Definition     A COMMAND is a program that performs an action other than returing a value.

Definition     An OPERATOR is a program that returns a value.

  Ex:
In the context of a the Circle class.
area () is an operator.
shrink () is a command.  

Ex:
In the context of the IO class...
println (< int >) is a command.
read_int () is an operator.  

Example operator definition.
The operator computes the real average of two integers.  

Definition of average:  

static public double average (int x,  int y)   {   int sum = x + y;   double average = sum / 2.0;   return average;   }

 

Using the operator...
Application:   double a = average (2,  5);  = 3.5  

The execution of average (2,  5)

1. Parameters are ``passed''.   This means bindings are established from the ``formal parameters'' in the definition to the ``actual parameters'' in the applicaiton.
   x - > 2
   y - > 5
   
2. The body of the method is executed in the context of the parametric bindings.
   sum - > 7
   average - > 3.5 (the value returned).
   

Here is what we use it for...  

Problem
Read two real numbers.   Interpret the first of the radius of a circle,  and the second as the side of a square.   Display the average value of the areas of the corresponding circle and square.  

class averageCS   {   static public void main (---)   double r = IO.read_double ();   double r = IO.read_double ();     double acs = averageCS (r,  s);     IO.println ( ``The average area is ``+ acs + ``square inches'');   }     static private double averageCS (double radius,  double side)   {   / /create the circle   Circle c = new Circle (radius);   Square s = new Square (side);     double ave = (c.area () + s.area () / 2.0;   return ave;   }   }

 

The ``block structure'' of this application
  ----------------------------------------
  | averageCS |
  | ------------------------------------ |
  | | main () main | |
  | | | |
  | ------------------------------------ |
  | | average () operator | |
  | | | |
  | ------------------------------------ |
  --------------------------------------- |
 

Notes on the execution....
Suppose 10.0 and 20.0 are numbers which will be read.  

in main ():   r - > 10.0
in main ():   s - > 20.0

``in'' averageCS (---):
radius - > 10.0
side - > 20.0
in averageCS (---):
c - > (circle) of 10.0 radius
s - > (square) of 20.0 side length
ave - > 357.   ?   < - value returned to the main method.
in main ():
acs - > 357.   ?   - the value is displayed  

Example of a command definition:  

Command which takes one integer and displays both the predecessor & the successor.  

~ ~ ~ = stuff
~ ~ ~ void < - indicates no value will be returned  

~ ~ ~ void printPS (int x)   {   IO.println (x-1);   IO.println (x + 1);   }

 

Example Use

~ ~ ~ main (--)   {   printPS (5);   printPS (10);   }

 

Output Stream
4 6 9 11  

Lab was discussed today as well.