1. Functions, Part 2 of 2 Topics Functions That Return a Value
Parameter Passing
Local Variables
Header Files
Reading
Sections

2. Functions Can Return Values
/****************************************************************************
** averageTwo - calculates and returns the average of two numbers
** Inputs: num1 - an integer value
** num2 - an integer value
** Outputs: the floating point average of num1 and num2
*****************************************************************************/
float averageTwo (int num1, int num2) {
float average ; /* average of the two numbers */
average = (num1 + num2) / 2.0 ;
return average ; }

3. Using averageTwo #include <stdio.h>
float averageTwo (int num1, int num2) ;
int main ( ) {
float ave ;
int value1 = 5, value2 = 8 ;
ave = averageTwo (value1, value2) ;
printf (“The average of %d and %d is %f\n”, value1, value2, ave) ;
return 0 ; }
float averageTwo (int num1, int num2)
float average ;
average = (num1 + num2) / 2.0 ;
return average ;

4. Parameter Passing
Actual parameters are the parameters that appear in the function call.
average = averageTwo (value1, value2) ;
Formal parameters are the parameters that appear in the function header.
float averageTwo (int num1, int num2)
Actual and formal parameters are matched by position. Each formal parameter receives the value of its corresponding actual parameter. (I.e.: The variables that are passed to the function are matched with the formal parameters in the order they are passed)

5. Parameter Passing (con’t)
Corresponding actual and formal parameters do not have to have the same name, but they may.
Corresponding actual and formal parameters must be of the same data type, with some exceptions.

6. Local Variables
Functions only “see” (have access to) their own local variables. This includes main( ) .
Formal parameters are declarations of local variables. The values passed are assigned to those variables.
Other local variables can be declared within the function body.

7. Parameter Passing and Local Variables
#include <stdio.h> float averageTwo (int num1, int num2)
float averageTwo (int num1, int num2) ; {
int main ( ) float average ; {
float ave ; average = (num1 + num2) / 2.0 ;
int value1 = 5, value2 = 8 ; return average ; }
ave = averageTwo (value1,
value2) ;
printf (“The average of “) ;
printf (“%d and %d is %f\n”,
value1, value2, ave) ;
return 0 ;
value value ave num num average
int int float int int float

8. Same Name, Still Different Memory Locations
#include <stdio.h> float averageTwo (int num1, int num2)
float averageTwo (int num1, int num2) ; {
int main ( ) float average ; {
float average ; average = (num1 + num2) / 2.0 ;
int num1 = 5, num2 = 8 ; return average ; }
average = averageTwo (num1,
num2) ;
printf (“The average of “) ;
printf (“%d and %d is %f\n”,
num1, num2, average) ;
return 0 ;
num num average num num average
int int float int int float

9. Changes to Local Variables Do NOT Change Other Variables with the Same Name
#include <stdio.h>
void addOne (int number) ; void addOne (int num1) {
int main ( ) num1++ ;
{ printf (“In addOne: “) ;
int num1 = 5 ; printf (“num1 = %d\n”, num1) ;
addOne (num1) ; }
printf (“In main: “) ;
printf (“num1 = %d\n”, num1) ; num1
return 0 ;
} int
num1
OUTPUT
int In addOne: num1 = 6
In main: num1 = 5

10. Header Files
Header files contain function prototypes for all of the functions found in the specified library.
They also contain definitions of constants and data types used in that library.

11. Commonly Used Header Files
Header File Contains Function Prototypes for:
<stdio.h> standard input/output library functions
and information used by them
<math.h> math library functions
<stdlib.h> conversion of numbers to text, text to numbers, memory allocation, random numbers, and other utility functions
<time.h> manipulating the time and date
<ctype.h> functions that test characters for certain properties and that can convert case
<string.h> functions that manipulate character strings
others see Chapter 5 of text

12. Using Header Files #include <stdio.h> #include <stdlib.h>
#include <math.h>
int main ( ) {
float side1, side2, hypotenuse ;
printf(“Enter the lengths of the right triangle sides: “) ;
scanf(“%f%f”, &side1, &side2) ;
if ( (side1 <= 0) || (side2 <= 0) {
exit (1) ; }
hypotenuse = sqrt ( (side1 * side1) + (side2 * side2) ) ;
printf(“The hypotenuse = %f\n”, hypotenuse) ;
return 0 ;

13. Math Library double sqrt (double x); returns the square root of x
double pow (double x, double y)
x raised to the y power
pow (3.0, 2.0) is 9.0
pow (8.0, 1.0 / 3) is 2.0
double sin (double x)
trigonometric sine of x (x in radians)
All math library functions take doubles as arguments and return doubles

14. Common stdlib functions
void exit (int x);
prematurely ends program execution
void srand (unsigned int x);
“seeds” the random number generator with an unsigned integer that is used to start the calculations that generate the pseudo-random number
srand (200);
int rand (void);
returns an unsigned pseudo-random integer in the range of 0 to or 0 to depending on the size of an integer on the system your on
num = rand( );

15. Manipulating what rand() returns
Since rand( ) returns unsigned integers in a large range, we often have to manipulate the return value to suit our purposes
Suppose we want only random numbers in the range from 0 to 5
num = rand ( ) % 6
How about 1 to 6?
num = 1 + rand( ) % 6;
How about 5 to 20?
num = 5 + rand ( ) % 16;

16. srand ( ) and rand ( )
The pseudo-random number generator needs an unsigned int as it’s seed
Although it produces what appear to be random numbers, if we use the same seed, we get the same sequence of random numbers
To get different random numbers each time we run our program, we have to give a different seed each time

17. srand ( ) and rand ( ) #include <stdio.h> Since we are always
#include <stdlib.h> using the value 67 to seed the
#define SEED 67 generator, the same numbers will be produced whenever we
main ( ) run our program. {
int i, num;
srand (SEED);
for (i = 0; i < 5; i++)
num = rand ( );
num = 1 + num % 6;
printf (“%d\n”, num); }

18. <time.h>
One of the most useful functions in the time library is the time( ) function
It returns the time of day as seconds
Since this number is different every time we call it, a common use is as a seed for the random number generator
Each time we run our program, a different sequence of random numbers will be produced
srand (time ( NULL) ) ;

19. Data Types and Conversion Specifiers
Data Type printf scanf
conversion conversion
float %f %f
double %f %lf
long double %Lf %Lf
int %d %d
long int %ld %ld
unsigned int %u %u
unsigned long int %lu %lu
short int %hd %hd
char %c %c