – sets all bits to zero
reset(int) – set just this position to zero
Copyright © Curt Hill

24. More methods set() – sets all bits to one
set(int) – sets just one bit to one
size() – returns the template parameter
to_string() – returns a string that has a 0 or 1 for each position
There is a contructor that allows this to be constructed back into a set
test(int) – returns true if this position is one
Copyright © Curt Hill

25. Finally
The STL Set is a container class that maintains its unique items in sorted order
It is easy to insert to and query
The algorithms set_union, set_intersection, set_difference may be applied to any sorted container class
They are very general, but not near as easy as they should be
Copyright © Curt Hill