Chapter 16: An Introduction to Data Structures

Chapter Goals

• To be able to write programs with standard lists and iterators
• To understand the advantages and disadvantages of the list data structure
• To be able to implement linked lists
• To learn about stacks and queues
• To become familiar with common containers and algorithms from the Standard Template Library

Linked Lists

• Suppose you are maintaining a vector of sorted objects (such as employees)
  • Many elements will need to be shifted back if an new object is inserted in the middle.
  • Many elements will need to be shifted forward if an object is deleted from the middle.
• Moving a large number of records can involve a substantial amount of computer time.
• Rather than store a sequence of values in one long block of memory (like a vector or an array) a linked list stores each value in its own memory block, together with the locations of the neighboring blocks in the sequence.

• Inserting an element into a list now requires no shifting, merely reassigning new locations to adjacent objects.

• Removing an element from the list doesn't require shifts either.

• The standard C++ library has an implementation of the linked list structure.
  • First we will learn how to use the standard list.
  • Later we will find out how to implement lists.
• Doubly linked lists (as shown in the illustrations) have links going in both directions.
• Singly linked lists only link elements in one direction.

• Just like the vector, the standard list is a template.
• You can use push_back to add elements to the list.

  list<string> staff;
  
  staff.push_back("Cracker, Carl");
  staff.push_back("Hacker, Harry");
  staff.push_back("Lam, Larry");
  staff.push_back("Sandman, Susan");

• You cannot directly access elements using subscript access (e.g. staff[3]) - the values are not stored in one contiguous block in memory and there is no fast way to access them.
  
• Instead you must start at the beginning of the list, and visit each element in turn using a list iterator. An iterator marks a position in the list.

  list<string>::iterator pos;
  pos = staff.begin();
  

• To move an iterator forward in the list, use the ++ operator.

  pos++;

• To move an iterator backward in the list, use the -- operator.

  pos--;

• You can find the value that is stored in the position with the * operator.

  string value = *pos;

• The value *pos represent the value that is stored in the list.

  *pos = "Van Dyck, Vicki";

• To insert another string before the iterator position, use the insert function.

  staff.insert(pos, "Reindeer, Rudolph");

• Each list has an end position that does not correspond to any value in the list but that points past the list's end.

  pos = staff.end(); /* points past the end of the list */
  staff.insert(pos, "Yaglov, Yvonne");
     /* insert past the end of list */

• The end position does not point to any value, so you cannot look up the value at that position.

  string value = *(staff.end()); /* ERROR */

• Compare to accessing s[10] in a vector with 10 elements.

• The end position is useful for stopping a traversal of the list.

  pos = staff.begin();
  while (pos != staff.end())
  {  cout << *pos << "\n";
     pos++;
  }

• The traversal can be described more concisely with a for loop:

  for (pos = staff.begin(); pos != staff.end(); pos++)
     cout << *pos << "\n";

• Compare this to a traversal of a vector.

  for (i = 0; i < s.size(); i++)
     cout << s[i] << "\n";

• To remove an element from a list, you move an iterator to the position you want to remove, then call the erase function.

  pos = staff.begin();
  pos++;
  staff.erase(pos);

Linked Lists(list1.cpp)

Implementing Linked Lists (The Classes for Lists, Nodes, and Iterators)

• We will start with a linked list of strings.
• A linked list stores each value in a separate object called a node.

  class Node {
  public:
     Node(string s);
  private:
     string data;
     Node* previous;
     Node* next;
  friend class List;
  friend class Iterator;
  };

• The friend declarations indicate the List and Iterator member functions are allowed to inspect and modify the data members of the Node class.
• A class should not grant friendship to another class lightly, because it breaks the privacy protection.

• The list object holds the locations of the first and last nodes in the list.

  class List {
  public:
     List();
     void push_back(string s);
     void insert(Iterator pos, string s);
     Iterator erase(Iterator pos);
     Iterator begin();
     Iterator end();
  private:
     Node* first;
     Node* last;
  };

• When the list is empty the first and last pointers are NULL.
• A list object stores no data; it just knows where to find the node objects that store the list contents.

• An iterator denotes a position in the list.

  class Iterator {
  public:
     Iterator();
     string get() const;     /* use instead of * */
     void next();            /* use instead of ++ */
     void previous();        /* use instead of -- */
     bool equals(Iterator b) const; /* use instead of == */
  private:
     Node* position;
     Node* last;
  friend class List;
  };

• We will enable the operators ++, --, * and == in the next chapter.
• If the iterator points past the end of the list, then the position pointer is NULL.
• The iterator stores a pointer to the last element of the list, so that the previous method can move the iterator back from the past-the-end position to the last element of the list.

Implementing Linked Lists (Implementing Iterators)

• Iterators are created by the begin and end functions of the List class.

  Iterator List::begin() {
     Iterator iter;
     iter.position = first;
     iter.last = last;
     return iter;
  }
  
  Iterator List::end() {
     Iterator iter;
     iter.position = NULL;
     iter.last = last;
     return iter;
  }

• The next function advances the iterator to the next position.

• Note that it is illegal to advance the iterator once it is in the past-the-end position.

  void Iterator::next()
  {  assert(position != NULL)
     position = position->next;
  }

• The previous function is similar.
• When the iterator points to the first element in the list, it is illegal to move it further backward.

  void Iterator::previous()
  {  if (position == NULL) position = last;
     else    position = position->previous;
     assert(position != NULL);
  }

• The get function simply returns the data value of the node to which position points.

  string Iterator::get() const
  {  assert(position != NULL);
     return position->data;
  }

• The equals function compares two position pointers.

  bool Iterator::equals(Iterator b) const
  {  return position == b.position; }

Implementing Linked Lists (Implementing Insertion and Removal)

• For the push_back function, we must first make a new node.

