Write c language compiler by yourself (5)

 

C code

% {

# Define YYSTYPE double

# Include <math. h>

Int yylex (void );

Void yyerror (char const *);

%}

 

% Token NUM

 

%

 

Input:

| Input line

;

 

Line: '\ N'

| Exp '\ n' {printf ("\ t %. 10g \ n", $1 );}

;

 

Exp: NUM {$ = $1 ;}

| Exp '+' {$ = $1 + $2 ;}

| Exp '-' {$ = $1-$2 ;}

| Exp '*' {$ = $1*$2 ;}

| Exp '/' {$ = $1/$2 ;}

/* Unary minus */

| Exp 'n' {$ =-$1 ;}

;

%

# Include <stdio. h>

Int yylex (void)

{

Int c;

/* Skip white space .*/

While (c = getchar () = ''| c = '\ t ')

;

/* Process numbers .*/

If (c = '.' | isdigit (c ))

{

Ungetc (c, stdin );

Scanf ("% lf", & yylval );

Return NUM;

}

/* Return end-of-input .*/

If (c = EOF)

Return 0;

/* Return a single char .*/

Return c;

}

Void yyerror (char const * error)

{

}

Int main ()

{

Return yyparse ();

}

Install Bison2.5 in Ubuntu linux and run:

Bison first. y

Gcc-o first. tab. c

 

Run

./Fisrt

 

1 4 +

5

3 10 *

30

 

OK.