C++ Function Example[2] [top]

At line 17,

                       argument
                          |
                          | 
          y    =    sqrt( i );
                    |       |
                    +-------+
                        |
                   function call
                
                    

• A function call is treated as a single expression.
• The value of a function call is the value computed and returned by the function.

    1	#include <iostream>
    2	#include <iomanip>
    3	#include <cmath>
    4	
    5	using namespace std;
    6	
    7	int main()
    8	{
    9	  double y;
   10	
   11	  cout.setf(ios::fixed);
   12	  cout.precision(4);
   13	
   14	  cout << setw(10) << "Number"
   15	       << setw(13) << "Square Root" << endl;
   16	  for(int i = 1; i <= 10; i++) {
   17	    y = sqrt((double) i);
   18	    cout << setw(10) << i
   19		 << setw(13) << y << endl;
   20	  }
   21	
   22	  cout << endl << endl;
   23	  return 0;
   24	}

Predefined Functions[3] [top]

• The C/C++ libraries have many predefined functions you can use in your program.

• You generally need to include one or more header files (with the #include directive) to use these functions.

• The <cmath> header includes the declaration of the sqrt function used in the example and the pow function that computes powers of a value.

• A function must be declared and defined in order to use it.
• The sqrt function is declared in the <cmath> header and it is defined in the library.

User Defined Functions[4] [top]

• You can write your own functions

• The code for a function can serve as both the declaration and the defintion.

• As for variables, if you are to going to call your function, it must be declared before the point that it is called.

• The function definition can occur in a file after the main function that calls it, but a declaration must come before it is called.

Skeleton for Writing a Function[5] [top]

return_type  function_name (  argument_list ) {
   local declarations

   initializations

   processing statements

   return statement
}

• The return statement can occur among the processing statements instead of, or as well as at the end of the {} block which is the function body.
• The return statement should be

  return expression;

  where the expression is of the same type as the declared return_type (or at least can be converted implicitly to that type).

Example: No Function[6] [top]

Here is the example program that computed square roots without using the predefined sqrt function:

    1	int main()
    2	{
    3	
    4	  double x, y;
    5	  cout << "Enter a positive number: ";
    6	  cin >> x;
    7	
    8	  y = ( 1.0 + x ) /2;
    9	
   10	  while( fabs(x - y * y) > 0.0001 ) {
   11	    y = (y + x/y)/2;
   12	  }
   13	  cout.setf(ios::fixed);
   14	  cout.precision(4);
   15	
   16	  cout << "x = " << x << endl;
   17	  cout << "y = " << y << endl;
   18	  cout << "y*y = " << y * y << endl;
   19	
   20	  return 0;
   21	}

Writing a Square Root Function[7] [top]

(In class)

Terms Associated with Calling a Function[8] [top]

• Function arguments
• Function return type
• Function return value

Terms Associated with Writing a Function[9] [top]

• Function declaration (or Function prototype)
• Function definition
• Functin head
• Function parameters
• Function return type
• Return statement

Function Declarations[10] [top]

• Include parameter names and parameter types
• Include only the parameter types.

Function Definitions[11] [top]

• Cannot be in the body of another function;
• So a function definition (other than main) must occur either before or after main, not inside main.
• If a function definition occurs before main, it is simultaneously a definition and a declaration from main's point of view.

Functions that don't return a value[12] [top]

• A function is written to simply print the directions; no computations. What does it return?
• The void type.
• The return statement again.

What are "local" variables?[13] [top]

• What is the scope of variables declared in a function?
• Can a separate function access the variables in main directly?
• Can a separate function modify the variables in main?
• Is this a good thing?