Chapter 9: Vectors and Arrays I

Chapter Goals

• To become familiar with using vectors to collect objects
• To be able to access vector elements and resize vectors
• To be able to pass vectors to functions
• To learn about common vector algorithms

Using Vectors to Collect Data Items

• Example: Suppose you write a program that reads in a list of salary figures and prints out the list, marking the highest value, like this:

  32000
  54000
  67500
  29000
  35000
  80000
  highest value => 115000
  44500
  100000
  65000

• All the values need to be read to find the highest one.
• If you know there are 10 inputs, you could use 10 variables salary1, salary2, ... , salary10. But you would have to write a lot of code 10 times to handle each variable.
• This technique becomes prohibitive as the list gets larger (100 salaries).

• A vector is a collection (a sequence) of data items of the same type.

  vector<double> salaries(10);

• This vector holds 10 successive values of double type.

  

vector<type_name> variable_name;
vector<type_name> variable_name(initial_size);

vector<int> scores;
vector<Employee> staff(20);

• You must specify which slot (element, memory location) you want to use with the [] (subscript, index) operator.

  salaries[4] = 35000;

• The number inside the brackets is called an index.
• Because salaries is a vector of double values, a slot such as salaries[4] can be used just like any variable of type double.

  cout << salaries[4] << "\n";

• In C++, the slots (elements) of vectors are numbered starting at 0.

  

Vector Subscripts

vector_expression[integer_expression]

salaries[i + 1]

• Trying to access a slot that does not exist in the vector is an error.

  vector<double> salaries(10);
  cout << salaries[10]; /* COMMON ERROR! Legal subscripts are 0 until 9 */

• The C++ standard implementation of vector generates no error message.
• If you make an index error, you silently read or overwrite another memory location.
• When a vector is defined without a size parameter, it is empty and can hold no elements.

  vector<double> salaries; /* no size given */
  salaries[0] = 35000; /* ERROR ! */

• You can find the size vector by calling the size()member function.

  for(i = 0; i < salaries.size(); i++)
     do something with salaries[i];

• Using size is actually better than using a constant (so called magic number).
• The function push_back allows you to start out with an empty vector and grow the vector whenever another element is added.

  vector<double> salaries;
  . . . 
  double s;
  cin >> s;
  . . .
  salaries.push_back(s);

• The push_back member function resizes the vector by adding one element to its end.
• If you already know how many elements you need in a vector, you should specify that size when you define it.
• Another member function, pop_back, removes the last element of a vector, shrinking its size by one.

  salaries.pop_back(); /* Now salaries has size 9 */

• The C++ standard (STL) defines many more useful functions for vectors, but we will use only size, push_back and pop_back.

Vector Subscripts (salvect.cpp)

Vector Parameters and Return Values

• Functions and procedures often have vector parameters.
  
• Example: This function computes the average of a vector of floating-point numbers. 

  double average(vector<double> v)
  {  if (v.size() == 0) return 0;
     double sum = 0;
     for (int i = 0; i < v.size(); i++) sum = sum + v[i];
     return sum / v.size();
  }

• average.cpp

• A vector can be passed by value or by reference.
• Pass by reference is used for modifying individual elements of the vector.
• Example: The function raises all values in the vector by the given percentage.

  void raise_by_percent(vector<double>& v, double p)
  {  for (int i = 0; i < v.size(); i++)
             v[i] = v[i] * (1 + p / 100);
  }

• raisesal.cpp

• A function can return a vector.
• Example: Here is a function that collects all values that fall within a certain range.

  vector<double> between(vector<double> v, double low, double high)
  {  vector<double> result;
     for (int i = 0; i < v.size(); i++)
        if (low <= v[i] && v[i] <= high) result.push_back(v[i]);
     return result;
  }

• between.cpp

• Example: Here is a function that collects the positions (indexes) of all matching values in a vector of integers.

  vector<int> find_all_between(vector<double> v, double low, double high)
  {  vector<int> pos;
     for (int i = 0; i < v.size(); i++)
        if (low <= v[i] && v[i] <= high) pos.push_back(i);
     return pos;
  }

Vector Parameters and Return Values (matches.cpp)

Removing and Inserting Elements

• How do you remove an element from a vector?
• If the order is not important, overwrite the element to be removed with the last element of the vector, then shrink the size of the vector.

  void erase(vector<string>& v, int pos)
  {  int last_pos = v.size() - 1;
     v[pos] = v[last_pos];
     v.pop_back();
  }

  

• erase1.cpp
  
• If the order matters, you must move all elements down by one slot (to the index 0), then shrink the size of the vector.

  void erase(vector<string>& v, int pos)
  {  for (int i = pos; i < v.size() - 1; i++) v[i] = v[i + 1];
     v.pop_back();
  }

  

Removing and Inserting Elements (erase2.cpp)

• To insert an element in the middle of a vector, you must add a new element at the end of the vector and move all elements above (to the end of the vector) the insertion location up by one slot.

  void insert(vector<string>& v, int pos, string s)
  {  int last = v.size() - 1;
     v.push_back(v[last]);
     for (int i = last; i > pos; i--) v[i] = v[i - 1];
     v[pos] = s;
  }

• Note that when you insert an element you start at the end of the vector, move that element up, then go to the one before that.

Removing and Inserting Elements (insert.cpp)

Parallel Vectors

• Example: Suppose you want to process a series of product data, and the display the product information, making the best value (with the best price/score ratio).

                 ACMA P600 Price: 995 Score75
                 ALaris Nx686 Price 798 Score 57
                 AMAX Powerstation 600 Price: 999 Score 75
                 AMS Infogold P600 Price: 795 Score: 69
                 AST PRemmia Price: 2080 Score: 80
                 Austin 600 Price: 1499 Score: 95
  best value =>  Blackship NX-600 Price 598 Score: 60
                 Kompac 690 Price: 695 Score: 60

• One possibility is to create three vectors (names, price, scores) of the same length.
  
• These vectors are called parallel vectors because they must be processed together.
• Each slice - names[i], prices[i], scores[i] - contains data that needs to be processed together.

Parallel Vectors (bestval1.cpp)

• Parallel vectors become a headache in larger programs.
  • Each vector must be the same length.
  • Each slice is filled with values that belong together.
  • Any function that operates on a slice must get several vectors as parameters.
• To remove parallel vectors, look at the slice and find the concept it represents. Make the concept into a class.
• Eliminate parallel vectors and replace them with a single vector of objects.

Parallel Vectors (bestval2.cpp)