Functions, Part 2 of 2 Topics Functions That Return a Value Parameter Passing Local Variables Header Files Reading Sections 5.1 - 5.7

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 ; }

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 ;

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)

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.

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.

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 ; value1 value2 ave num1 num2 average 5 8 int int float int int float

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 ; num1 num2 average num1 num2 average 5 8 int int float int int float

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 5 OUTPUT int In addOne: num1 = 6 In main: num1 = 5

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.

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

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 ;

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

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 65535 or 0 to 4294967295 depending on the size of an integer on the system your on num = rand( );

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;

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

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); }

<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) ) ;

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