increment(++)/decrement(--) operators

• unary operators
• 2 modes:
  • postfix (operator follows operand)
    E.g.,
    AخA

     

    x--;
  • prefix (operator precedes operand)
    E.g.,
     

    --x;
• Example
  ​x

   

  for (int i=0; i<10; ++i)
• increment
  • increase by one
• decrement
  • decrease by one
• Different syntax, same semantics
   

  x = x + 1;
   

  x += 1;
   

  ++x;
   

  x++;
• increment and decrement operators
• Examples
   

  #include <iostream>

  using namespace std;

  ​

  int main()

  {

      for (int i=0; i<10; ++i) {

          cout << i << endl;

      }

  ​

      int x = 5;

      cout << x << endl; // 5

      cout << x++ << endl; // 5

      cout << x << endl; // 6

  ​

      int y = 5;

      cout << y << endl; // 5

      cout << ++y << endl; // 6

      cout << y << endl; // 6

  ​

      return 0;

  }

lvalue and rvalue

• A C++ expression is either an lvalue or an rvalue
• lvalue (locator value)
  • has an identity (i.e., an address that is accessible though code)
  • all variables, even those marked with const; a variable that is not const is called a modifiable lvalue
  • persists after the evaluation of a single expression
• rvalue
  • any expression that is not an lvalue
  • cannot be referred to (does not have an identity)
  • temporary values for intermediate values in a computation are often rvalues
  • does not persist after the evaluation of the expression that uses it
• More in-depth description (will not be on the final exam)
  • http://en.cppreference.com/w/cpp/language/value_category
  • http://www.stroustrup.com/terminology.pdf
•  

  #include <iostream>

  using namespace std;

  ​

  int& return_by_reference(int& arg)

  {

      return arg;

  }

  ​

  int main()

  {

      int x = 1;

  ​

      // 1 = x; // error: lvalue required as left operand of assignment

  ​

      return_by_reference(x) = 2; // valid - lvalue returned from function

      cout << x << endl;

  ​

      const int y = 3; // valid - initialized y

      //y = 4; // error: assignment of read-only (lvalue) variable

  ​

      //cout << &(&y) << endl // error: lvalue required as unary ‘&’ operand

      int a, b;

      ((y < 3) ? a : b) = 7; // valid - conditional evaluates to an lvalue

  ​

      return 0;

  }

More on struct

• Introduce new type into the program
• Why not use arrays?
• Example
   

  #include <iostream>

  #include <string>

  #include <sstream>

  #include <fstream>

  using namespace std;

  ​

  int main()

  {

      ifstream input("students.txt");

      for (string line; getline(input, line);) {

          string name;

          double numeric_grade;

          char letter_grade;

          double height_in_feet;

  ​

          stringstream ss(line);

  ​

          ss >> name;

          ss >> numeric_grade;

          ss >> letter_grade;

          ss >> height_in_feet;

  ​

          cout << name << " " 

              << numeric_grade << " "

              << letter_grade << " "

              << height_in_feet << endl;

      }

  ​

      return 0;

  }

In-Class Modifications to struct Example

•  

  #include <iostream>

  #include <string>

  #include <vector>

  #include <sstream>

  #include <fstream>

  using namespace std;

  ​

  struct student_info {

      string name;

      double numeric_grade;

      char letter_grade;

      double height_in_feet;

  };

  ​

  int main()

  {

      ifstream input("students.txt");

      vector<student_info> all_students;

  ​

      for (string line; getline(input, line);) {

          student_info s;

          stringstream ss(line);

  ​

          ss >> s.name;

          ss >> s.numeric_grade;

          ss >> s.letter_grade;

          ss >> s.height_in_feet;

  ​

          cout << s.name << " " 

              << s.numeric_grade << " "

              << s.letter_grade << " "

              << s.height_in_feet << endl;

  ​

          all_students.push_back(s);

      }

  ​

      return 0;

  }