chapter 9. Functions

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민기복

Ki-Bok Min, PhD

서울대학교 에너지자원공학과 조교수

Assistant Professor, Energy Resources Engineering

Fundamentals of Computer System

- chapter 9. Functions

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Last week

Chapter 7. C control statements: Branching and Jumps

• C control statements: Branching (분기)

– if, else : important! & Easy!

– Switch ~ case

• C control statements: Jumps

– Continue, break, goto

• Logical operators (논리연산자): && ||

• Character I/O functions (문자 입출력함수)

– getchar() and putchar()

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if statement

three general forms

if (expression) statement

if (expression1) statement1

else if (expression2) statement2

else

statement3 if (expression)

statement1

else (expression2) statement2

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if statement

general form (1)

if (expression) statement

• Branching statement (분기문)

• Test/execution is done only once

• while???

value > 0

printf (“양수입니다n”) if

false true

a single statement or a single block with { }

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if statement

general form (2)

if (expression) statement1 else (expression)

statement2

value > 0

true else

printf (“음수입니다.\n”)

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if statement

general form (3)

if (expression1) statement1

else if (expression2) statement2

else

statement3

value > 0

true else if

printf (“음수입니다.\n”) value < 0

true else

printf (“0입니다.\n”)

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if statement

general form (3)

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if statement

general form (3)

if (score < 1000) bonus = 0;

else if (score < 1500) bonus = 1;

else

bonus = 4;

Multiple levels up to 127

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if statement

pairing else with if

if (number > 6)

if (number <12)

printf(“You are close!\n”);

else

printf(“Sorry, you lose a turn!\n”);

• When number is 5???

• Very important to have proper indentations (띄어쓰기)!

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Character I/O functions ( 문자입출력함수 ) getchar() & putchar() - 단일문자

• getchar()

– returns the next character from input (입력으로부터 다음 문자를 리턴)

– No argument

– ch = getchar();  scanf(“%c, &ch);

• putchar()

– prints its argument.

– putchar(ch);  printf(“%c”, ch);

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Logical operator

&& || !

Operator Meaning

&& And

|| Or

! Not

X Y X && Y X || Y X !

T T T T F

T F F T F

F T F T T

F F F F T

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Logical operator

order of evaluation and range

if (range >= 90 && range <= 100 ) printf(“A\n”);

if (90<= range <= 100 ) printf(“A\n”);

Semantic error!

(90<= range) <= 100  always true Don’t do this!

0 or 1

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Conditional operator ( 조건연산자 )

?:

• ?: the only tertiary operator (삼항연산자)

• General form

Expression1 ? Expression2 : Expression 3 ex) max = (a > b) ? a : b;

x = (y < 0) ? –y : y;

If (y<0) x = -y;

elsex = y;



true false

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Loop Aids: Continue and break

• 처리의 순서나 흐름을 조절하는 제어문

• continue: 해당루프의 나머지를 건너뛰고, 해당

루프의 다음 사이클을 시작.

• break: 어떤 처리의 순서나 흐름을 중단하고, 루프를

탈출.

• 중첩구조에서는 내부구조만 영향을 받음.

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Continue and break

while (( ch = getchar() ) ! EOF) {

blabla(ch);

if (ch == ‘\n’) break;

yakyak(ch);

}

blunder(n,m);

while (( ch = getchar() ) ! EOF) {

blabla(ch);

if (ch == ‘\n’)

continue yakyak(ch);

}

blunder(n,m);

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Multiple choices: switch ~ case

switch (integer expression) {

case constant1:

statement1;

case constant2:

statement2;

default:

statements;

}

• expression & case label : integer (including char)

• No case label matches expression: default

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Multiple choices: switch ~ case

Switch~case with break

Switch~case without break

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Multiple choices: switch ~ case

• Ex)

Switch (choice) {

case 1:

case 2: printf(“파이팅!\n”); break case 3: printf(“잘했다!\n”)

case 4: printf(“멋지다!\n”); break default: printf(“잘 지내라.\n”);

}

choice = 1, 2, 3, 4, 5?

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goto 조심해서 쓸 것

goto label;

..

label: statement

while (funct > 0) {

for (I = 1; I <=100; i++) {

for (j = 1; j <=50; j++) {

… if(xxx)

goto help;

… }

… } ..

}…

help: xxx

변수이름 규칙과 동일

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Today

Chapter 9. Functions ( 함수 )

• Functions and how to define them

• Arguments and return values (전달인자와 리턴값)

• Function types (함수의 데이터형)

• Declaring a function (함수 선언)

• Recursion (재귀)

• Pointers : a first look

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Functions What is it?

• Functions: a self-contained unit of program code designed to accomplish a particular task.

• 함수: 하나의 특정 작업을 수행하도록 독립적으로 설계된 프로그램 코드의 한 단위

– Ex) printf(): causes data to be printed on your screen – Strlen(): tells a program how long a certain string is.

• Why do we need it?

– Only one function can be used many times for repeated work.

– Imagine we write a program for printf() each time you need it. – terrible!

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Functions Why use it?

• Suppose you want to write a program that does the following.

