Chapter 4 Function

Chapter 4 Function

By C. Shing

ITEC Dept

Radford University

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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

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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?(1) while(scanf(“%c”, &character) == ‘\n’)

(2) while(scanf(“%c”, &character) >0 && character == ‘\n’)

Another way to write it iswhile(scanf(“%c”, &character) != EOF && character == ‘\n’)

User-defined functions

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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

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Library Functions (Cont.) Defined in stdlib.h

rand(): random number between 0 and RAND_MAX

srand(seed): starts from random seed

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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

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Library Functions (Cont.) Example.

atoi (“10”) returns 10or 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

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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

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Library Functions (Cont.) Defined in

ctype.h isspace(character): macro to check whether character is white space

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

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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

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Library Functions (Cont.) time.h

time(NULL): number of seconds since Jan 1, 1970

Example: dice.c

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User Defined Function Need function prototype (function header)

Return_type function_name (formal parameter_types);

Declare before function definition,

(i.e. header and body) put after #include lines and before main function

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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;}

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Example: Skip blank Skip any character

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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

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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

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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

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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

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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:int i; // same as

//auto int i;

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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

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Storage Class – static (Cont.)

1. Within a function

Example: dice_static.c

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Storage Class – static (Cont.)

2. Between functions within the same file:

global to functions after variable declaration,

Example: dice_static_global.c

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Example Note: the static storage class must occur in the

same file Count # of skipped blanks

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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

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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

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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

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Side Effect Every variable used in a function must

be defined either in formal parameters

or locally

Example: side.c

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Recursion C supports recursion

Example: recursion.c

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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

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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 smallerproblems (smaller functions) until the smallerproblems become simple enough to be solved(use function with one task). The structure design is a tree.

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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.

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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_________ ______212.00 100.00Fahrenheit Celsius_________ ______ 32.00 0.00

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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

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Top-Down Design Example (Cont.)

Problem (1st level)

InputData (2nd level) ProcessData (2nd level) OutputResult (2nd level)

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Top-Down Design Example (Cont.) We can divide OutputResult into

2 parts:

PrintHeading

DisplayResult

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Top-Down Design Example (Cont.)

PrintResult (2nd level)

PrintHeader (3rd level) DisplayResult (3rd level)

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Top-Down Design Example (Cont.) Functions:

1st level: main(void) 2nd level:

InputData: this function reads data from keyboard and outputs fahrenheit degree

precondition: nothingpostcondition: 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

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Top-Down Design Example (Cont.) Functions: (Cont.)

3rd level:PrintHeader: this function prints header and __

in screenprecondition: nothingpostcondition: nothing

DisplayResult: this function displays fahrenheit and celsius

in screenprecondition: fahrenheit (double),

celsius (double)postcondition : nothing

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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);

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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;

}

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double InputData (void) {double fahrenheit;

if (scanf(“%lf”, &fahrenheit) != EOF)return fehrenheit;

elseexit(-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);

}

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void PrintHeader (void) {printf(“Fahrenheit\tCelsius\n”);

}void DisplayResult (double fahrenheit,

double celsius) {printf(“%10.2lf\t%10.2lf\n”, fahrenheit, celsius);

}

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Top-Down Design Example (Cont.) Example:

Hint on HW #2

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References Herbert Schildt: C: The Complete Reference,

4th ed, Osborne Publishing Deitel & Deitel: C How to Program, 4th ed.,

Chapter 5 & 8, Prentice Hall

Chapter 4 Function

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