  Node* newnode = new Node(s);

• The node must point back to the old end of the list, while the old end must point to it.

  newnode->previous = last; // (1)
  last->next = newnode;     // (2)
  last = newnode;           // (3) !

• There is a special case when last is NULL (the list is empty).

  if (last == NULL)
  {  first = newnode;
     last = newnode;
  }

• To insert an element in the middle of the list requires manipulating both elements on either side of the new node.

• There is a special case when the list is empty.

  if (iter.position == NULL)
  {  push_back(s);
     return;
  }

• Otherwise we create pointers to track the surrounding nodes.

  Node* after = iter.position;
  Node* before = after->previous;

• We tell the new node where it belongs in the list.

  newnode->previous = before; // (1)
  newnode->next = after;      // (2)

• Then update the surrounding nodes.

  after->previous = newnode;  // (3)
  if (before == NULL) /* insert at beginning */
     first = newnode;
  else
     before->next = newnode;  // (4)

• When know that after is not NULL, but before could be.

• As with insertion, we will need to work with both the element before and after the one to be removed.

Of course we cannot remove a node that is not there.

assert(iter.position != NULL)

We create pointers to track all three nodes we need to work with.

Node* remove = iter.position;
Node* before = remove->previous;
Node* after = remove->next;

We disconnect the node to be removed from the one before it; note the special case when we delete from the front of the list.

if (remove == first) first = after;
else   before->next = after;

We repeat the process with the node after the one to delete.

if (remove == last) last = before;
else   after->previous = before;

Finally we delete the node.

delete remove;

Implementing Linked Lists (list2.cpp)

Stacks and Queues

• Stacks and queues are special data types that allow insertion and remove of items at the ends only, not in the middle.
• A stack lets you insert and remove elements at one end only, traditionally called the top of the stack.
• To visualize a stack, think of a stack of books.
• Since items can only be added or removed from the top of the stack, they are removed in the order that is opposite from the order they were added.
• This is called last in, first out or LIFO order.
• The addition and removal operations are called push and pop.

• The is a stack template in the standard C++ library.

  stack<string> s;
  s.push("Tom");
  s.push("Dick");
  s.push("Harry");
  while (s.size() > 0)
  {  cout << s.top() << "\n";
     s.pop();
  }

• A queue is similar to a stack, except that you add items to one end of the queue (the back) and remove them from the other end of the queue (the front).
• To visualize a queue, think of people lining up.
• Queues store items in a first in, first out or FIFO fashion.
• The standard queue template implements a queue in C++.

  queue<string> q;
  q.push("Tom");
  q.push("Dick");
  q.push("Harry");
  while (q.size() > 0)
  {  cout << q.front() << "\n";
     q.pop();
  }

Stacks and Queues (fifolifo.cpp)

Other Standard Containers

• The standard library contains several other useful containers.
• The set always keeps its elements in order, no matter in which order you insert them.

  set<string> s;
  
  s.insert("Tom");
  s.insert("Dick");
  s.insert("Harry");
  
  set<string>::iterator p;
  for (p = s.begin(); p!= s.end(); p++)
     cout << *p << "\n";

• The above code displays the strings in sorted order: Dick, Harry, Tom.
• The set data structure keeps the values in a special tree shaped structure; each time an element is inserted, the tree is reorganized.

• The C++ set ignore duplicates; if you insert an element into the set that is already present, the insertion has no effect.
• The count function returns the number of times that an element is contained in a set (should always be 1).

  set<string> s;
  
  s.insert("Tom");
  s.insert("Tom");
  cout << s.count("Tom") << "\n"; /* displays 1 */

• If you want to be able to keep track of multiple occurrences of identical values, use a multiset instead.
• The count function works for multiset, with expected results.

  multiset<string> m;
  
  m.insert("Tom");
  m.insert("Tom");
  cout << m.count("Tom") << "\n"; /* displays 2 */

• A map is similar to a vector, but you can use another data type for the indices.

  map<string, double> scores;
  scores["Tom"] = 90;
  scores["Dick"] = 86;
  scores["Harry"] = 100;

Standard Algorithms

• Why iterators?
• It is possible to supply generic functions that can carry out a task with the elements in any container that uses iterators.
• Example: The accumulate function can compute the sum of all elements in a vector or a list.

  vector<double> data;
  /* do something with data */
  
  double vsum = 0;
  accumulate(data.begin(), data.end(), vsum);
  /* now vsum contains the sum of the elements in the vector */
  list<double> salaries;
  /* do something with salaries */
  
  double lsum = 0;
  accumulate(salaries.begin(), salaries.end(), lsum);
  /* now lsum contains the sum of the elements in the list */

• The standard library also supplies a find algorithm.
• The find algorithm returns the second iterator (from the parameter list) if the search fails.

  /* search for a certain name on the staff */
  list<string>::iterator it =
     find(staff.begin(), staff.end(), name);

• The first two parameters can be other iterators for the list (if you don't want to search the entire list).

• The standard library provides a wealth of ready-to-use and fully debugged data structures and algorithms.
  • for_each applies a function to each element
  • find (as above)
  • find_if locates the first element fulfilling a condition
  • count (as above)
  • equal tests if containers have the same elements in the same order
  • replace/replace_if replace all matching elements with a new one
  • unique remove adjacent identical values
  • min_element, max_element finds the smallest and largest elements
  • next_permutation rearranges the elements; call it n! times iterates through all permutations
  • sort sorts the elements; stable_sort performs better if the container is already almost sorted
  • random_shuffle randomly rearranges the elements
  • nth_element find the nth element without sorting the container.
  • plus many more...
• Before writing a lot of code from scratch, check whether the standard library already has what you need.