– Read in a list of numbers – Sort the numbers

– Find their average – Print a bar graph

• Functions helps you concentrate on the program’s overall design

• Using functions is more important than making them. ex) printf()

#include <stdio.h>

#define SIZE 50 Int main(void) {

Float list (SIZE);

Readlist(list, SIZE);

Sort(list, SIZE);

Average(list, SIZE);

Bargraph(list, SIZE);

Return 0;

}

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Analyzing the program

Function prototype (함수 프로토타입):

tells the compiler what sort of function starbar() is

Function call (함수호출):

causes the function to be executed

Function definition (함수정의):

specifies exactly what the function does

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Functions Declaration

• Any program that uses a function should declare the type before it is used – 함수의 데이터형 미리 선언.

void starbar(void);

– This line announces that the program uses a type void function called starbar() & compiler expect to find the definition for this function elsewhere. May put this inside main()

- Function type

- void indicate that there is no return value

-Default is ‘integer’

- indicates that there is no argument (전달인자)

- declaration use ; - definition don’t use ;

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Functions

Calling a function

• Calls the function starbar() from main() by using its name followed by a semicolon

• Starbar();

• When finished with the code within starbar(), the computer returns to the next line of the calling function – main()

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Functions Defining

• Defining starbar() is very similar to defining main()

• Uses { }, declare variables

• starbar(void) is not followed by ; - tell the compiler that you are defining.

• The variable count is a local variable. count can be used in main() without conflict.

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Functions

• The program includes starbar() and main() in the same file.

• You may use two separate files.

• no return value  no communication with calling function

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Function

Function argument ( 전달인자 )

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Function

Function argument ( 전달인자Argument () 전달인자)

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Functions

Argument ( 전달인자 )

• ch & num: argument (전달인자), formal argument

(형식전달인자) or formal parameter (형식매개변수)

• ch: char type, num: int

void show_n_char(ch, num);

char ch int num

Valid (but old style)

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Functions

Arguments ( 전달인자 )

• SPACE, 8: actual argument (실전달인자)

• Formal parameter (형식매개변수): called functioin – show_n_char()

• Actual argument (실질전달인자): calling function – main()

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Functions

Arguments ( 전달인자 ) & return value

• Argument send information to functions.

Calling function

ex) main() Function to be called

argument Return value

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Functions

Local and global variable (지역변수와 전역변수)

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Returning a value from a function with return

imin() returns min

imin()함수는 min값을 리턴함.

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Functions

Local and global variable (지역변수와 전역변수)

imin(n,m);

lesser = min;

Above expressions does not work  main() has no idea what min is.

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Functions

return (n<m) ? n : m

If (n < m)

return n;

else return m;

same same

‘return’ terminates the function

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Functions return

• Return value can be assigned to a variable, can be used as part of an expression.

– Ex) Answer = 2 * imin(z,zstar) + 25;

Printf(“%d\n”, imin(-32+answer, LIMIT));

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Function

Function types

• You generally inform the compiler about functions by

declaring them in advance – should come before it is used.

• Function should be declared by type.

• A function with a return value should be declared the same type as the return value.

• If no type is given  default is ‘integer’.

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Functions prototype

• One way to omit a prototype  putting entire definition before the first use

int imax(int a, int b) { return a > b? a : b;}

Int main() { …

z=imax(x,50);

… }

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Recursion

• Recursion: calling itself (함수가 자기자신을 호출)

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Recursion

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Recursion

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Recursion

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Recursion

Recursion fundamentals

• Each level of function call has its own variables.

• Recursive function must contain soethingto halt the recursive calls.

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Recursion

tail recursion

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Compiling programs with two or more source code files

hotel.h hotel.c

- function definitions usehotel.c

- Contains main()

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Compiling programs with two or more source code files

• Place the #define in a header file and use #include in each source code

• Header file contains the defined constants and function prototypes

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Pointers: a first look What is it?

• Pointer: a variable whose value is a memory address

• 포인터는 주소를 값으로 가지는 변수이다.

– Char 형 변수  문자

– int형 변수  정수

– 포인터변수  주소

• People: a name and a value

• Computer: an address (a computer’s version of name) and a value

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Pointers: a first look What is it?

p의 값은 a의 주소이고, a를 가리킨다.

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Pointers: a first look What is it?

• & operator gives you the address for a variable

• pooh = 24;

• pooh: 변수의 이름, &pooh : 변수의 주소(ex. 0B76)

• Printf(“%d %p\n”,pooh,&pooh);

• 24 0B76

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Pointers: a first look

the indirection operator( 간접연산자 )

ex) a = 3;

p = &a; //a를 가리키는 포인터

b = *p; //p이 가리키고 있는주소의

값을 b에 대입

b = a p = &a; 

b = *p;

p가 가리키는 주소의 값

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Pointers: a first look declaring pointers

pointer a;

• Above does not provide sufficient information.

• Various types take different amount of memory and some pointer operations require knowledge of that size.

– Int *a – Char *a – Float *a

• Space between * and the pointer name is optional.

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Pointers: a first look

Why do we need it? ( 왜 필요할까 ?)

How do we alter values in calling function?

– use pointer!

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Pointers: a first look

Why do we need it? ( 왜 필요할까 ?)

Send ‘address’ instead of the values.

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Pointers: a first look

This is why…( 이래서 필요하다 )

• X 와 y의 주소를 전달함으로써 interchange()가 그변수들에 접근할 수 있게 한다.

• 포인터와 간접연산자 *를 사용함으로써 함수는

각각의 주소에 저장되어 있는 값들을 구하고 변경할 수 있게 된다.

• 포인터는 interchange()함수의 변수들이 지역적이라는

사실을 극복해준다. Main()에 손을뻗쳐 저장되어 있는 값을 변경.

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Today

Chapter 9. Functions ( 함수 )

• Functions and how to define them

• Arguments and return values (전달인자와 리턴값)

• Function types (함수의 데이터형)

• Declaring a function (함수 선언)

• Recursion (재귀)

• Pointers : a first look

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Next week

Chapter 10. Arrays and Pointers (배열과 포인터)

• Arrays – initialization, assignment,

• Multidimensional arrays

• Pointers and arrays

• Functions, Arrays, and Pointers

• Pointer operations