Sunday, November 27, 2011

stl set example

using set in c++

The first example shows how to insert elements into a set and to use iterators to print them


As usual, the include directive

#include <set>

defines all necessary types and operations of sets.

The type of the container is used in several places, so first a shorter type name gets defined:

typedef set<int> IntSet;

This statement defines type IntSet as a set for elements of type  int. This type uses the default sorting criterion, which sorts  the elements by using operator <. This means the elements are sorted in ascending order. To sort in descending order or use a  completely different sorting criterion, you can pass it as a  second template parameter. For example, the following statement defines a set type that sorts the elements in descending order

Note that you have to put a space between the two ">" characters.  >>" would be parsed as shift operator, which would result in a syntax error.

typedef set<int, greater<int> > IntSet;

greater<> is a predefined function object. For a sorting criterion that uses only a part of the data of an object (such as he ID).

All associative containers provide an insert() member function to  nsert a new element:


The new element receives the correct position automatically  according to the sorting criterion. You can't use the push_back() or push_front() functions provided for sequence containers. They make no sense here because you can't specify the position of the  new element.

After all values are inserted in any order, the state of the  container is as shown in Figure below. The elements are sorted  into the internal tree structure of the container so that the  value of the left child of an element is always less (with  respect to the actual sorting criterion) and the value of the  right child of an element is always greater. Duplicates are not  allowed in a set, so the container contains the value 1 only  once.

Figure . A Set that Has Six Elements

To print the elements of the container, you use the same loop as  in the previous list example. An iterator iterates over all  elements and prints them:

IntSet::const_iterator pos;
for (pos = coll.begin(); pos != coll.end(); ++pos) {
   cout << *pos << ' ';

Again, because the iterator is defined by the container, it does  the right thing, even if the internal structure of the container  is more complicated. For example, if the iterator refers to the  third element, operator ++ moves to the fourth element at the  top. After the next call of operator ++ the iterator refers to the fifth element at the bottom (Figure below).

Figure. Iterator pos Iterating over Elements of a Set

The output of the program is as follows:

1 2 3 4 5 6

If you want to use a multiset rather than a set, you need only  change the type of the container (the header file remains the  same):

typedef multiset<int> IntSet;

A multiset allows duplicates, so it would contain two elements  that have value 1. Thus, the output of the program would change  to the following:

1 1 2 3 4 5 6

See Also:

No comments:

Post a Comment