Chapter 4 Function

1. Chapter 4 Function By C. Shing ITEC Dept Radford University

2. Objectives • Understand how to use library functions • Know how to write user functions • Understand the scope rule • Understand pass by value • Understand how variables and functions are stored

3. Functions • Library functions: • any function ,that returns int, must return -1 (EOF) if not successful e.g. scanf(“%c”, &character) returns -1 if reach end of file Question: Read in characters until the end of file and check whether it is a newline character. Which one is correct? • while(scanf(“%c”, &character) == ‘\n’) • while(scanf(“%c”, &character) >0 && character == ‘\n’) Another way to write it is while(scanf(“%c”, &character) != EOF && character == ‘\n’) • User-defined functions

4. Library Functions • Defined in • stdio.h • printf(): format output • getchar(): macro to read a character from keyboard, buffered • putchar(): macro to print a character to screen, buffered Example: readchar.c

5. Library Functions (Cont.) • Defined in • stdlib.h • rand(): random number between 0 and RAND_MAX • srand(seed): starts from random seed

6. Library Functions (Cont.) • Defined in • stdlib.h (Cont.) • atoi (string_var): convert string variable and return an intege • atof (string_var): convert string variable and return a float • atol(string_var): convert string variable and return a long int • strtod(string_var, endstring): convert string variable and return a double and stores the rest non-double in endstring • strtol(string_var, endstring,0): convert string variable and return a long int and stores the rest non-long int in endstring • strtoul(string_var, endstring,0): convert string variable and return an unsigned long and stores the rest non-long int in endstring

7. Library Functions (Cont.) • Example. atoi (“10”) returns 10 or int j; char c; scanf(“%c”, &c); // type in 9 j=atoi(&c); // j also contains 9 or j= c – ‘0’; // character stored as ASCII code

8. Library Functions (Cont.) • Defined in • assert.h • assert(condition): macro to check whether condition is true if false, print error message • limits.h • UINT_MAX: unsigned int max • INT_MAX: int max Example: limit.c

9. Library Functions (Cont.) • Defined in • ctype.h • isspace(character): macro to check whether character is a white space(‘\t’, ’\n’, ‘ ‘, ‘\r’, ‘\f’, ‘\v’) • isupper(character): macro to check whether character is an upper case • isdigit(character): return 1 if character is a digit • isalpha(character): returns 1 if character is a letter • isalnum(character): returns 1 if character is either a digit or a letter • tolower(character): change character to lower case • iscntrl (character): returns 1 if it is a control character

10. Library Functions (Cont.) • Defined in (Cont.) • math.h • sin(), cos(), tan(), sqrt(), pow(), exp(), log() • fabs(): absolute value of a float • ceil(): smallest integer upper bound • floor(): greatest integer lower bound • pow(x, y): x raised to power y • fmod(x,y): remainder of x/y as float

11. Library Functions (Cont.) • time.h • time(NULL): number of seconds since Jan 1, 1970 Example: dice.c

12. User Defined Function • Need function prototype (function header) • Return_typefunction_name (formal parameter_types); • Declare before function definition, (i.e. header and body) • put after #include lines and before main function

13. User Defined Function (Cont.) • Function definition • Header: Return_type function_name (formal parameter list) • Body or block: { //declaration_of_variables_not_in_formal_parameter_list … // function call // returned function value is stored under function name variable=called_function_name (actual parameters); … // needed if any expression to be returned return expression; }

14. Example: • Skip blank • Skip any character

15. Scope Rule • The variable is meaningful and unique in its defined block • The local variable redefined scope precedes the global variable if use the same name Example: scope.c

16. Scope Rule (Cont.) • Answer: Before 1st nested block: x= 1, y= 2, z=3 1st nested block: x= 4, y= 5, z=6 After 1st nested block: x= 1, y= 2, z=3 Nest in 2nd nested block: x= 8, y= 7, z=8 2nd nested block: x= 8, y= 7, z=7 After 2nd nested block: x= 8, y= 2, z=7 • Can you try this one? scope2.c

17. Pass By Value • A copy of function actual parameters are passed to formal parameters in the called function according to parameter positions Example: byVal.c

18. Storage Class • Represent how to store variables and functions in program • Global variable: permanent storage • Local variable: temporary storage • 4 classes • auto • Static: used in the same file • register • Extern: used in different file

19. Storage Class - auto • Local variable: allocate memory when declared, destroy when exit the scope of the variable • Default when declare a variable • Store in stack space Example: inti; // same as //auto inti;

20. Storage Class - static • Retain value when exit the scope of the variable (permanent storage) • Automatically initialized once when declared • Used as a global variable and a default • Visible in the same file only • Declare as static type variable[=initial value]; The initial value only initialize the variable once when called

