computer science, korea university chapter2 lexical elements, operators,and the c system rim,...

COMPUTER SCIENCE, KOREA UNIVERSITY

Chapter2 Lexical Elements, Operators,and the C System

Rim, Hae-Chang

Department of Computer Science and Engineering

Korea University

Natural Language Processing Lab, Korea University 2

Compilers and Tokens

C is a language An alphabet Syntax: rules for putting together words and punctuations to

make legal programs Compiler

Checks the legality of the source code (program) Translate the source code into object code (object code is

translated into executable code (i.e. target language, machine language)

Collects the characters of the program into tokens Tokens in C (ANSI C)

The basic vocabulary of C keywords, identifiers, constants, string constants, operators,

punctuators


Contents

2.1 Characters and Lexical Elements2.2 Syntax Rule( 구문 규칙 )2.3 Comments( 주석 ), 2.4 keywords2.5 Identifiers( 식별자 ), 2.6 Constants( 상수 )2.7 String Constants( 문자열 상수 )2.8 Operators and Punctuators( 연산자와 구두점 )2.9 Precedence and Associativity of Operators ( 연산자의 우선 순위와 결합 법칙 )2.10 Increment and Decrement Operators ( 증가 연산자와 감소 연산자 )2.11 Assignment Operators ( 배정 연산자 )2.12 An example: Computing Powers of 22.13 The C system


2.1 Characters and Lexical Elements

A programmer construct C program as a sequence of characters

Characters that can be used in a C program Lowercase letters : a b c ... z

Uppercase letters : A B C ... Z

Digits : 0 1 2 3 4 5 6 7 8 9

Other characters : + - * / = ( ) { } [ ] < >

’ ”! @ # $ % ^ & _ | \ ~

. , ; : ?

White space characters : blank, newline, tab

Compiler collects characters into tokens


An Example C Source Code – sum.c

/* Read in two integers and print the sum. */

#include <stdio.h>

int main(void){ int a, b, sum; printf(”Input two integers : ”); scanf(”%d%d”, &a, &b); sum = a + b; printf(”%d + %d = %d\n”, a, b, sum); return 0;}


An Example C Source Code – sum.c

/* Read in two integers and print their sum. */ comments : /* 부터 */ 까지는 공백으로 대치

#include <stdio.h> 전처리 지시자 : the standard header file stdio.h is included stdio.h contains the function prototypes for printf() and scanf()

int main(void)

{

int a, b, sum;

=> The compiler groups these characters into four kinds of tokens: 키워드 : int, void 식별자 : main, a, b, sum 연산자 : ( ) -> tells the compiler that main is a function 구두점 : “{”, “,”, “;”


An Example C Source Code – sum.c printf(“Input two integers: “);

scanf(“%d%d”, &a, &b); printf and scanf are identifier () after them tells the compiler that they are functions

"Input two integers : " String constant : 큰 따옴표로 둘러싸인 문자들

&a, &b & is the address operator

sum = a + b; = and + are operators


2.2 Syntax Rules

BNF(Backus-Naur Form) Can be used to describe a programming language Productions or rewriting rules

Symbols used in productionsitalics non-terminals (syntactic categories)

::= LHS can be rewritten as RHS

| choice

{ }1 choose one item

{ }0+ repeat 0 or more times

{ }1+ repeat 1 or more times

{ }opt optional

others terminal symbols


BNF Examples - 1

Any lowercase or uppercase alphabet or any digitdigit ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

lowercase_letter ::= a | b | c | ... | z

uppercase_letter ::= A | B | C | ... | Z

letter ::= lowercase_letter | uppercase_letter

letter_or_digit ::= letter | digit e.g) “a”, “3”, …

Any sequence of letter or digits

alphanumeric_string ::= {letter | digit}1+

e.g) “3”, “ab777c”, …


BNF Examples - 2

Any sequence of letter or digits that start with a uppercase letter

Any sequence of letter or digits that start with either a uppercase letter or a digit

Any sequence of letter or digits that may end with “.”

u_alpha_string ::= uppercase_letter {letter | digit}0+

p_name ::= {uppercase_letter | digit}1{letter | digit}0+

f_word ::= {letter | digit}1+ {.}opt


2.3 Comments( 주석 )

C style comments Any strings placed between “/*” and “*/” /* here is comments */ /** this is also comments **/ /* what if ”*/” comes in the middle */

C++ style comments // form here to the end of the line


2.4 Keywords( 키워드 )

Reserved words in C (They can not be redefined or used in other contexts)

Additional keywords in Borland C asm, cdecl, far, huge, interrupt, near, pascal

autobreakcasecharconstcontinuedefaultdo

doubleelseenumexternfloatforgotoif

intlongregisterreturnshortsignedsizeofstatic

structswitchtypedefunionunsignedvoidvolatilewhile


2.5 Identifiers( 식별자 )

Identifiers

Names of variables or functions

identifier ::= { letter | _ }1{ letter | digit | _ }0+

e.g.) k, _id, iamanidentifier2, so_am_i

not#me, 101_south, -plus (X)

First 31 characters of identifier are discriminated (ANSI C)

Good programming style: choose meaningful names

Caution: Identifiers that begin with _ can conflict with

system names.


