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关于Basic程序解释器及编译原理的简单化(1)---Basic器的语法分析及主要代码

王朝c/c++·作者佚名  2006-01-08
窄屏简体版  字體: |||超大  

这就是Basic解释器的主代码,其中用到上节讲的词法提取get_token()和代数式求值get_exp(int *result)函数.

这一节的代码更简单,就是随心所欲地将得到的token组装.

譬如在get_token后如果token装PRINT,你就调用一次get_token将下一个token答应出来就是了,很简单的,或许你自己也能搞定的.

在下一节你,我会给你完整的C++封装好了的源代码.

/* A tiny BASIC interpreter */

#include <stdio.h>

#include <setjmp.h>

#include <math.h>

#include <ctype.h>

#include <stdlib.h>

#define NUM_LAB 100

#define LAB_LEN 10

#define FOR_NEST 25

#define SUB_NEST 25

#define PROG_SIZE 10000

#define DELIMITER 1

#define VARIABLE 2

#define NUMBER 3

#define COMMAND 4

#define STRING 5

#define QUOTE 6

#define PRINT 1

#define INPUT 2

#define IF 3

#define THEN 4

#define FOR 5

#define NEXT 6

#define TO 7

#define GOTO 8

#define EOL 9

#define FINISHED 10

#define GOSUB 11

#define RETURN 12

#define END 13

char *prog; /* holds expression to be analyzed */

jmp_buf e_buf; /* hold environment for longjmp() */

int variables[26]= { /* 26 user variables,A-Z */

0,0,0,0,0,0,0,0,0,0,

0,0,0,0,0,0,0,0,0,0,

0,0,0,0,0,0

};

struct commands { /* keyword lookup table */

char command[20];

char tok;

} table[] = { /* command must be entered lowercase */

"print",PRINT, /* in this table */

"input",INPUT,

"if",IF,

"then",THEN,

"goto",GOTO,

"for",FOR,

"next",NEXT,

"to",TO,

"gosub",GOSUB,

"return",RETURN,

"end",END,

NULL,END

};

char token[80];

char token_type,tok;

struct label {

char name [LAB_LEN];

char *p; /* point to place to go in source */

};

struct label label_table[NUM_LAB];

char *find_label(),*gpop();

struct for_stack {

int var; /* counter variable */

int target; /* target value */

char *loc;

} fstack[FOR_NEST]; /* stack for FOR/NEXT loop */

struct for_stack fpop();

char *gstack[SUB_NEST]; /* stack for gosub */

int ftos; /* index to top of FOR stack */

int gtos; /* index to top of GOSUB */

void print(),scan_labels(),find_eol(),exec_goto();

void gosub(),greturn(),gpush(),label_init(),fpush();

/* Load a program */

load_program (char *p,char *fname)

{

FILE *fp;

int i=0;

if (!(fp=fopen(fname,"rb"))) return 0;

i=0;

do {

*p = getc(fp);

p++;i++;

} while (!feof(fp)&&i<PROG_SIZE);

*(p-2) = '\0'; /* null terminate the program */

fclose (fp);

return 1;

}

/* assign a variable a value */

assignment()

{

int var,value;

/* getthe variable name */

get_token();

if (!isalpha(*token)) {

serror(4);

return;

}

var = toupper(*token)-'A';

/* get the equals sign */

get_token();

if (*token!='=') {

serror(3);

return;

}

/* get the value to assign to var */

get_exp(&value);

/* assign the value */

variables[var] = value;

}

/* execute a simple version of the BASIC PRINT statement */

void print()

{

int answer;

int len=0,spaces;

char last_delim;

do {

get_token(); /* get next list item */

if (tok==EOL||tok==FINISHED) break;

if (token_type==QUOTE) { /* is string */

printf ("%s",token);

len+=strlen(token);

get_token();

}

else { /* is expression */

putback();

get_exp(&answer);

get_token();

len += printf ("%d",answer);

}

last_delim = *token;

if (*token==',') {

/* compute number of move to next tab */

spaces = 8-(len%8);

len += spaces; /* add in the tabbing position */

while (spaces) {

printf (" ");

spaces--;

}

}

else if (*token==';') {

printf (" ");

}

else if (tok != EOL && tok != FINISHED) serror (0);

} while (*token==';'||*token==',');

if (tok==EOL||tok==FINISHED) {

if (last_delim != ';' && last_delim != ',') printf ("\n");

}

else serror(0); /* error is not, or ; */

}

/* find all labels */

void scan_labels()

{

int addr;

char *temp;

label_init(); /* zero all labels */

temp = prog; /* save poiter to top of program */

/* if the first token in the fike is a label */

get_token();

if (token_type==NUMBER) {

strcpy (label_table[0].name,token);

label_table[0].p=prog;

}

find_eol();

do {

get_token();

if (token_type==NUMBER) {

addr = get_next_label(token);

if (addr==-1||addr==-2) {

(addr==-1) ? serror(5):serror(6);

}

strcpy (label_table[addr].name,token);

label_table[addr].p = prog; /* current point in program */

}

/* if not on a blank line , find next line */

if (tok!=EOL) find_eol();

} while (tok!=FINISHED);

prog = temp; /* restore to original */

}

/* find the start of next line */

void find_eol()

{

while (*prog!='\n'&&*prog!='\0') ++prog;

if (*prog) prog++;

}

/* return index of next free posion in the label array

-1 is returned if the array is full.

-2 is returned when duplicate label is found.

