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

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) = '