2.6 Constants( 상수 )

Integer e.g.) 0, 17, 017, 0x17

Floating numbers e.g.) 1.0, 3.141592, 3.14E+03

Character constants e.g.) ’a’, ’b’, ’+’, ’\n’

String constants e.g.) ”abc”, ””, ” ”, ”\n”, ”\””, ”\\”


2.8 Operators,Punctuators( 연산자 , 구두점 )

Operators e.g.) +, - , *, /, %

Punctuators e.g.) “(”, “)”, “{”, “}”, “,”, “;”

Exampleint main(void){ int a, b, c = 3; a = 17 * ( b + c ); printf(”%d\n”, a);}


2.9 Precedence and Associativity ( 우선순위와 결합법칙 )

Precedence and associativity The order in which operations are performed e.g.) 1 + 2 * 3 1 + (2 * 3)

1 + 2 – 3 ((1 + 2) – 3)

Precedence and associativity of arithmetic operators

Operator Associativity

() ++(postfix) --(postfix)+(unary) –(unary) ++(prefix) --(prefix)* / %+ -= += -= *= /= etc.

left to rightright to leftleft to rightleft to rightright to left


2.10 Increment and Decrement Operators ( 증가 연산자와 감소 연산자 )

Increment and decrement operators ++i; i = i + 1; i++; --i; i = i – 1; i--; Both operators can be either prefix or postfix

Exerciseint a, b, c=0, d=0;a = ++c;b = d++;printf(”%d %d %d %d\n”, a, b, c--, --d);

a=1, b=0, c=1, d=1 (before printf)

1 0 1 0 is printed a=1, b=0, c=0, d=0 (after printf)


2.11 Assignment Operators ( 배정연산자 )

Assignment operators Change the value of a variable e.g.) a = 1; a = 2 + 3; a = b + c;

a = ( b = 2 ) + ( c = 3 );a = b = c = 0;

More assignment operators +=, -=, *=, /=, %=, <<=, >>=, |=, &=, ^= e.g.) k += 2; k = k + 2;

j *= k + 3; j = j * (k + 3);


Assignment operator( 배정 연산자 )

다른 언어와는 달리 C 는 = 를 연산자로 다룸a = ( b = 2 ) + ( c = 3);

배정 연산자=, +=, -=, *=, /=, %=, >>=, <<=, &=, ^=, |=

( 주의 ) j *= k + 3 은 j = j * k + 3 이 아니라 , j = j * (k + 3) 임선언 및 초기화int i ＝ 1, j ＝ 2, k ＝ 3, m ＝ 4;수식 동일한 수식 동일한 수식 결과i ＋＝ j ＋ k i ＋＝ (j ＋ k) i ＝ (i ＋ (j ＋ k)) 6 j ＊＝ k ＝ m ＋ 5 j ＊＝ (k ＝ (m ＋ 5)) j ＝ (j ＊ (k ＝ (m ＋ 5))) 18


2.12 Example – Computing Powers of 2

/* Some powers of 2 are printed. */

#include <stdio.h>

int main(void)

{

int i = 0, power = 1;

while (++i <= 10)

printf("%-6d", power *= 2);

printf("\n");

return 0;

}

2 4 8 16 32 64 128 256 512 1024


2.13 The C system Preprocessor and Standard Library

C 시스템 C 언어 , 전처리기 , 컴파일러 , 라이브러리 , 편집기 등으로

구성

전처리기 # 으로 시작하는 행을 전처리 지시자라고 함

#include <filename>#include "filename"#define PI 3.141592

표준 라이브러리 프로그램에 유용한 함수들로 C 시스템이 제공함 printf(), scanf(), 등 사용자가 알아서 해당 헤더파일을 포함시켜야함


Practice – prn_rand.c#include <stdio.h>#include <stdlib.h>

int main(void){ int i, n;

printf("\n%s\n%s", "Random integers will be printed.", "How many do you want to see?"); scanf("%d", &n); for (i = 0; i < n; ++i) { if (i % 10 == 0) putchar('\n'); printf("%7d", rand()); } printf("\n\n"); return 0;}

computer science, korea university chapter2 lexical elements, operators,and the c system rim,...

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