More Operators

• Initialization of Variables.
  • Often we write code like this

  • int a;  // a does something
    
    a = 3;
           

  • C++ gives us a shortcut for this.

  • int a=3;  // a does something
           

  • This can be done with multiple variables as well.

  • int a,   // a does something
        b,   // b does something
        c;   // c does something
    
    a = 3; 
    b = 7;   
    d = 9;   
           
     becomes:
           
    int a=3,  // a does something
        b=7,  // b does something
        c=9;  // c does something
           
           
     Only to literal values right now.
           
     This helps eliminate initialization problems
           
     I like this.
       

• Increment and Decrement
  • We perform i = i + 1 often
  • Often enough that we give it it's own operation.
  • i++
    • Is the same as i = i + 1;
    • If we use this in an expression, it returns the value of i and then increments i.

    • i = 0;
      j = i++;
      
      is the same as:
      
      i = 0;
      j = i;
      i = i + 1;
                 

    • This is called a postfix operator.
  • ++i
    • Increments i then returns the value.

    • i = 0;
      j = ++i;
      
      is the same as:
      
      i = 0;
      i = i + 1;
      j = i;
                 

    • This is called a prefix operator.
  • What will this code doe?

    int main () {
        int i;
    
        i = 0;
    
        i++;
        cout << "I is " << i << endl;     // 1 
        cout << "I is " << i++ << endl;   // 1
        cout << "I is " << i << endl;     // 2
        cout << "I is " << ++i << endl;   // 3
        cout << "I is " << i << endl;     // 3
    }
           

  • This works for float types as well.
  • you can also use -- to subtract one.
• Compound Statements
  • How do you evaluate:

    10/5*3+8/4 = 2 * 3 + 8/4
               = 6 + 8/4
    	   = 6 + 2
    	   = 8
           

  • My Dear Aunt Sally - {Multiply Divide} {Add Subtract}
  • This is called operator precedence.

  • Precedence level	Operator
    High	unary +,-
    Medium	* , / , %
    Low	+,-

  • Jeff's First Rule: When in doubt, use ().
    • () have the highest precedence.
    • 10/(5*3) + 8/4
• Coercion
  • As long as we stick to the same types there are no problems.
    • int = int operation int operation int ...
    • float = float operation float operation float ...
  • Mixing types becomes a problem.
    • int = float operation int
    • i = 4.8 * 2;
  • The compiler will change the result into the desired type for us.
  • This is called coercion.
  • int to floating point
    • no problem, just add .0
  • floating point to int
    • No problem, just truncate.
    • Problem, information is lost.
  • Example:

    int main () {
    
       int i;
       float x;
    
       i = 4.999;
       x = 8;
    
       cout << "i is "<< i << endl;
       cout << "x is "<< x << endl;
    }
    
    Will compile to produce
    
    [bennett@douc54 three]$ g++ foo.cpp
    foo.cpp: In function `int main ()':
    foo.cpp:8: warning: assignment to `int' from `double'
    foo.cpp:8: warning: argument to `int' from `double'
    
    [bennett@douc54 three]$ a.out
    i is 4
    x is 8
    

  • expressions that contain both ints and floats are called Mixed type or mixed mode expressions.
  • When the compiler encounters mixed mode expressions.
    • Ints are coerced to floats
    • The operation is performed
    • a float is returned.
    • This is coerced if necessary
• Type Casting
  • Coercion removes control from us.
  • Sometimes we need to do the type changes ourselves.
  • type casting allows us to change datatypes ourselves.
  • Two formats
    • (type) value
      • (int) 4.999 // gives us 4
      • This is old C style
    • type (value)
      • int (4.999) // gives us 4 too.
      • This is a C++ thing.
  • A rounding technique is
    • intvar = int (floatvar + 0.5);
    • intvar = int (4.49999 + 0.5) = int (4.99999) = 4
    • intvar = int (4.5000 + 0.5) = int (5.0000) = 5
  • Coercion can lead to errors.
    • Consider the code:

      	   
      int main () {
         int item_count = 5,
             sum = 23 ;
         float average;
      
         average = sum/item_count;
      
         cout << "The average is  "<< average << endl;
      }
      
      Will produce
      
      [bennett@douc54 three]$ a.out
      The average is  4
      
      	   

    • But:

      int main () {
         int item_count = 5,
             sum = 23 ;
         float average;
      
         average = sum/ float(item_count);
      
         cout << "The average is  "<< average << endl;
      }
      
      Will produce
      [bennett@douc54 three]$ a.out
      The average is  4.6