Chapter 3 Lists, Stacks, and Queues Reading: Sections 3.1, 3.2, 3.3, 3.4 Abstract Data Types (ADT) Iterators Implementation of Vector

Abstract Data Type (ADT) High-level definition of data types An ADT specifies A collection of data A set of operations on the data or subsets of the data ADT does not specify how the operations should be implemented Examples list, stack, queue, deque, priority queue, table (map), associative array, set, graph, digraph How are these different? Class Class template ADT

List ADT Objects/data Operations A0, A1, A2, … A N-1 Size of the List is N Operations Up to the designer of a List, for example, printList() makeEmpty() Find() Insert() Remove() findKth() etc

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; However, doubly-linked list would need a different form We want a general approach to navigate elements for different implementations of an ADT

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 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 list<string>::iterator itr;

Getting an Iterator Two methods in all STL containers Example: iterator begin ( ) Returns an iterator to the first item in the container iterator end ( ) Returns an iterator representing end marker in the container (i.e. the position after the last item) 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; What about ??? Methods associated with iterators

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 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; Since C++11, we can use auto instead of specifying type auto itr = v.begin();

Container operations requiring iterators Adding element iterator insert(iterator pos, const object &x) 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)

Iterator example Removing every other elements in a list Before C++11 typename Container::iterator itr = lst.begin(); template <typename Container> void removeEveryOtherItem( Container & lst ) { auto itr = lst.begin( ); // C++11 while( itr != lst.end( ) ) itr = lst.erase( itr ); if( itr != lst.end( ) ) ++itr; }

const_iterator The following code is illegal Two versions of begin() and two versions of end() iterator begin() const_iterator begin() iterator end() const_iterator end() template <typename Container> void print (const Container &lst, ostream &out = cout) { typename Container::iterator itr = lst.begin(); while (itr != lst.end()) { out << *itr << endl; *itr = 0; // attempt to change the list itr++; }

const_iterator Note that c.begin() and c.end() functions in the example return const_iterator type. Returns a constant reference for operator* So that a function does not try to modify the elements of a constant container object. template <typename Container> void print( const Container & c, ostream & out = cout ) { if( c.empty( ) ) out << "(empty)"; else auto itr = begin( c ); // a const iterator // auto itr = c.begin(); // typename Container::const_iterator itr = c.begin(); out << "[ " << *itr++; // Print first item while( itr != end( c ) ) // while (itr != c.begin()) out << ", " << *itr++; out << " ]" << endl; }

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.

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 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 Object & back ( ) Returns the object at the end of the vector Object & front ( ) Returns the object at the front of the vector

vector in C++ STL (contd.) More Operations Object & operator[] ( int index ) Returns the object at location index (without bounds checking) Both accessor and mutator versions Object & at ( int index ) Returns the object at location index (with bounds checking) int capacity ( ) Returns the internal capacity of the vector Number of elements the container can hold without further memory allocation void reserve ( int newCapacity) Sets the new capacity of the vector Number of elements that the container can hold void resize( int newSize, const Object& val = Object() ) Change the size of the vector Newly created elements will be initialized to val

Implementing Vector Class Template Implementing Vector as first-class type Can be copied Memory it uses automatically reclaimed Vector maintains A primitive C++ array The array capacity The current number of items stored in the Vector Operations: Copy and move constructor Copy and move assignment operator= Destructor to reclaim primitive array. All the other operators we saw earlier.

Vector Implementation (Part 1) template <typename Object> class Vector { public: explicit Vector( int initSize = 0 ) // constructor : theSize{ initSize }, theCapacity{ initSize + SPARE_CAPACITY } { objects = new Object[ theCapacity ]; } Vector( const Vector & rhs ) // copy constructor : theSize{ rhs.theSize }, theCapacity{ rhs.theCapacity }, objects{ nullptr } objects = new Object[ theCapacity ]; for( int k = 0; k < theSize; ++k ) objects[ k ] = rhs.objects[ k ]; } Vector & operator= ( const Vector & rhs ) // copy assignment operator= Vector copy = rhs; // calling copy constructor std::swap( *this, copy ); return *this;

Vector Implementation (Part 2) ~Vector( ) { delete [ ] objects; } Vector( Vector && rhs ) // move constructor : theSize{ rhs.theSize }, theCapacity{ rhs.theCapacity }, objects{ rhs.objects } { rhs.objects = nullptr; rhs.theSize = 0; rhs.theCapacity = 0; } Vector & operator= ( Vector && rhs ) // move assignment operator= std::swap( theSize, rhs.theSize ); std::swap( theCapacity, rhs.theCapacity ); std::swap( objects, rhs.objects ); return *this;

Vector Implementation (Part 3) bool empty( ) const { return size( ) == 0; } int size( ) const { return theSize; } int capacity( ) const { return theCapacity; } Object & operator[]( int index ) { return objects[ index ]; // no error checking } const Object & operator[]( int index ) const void resize( int newSize ) if( newSize > theCapacity ) reserve( newSize * 2 ); // memory allocation is expensive theSize = newSize;

Vector Implementation (Part 4) void reserve( int newCapacity ) { if( newCapacity < theSize ) return; Object *newArray = new Object[ newCapacity ]; for( int k = 0; k < theSize; ++k ) newArray[ k ] = std::move( objects[ k ] ); theCapacity = newCapacity; std::swap( objects, newArray ); delete [ ] newArray; } void push_back( const Object & x ) // copy x if( theSize == theCapacity ) reserve( 2 * theCapacity + 1 ); // memory allocation is expensive objects[ theSize++ ] = x;

Vector Implementation (Part 5) void push_back( Object && x ) // move x { if( theSize == theCapacity ) reserve( 2 * theCapacity + 1 ); objects[ theSize++ ] = std::move( x ); } void pop_back( ) --theSize; const Object & back ( ) const return objects[ theSize - 1 ]; // Iterator stuff: not bounds checked typedef Object * iterator; // defined as pointer to object, not real nested class typedef const Object * const_iterator;

Vector Implementation (Part 6) iterator begin( ) { return &objects[ 0 ]; } const_iterator begin( ) const iterator end( ) { return &objects[ size( ) ]; } const_iterator end( ) const static const int SPARE_CAPACITY = 16; private: int theSize; // number of actual elements int theCapacity; // number of elements that can be stored without reallocating memory Object * objects; };