数据结构DataStructure ch3_vector

• However, doubly-linked list would need a different form • We want a general approach to navigate elements for different implementations of an ADT
4
Iterators
• A generalized type that helps in navigating any container
– – – – A way to initialize at the front and back of a list A way to move to the next or previous position A way to detect the end of an iteration A way to retrieve the current value
– void push_back ( const Object &x )
• adds x to the end of the vector
– void pop_back ( )
• Removes the object at the end of the vector
3
Iterators: motivation
• Need a way to navigate through the items in a container. • An example: navigating over vector v.
for (int i = 0; i != v.size(); i++ ) cout << v[i] << endl;
•
template <typename Container> void print( const Container & c, ostream & out = cout ) { if( c.empty( ) ) out << "(empty)"; else { Returns a constant reference for auto itr = begin( c ); operator* // auto itr = c.begin(); // typename Container::const_iterator itr = c.begin(); So that a function does not try to out << "[ " << *itr++; // Print first item
• •
Implemented as nested types of containers in STL Examples:
– Iterator type for vector<int> defined as
• vector<int>::iterator itr;
– Iterator type for list<string> defined as
•
Two versions of begin() and two versions of end()
– – – – iterator begin() const_iterator begin() iterator end() const_iterator end()
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const_iterator
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vector in C++ STL
• Collection Elements of some proper type T • Operations
– int size ( ) returns the number of elements in the vector – void clear ( ) removes all elements from the vector – bool empty ( ) returns true if the vector has no elements
– Add x in list before iterator pos – Returns iterator representing position of inserted item
• Removing element
– iterator erase(iterator pos) – Remove element at position pos
– Returns iterator representing position of item following pos
• Removing elements in a range
– iterator erase(iterator start, iterator end)
– Remove elements from start to end (not including end)
2
List ADT
• Objects/data
– A0, A1, A2, … A
– Size of the List is N
N-1
• Operations
– – – – – – – – Up to the designer of a List, for example, printList() makeEmpty() Find() Insert() Remove() findKth() etc
iterator end ( )
•
Example:
for (int i = 0; i != v.size(); i++ ) cout << v[i] << endl; can be written using iterators as for(vector<int>::iterator itr=v.begin(); itr!=v.end(); itr.???) cout << itr.??? << endl;
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The Vector Implementation of List ADT
• Extends the notion of array by storing a sequence of arbitrary objects
– Informally, we call it Vector ADT
• Elements of vector ADT can be accessed by specifying their index.
Note that c.begin() and c.end() functions in the example return const_iterator type.
•
•
modify the elements of a constant container object.
while( itr != end( c ) ) // while (itr != c.begin()) out << ", " << *itr++; out << " ]" << endl; } }
•
Previous example becomes
for(vector<int>::iterator itr=v.begin(); itr!=v.end(); itr++) cout << *itr << endl;
•
Alternatively
vector<int>::iterator itr = v.begin( ); while( itr != v.end( ) ) cout << *itr++ << endl;
•
What about ???
– Methods associated with iterators
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Iterator Methods
• • Iterators have methods Many methods use operator overloading
– – – – itr++ and ++itr advance the iterator to next location *itr return reference to object stored at iterator itr’s location itr1 == itr2 true if itr1 and itr2 refer to the same location, else false itr1 != itr2 true if itr1 and itr2 refer to different locations, else false
• list<string>::iterator itr;
5
Getting an Iterator
• Two methods in all STL containers
–
–
iterator begin ( )
• • Returns an iterator to the first item in the container Returns an iterator representing end marker in the container (i.e. the position after the last item)
Chapter 3 Lists, Stacks, and Queues
• Reading: Sections 3.1, 3.2, 3.3, 3.4