21. Storage Class – static (Cont.) • Within a function Example: dice_static.c

22. Storage Class – static (Cont.) • Between functions within the same file: global to functions after variable declaration, Example: dice_static_global.c

23. Example • Note: the static storage class must occur in the same file • Count # of skipped blanks

24. Storage Class - register • Declare and use positions must be as close as possible • Use fast register to store loop control variable if available to improve execution speed • If no register is available, use auto class for the variable Example: dice_register.c

25. Storage Class - extern • The external variable defined are global from the point of declaration • Automatically initialized when declared • Visible in different files • Declared as extern type variable; Example: dice.extern1.c, dice_extern2.c

26. Example • Note: the static storage class must occur in the same file Count # of skipped blanks: use 2 separate files Example 1: count1.c, count2.c Example 2: countskipblank1.c, countskipblank2.c

27. Side Effect • Every variable used in a function must be defined either in formal parameters or locally Example: side.c

28. Recursion • C supports recursion Example: recursion.c

29. Programming Process • Requirements: what is wanted • Specification: program description (iterative process) • Program Design: algorithm, module definition, file format, data structure • Coding • Testing: write test plan, test • Debugging • Release • Maintenance: fix bugs • Revision and Updating

30. Program Design • Top-Down Design: (Most Common) treat the whole problem as a big problem (main function), then use step-wise refinement to divide the problem into smaller problems (smaller functions) until the smaller problems become simple enough to be solved (use function with one task). The structure design is a tree.

31. Program Design • Bottom-Up Design: starts from the bottom of the design tree (the smallest function) and work up the tree until to the root.

32. Top-Down Design Example • Example: write a program to convert a series of Fahrenheit degrees into Celsius degree (use sentinel -1 to stop) The Output will look like Fahrenheit Celsius _________ ______ 212.00 100.00 Fahrenheit Celsius _________ ______ 32.00 0.00

33. Top-Down Design Example (Cont.) • First, write a structure chart (design tree): usually divide the problem into 3 parts in the next level: InputData ProcessData OutputResult

34. Top-Down Design Example (Cont.)

35. Top-Down Design Example (Cont.) • We can divide OutputResult into 2 parts: PrintHeading DisplayResult

36. Top-Down Design Example (Cont.)

37. Top-Down Design Example (Cont.) • Functions: 1st level: main(void) 2nd level: InputData: this function reads data from keyboard and outputs fahrenheit degree precondition: nothing postcondition: returns fahrenheit (double) ProcessData: this function converts the fahrenheit to celsius degree precondition: inputs fahrenheit (double) from caller postcondition: stores celsius (double) PrintResult: this function prints out fahrenheit and celsius degrees in screen precondition: inputs fahrenheit (double), celsius (double) from caller postcondition: no effects on farenheit or celsius degrees

38. Top-Down Design Example (Cont.) • Functions: (Cont.) 3rd level: PrintHeader: this function prints header and __ in screen precondition: nothing postcondition: nothing DisplayResult: this function displays fahrenheit and celsius in screen precondition: fahrenheit (double), celsius (double) postcondition : nothing

39. Top-Down Design Example (Cont.) • The program will look like this: #include <stdio.h> #include <stdlib.h> // for using exit // list function prototypes double inputData (void); double ProcessData (double fahrenheit); void OutputResult (double fahrenheit, double celsius); void PrintHeader (void); void DisplayResult (double fahrenheit, double celsius);

40. Top-Down Design Example (Cont.) • The program (Cont.): int main (void) { const int SENTINEL=-1; double fahrenheit, celsius; while ((fahrenheit = InputData ()) != SENTINEL) { celsius = ProcessData (fahrenheit); OutputResult (fahrenheit, celsius); } return 0; }

41. Top-Down Design Example (Cont.) • The program (Cont.): double InputData (void) { double fahrenheit; if (scanf(“%lf”, &fahrenheit) != EOF) return fehrenheit; else exit(-1); } double ProcessData (double fahrenheit) { const double FACTOR=5.0/9.0; return FACTOR*(fahrenheit-32); } void OutputResult (double fahrenheit, double celsius) { PrintHeader(); DisplayResult (fahrenheit, celsius); }

42. Top-Down Design Example (Cont.) • The program (Cont.): void PrintHeader (void) { printf(“Fahrenheit\tCelsius\n”); } void DisplayResult (double fahrenheit, double celsius) { printf(“%10.2lf\t%10.2lf\n”, fahrenheit, celsius); }

43. Top-Down Design Example (Cont.) • Example: Hint on HW #2

44. References • Herbert Schildt: C: The Complete Reference, 4th ed, Osborne Publishing • Deitel & Deitel: C How to Program, 4th ed., Chapter 5 & 8, Prentice Hall