*/

get_next_label(char *s)

{

register int t;

for (t=0;t<NUM_LAB;++t) {

if (label_table[t].name[0]==0) return t;

if (!strcmp(label_table[t].name,s)) return -2; /* dup */

}

return -1;

}

/* find location of given label. A null is returned if

label is not found; ohtherwise a pointer to the position

of the label is returned.

*/

char *find_label(char *s)

{

register int t;

for (t=0;t<NUM_LAB;++t)

if (!strcmp(label_table[t].name,s)) return label_table[t].p;

return '\0'; /* error condition */

}

/* execute a GOTO statement. */

void exec_goto()

{

char *loc;

get_token(); /* get label to go to */

/* find the location of label */

loc = find_label (token);

if (loc=='\0')

serror(7); /* label not defined */

else prog=loc; /* start program running at that time */

}

/* initialize the array that holds the labels.

by convention , a null label name indicates that

array posiiton is unused.

*/

void label_init()

{

register int t;

for (t=0;t<NUM_LAB;++t) label_table[t].name[0]='\0';

}

/* execute an IF statement */

void exec_if()

{

int x,y,cond;

char op;

get_exp(&x); /* get left exapression */

get_token(); /* get the operator */

if (!strcmp("<>",*token)) {

serror(0); /* not a leagal oprator */

return;

}

op = *token;

get_exp(&y); /* get right expression */

/* determine the outcome */

cond = 0;

switch(op) {

case '<':

if (x<y) cond=1;

break;

case '>':

if (x>y) cond=1;

break;

case '==':

if (x==y) cond=1;

break;

}

if (cond) { /* is true so process target of IF */

get_token();

if (tok != THEN) {

serror(8);

return;

} /* else program execution starts on next line */

}

else find_eol(); /* find start of next line */

}

/* execute a FOR loop */

void exec_for()

{

struct for_stack i;

int value;

get_token(); /* read the control variable */

if (!isalpha(*token)) {

serror(4);

return;

}

i.var = toupper(*token) - 'A'; /* save its index */

get_token(); /* read the equal sign */

if (*token!='=') {

serror(3);

return;

}

get_exp(&value); /* get initial value */

variables[i.var]=value;

get_token();

if (tok != TO) serror(9); /* read an discard the TO */

get_exp(&i.target); /* get target value */

/* if loop can execute at least once, push into on stack */

if (value<=i.target) {

i.loc = prog;

fpush(i);

}

else /* otherwise, skip loop code altogether */

while (tok!=NEXT) get_token();

}

/* execute a NEXT statement */

void next()

{

struct for_stack i;

i = fpop(); /*read the loop info */

variables[i.var]++; /* increment control variable */

if (variables[i.var]>i.target) return; /* all done */

fpush(i); /* otherwise,return the info */

prog = i.loc; /* loop */

}

/* push function for the FOR stack */

void fpush(struct for_stack i)

{

if (ftos>FOR_NEST)

serror(10);

fstack[ftos]=i;

ftos++;

}

struct for_stack fpop()

{

ftos--;

if (ftos<0) serror(11);

return (fstack[ftos]);

}

/* exec a simple form of BASIC INPUT command */

void input()

{

char str[80],var;

int i;

get_token(); /* see if prompt string id=s present */

if (token_type == QUOTE) {

printf (token); /* if so , print it and check for command */

get_token();

if (*token != ',') serror(1);

get_token();

}

else printf ("? "); /* otherwise, prompt with / */

var = toupper(*token) - 'A'; /* get the input var */

scanf ("%d",&i); /* read input */

variables[var] = i; /* store it */

}

/* execute a GOSUB command */

void gosub()

{

char *loc;

get_token();

/* find the label to call */

loc = find_label(token);

if (loc=='\0')

serror(7); /* label not defined */

else {

gpush(prog); /* save place to return to */

prog = loc; /* start program running at that loc */

}

}

/* return from GOSUB */

void greturn()

{

prog = gpop();

}

/* GOSUB stack push function */

void gpush(char *s)

{

gtos++;

if (gtos==SUB_NEST) {

serror(12);

return;

}

gstack[gtos] = s;

}

/* GOSUB stack pop function */

char *gpop()

{

if (gtos==0) {

serror(13);

return 0;

}

return gstack[gtos--];

}

main (int argc,char *argv[])

{

char in[80];

int answer;

char *p_buf;

char *t;

if (argc!=2) {

printf ("usage: run <filename>\n");

exit (1);

}

/* allocate memory for the program */

if (!(p_buf=(char *)malloc(PROG_SIZE))) {

printf ("allocation failure");

exit (1);

}

/* load the program to execute */

if (!load_program(p_buf,argv[1])) exit(1);

if (setjmp(e_buf)) exit(1); /* initialize the long jump */

prog = p_buf;

scan_labels(); /* find the labels in the program */

ftos = 0; /* initialize the FOR stack index */

gtos = 0; /* initialize the GOSUB stack index */

do {

token_type = get_token();

/* check for assignment stack */

if (token_type==VARIABLE) {

putback(); /* return the varto the input stream */

assignment(); /* must 1: assignment statemnet */

}

else /* is command */

switch (tok) {

case PRINT:

print();

break;

case GOTO:

exec_goto();

break;

case IF:

exec_if();

break;

case FOR:

exec_for();

break;

case NEXT:

next();

break;

case INPUT:

input();

break;

case GOSUB:

gosub();

break;

case RETURN:

greturn();

break;

case END:

exit(0);

}

}while (tok != FINISHED);

}

 
 
 
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