下面完成一个简单的计算器通过语法树进行计算，首先定义一个语法树的结构，然后编写flex文件，解析数字或符号，对于 符号返回本身，对于数字，返回NUMBER，并对yylval的d进行赋值，yylval指向一个联合类型，接着，在语法分析器中完成语法树的节点的增加，分别对应数字和符号有不同的增加方式，最后有一个单独的C代码处理计算，以及语法树相关计算的函数。对结果的计算的方式是对语法树进行递归。

词法分析器为：

dp@dp:~/flexbison % cat myast.l%option noyywrap nodefault yylineno%{#include "myast.h"#include "myast.tab.h"char buffer[20];%}EXP ([Ee][-+]?[0-9]+)%%"+"|"-"|"*"|"/"|"("|")"|"|" {return yytext[0];}[0-9]+"."[0-9]*{EXP}?|"."?[0-9]+{EXP}? {yylval.d=atof(yytext);return NUMBER;}\n {return EOL;}"//".*[ \t] {}"Q" {exit(0);}. {sprintf(buffer,"invalid character %c\n",*yytext); yyerror(buffer);} %%

语法分析器为：

dp@dp:~/flexbison % cat myast.y%{#include <stdio.h>#include <stdlib.h>#include "myast.h"%}%union{struct myast *mya;double d;}%token <d> NUMBER%token EOL%type <mya> exp factor term%%calclist:|calclist exp EOL{printf("= %g\n",eval($2));treefree($2);printf("$");}|calclist EOL{printf("$");};exp:factor|exp '+' factor {$$=newast('+',$1,$3);} |exp '-' factor{$$=newast('-',$1,$3);};factor:term |factor '*' term {$$=newast('*',$1,$3);} |factor '/' term {$$=newast('/',$1,$3);};term:NUMBER{$$=newnum($1);}|'|' term{$$=newast('|',$2,NULL);}|'(' exp ')' {$$=$2;} |'-' term {$$=newast('M',$2,NULL);};%%

然后头文件 为：

dp@dp:~/flexbison % cat myast.hextern int yylineno;void yyerror(char *s);struct ast{int nodetype;struct ast *l;struct ast *r;};struct numval{int nodetype;double number;};struct ast *newast(int nodetype,struct ast *l,struct ast *r);struct ast *newnum(double d);double eval(struct ast *);void treefree(struct ast *);

C代码文件的内容为：

dp@dp:~/flexbison % cat myastfunc.c#include <stdio.h>#include <stdlib.h>#include <stdarg.h>#include "myast.h"struct ast * newast(int nodetype,struct ast *l,struct ast *r){struct ast *a=malloc(sizeof(struct ast));if (!a){yyerror("out of space");exit(0);}a->nodetype=nodetype;a->l=l;a->r=r;return a;}struct ast * newnum(double d){struct numval *a=malloc(sizeof(struct numval));if (!a){yyerror("out of space");exit(0);}a->nodetype='D';a->number=d;return (struct ast *)a;}double eval(struct ast *a){double v; switch(a->nodetype){case 'D':v=((struct numval *)a)->number;break;case '+':v=eval(a->l)+eval(a->r);break;case '-':v=eval(a->l)-eval(a->r);break;case '*':v=eval(a->l)*eval(a->r);break;case '/':v=eval(a->l)/eval(a->r);break;case '|':v=eval(a->l);v=v<0?v:-v;break;case 'M':v=-eval(a->l);break; defaut:printf("bad node:%c\n",a->nodetype); } return v;}void treefree(struct ast*a){switch(a->nodetype){case '+':case '-':case '*':case '/':treefree(a->r);case '|':case 'M':treefree(a->l);case 'D':free(a);break;default:printf("free bad node %c\n",a->nodetype);}}void yyerror(char *s){fprintf(stderr,"line %d error!:%s",yylineno,s);}int main(){printf("$ ");return yyparse();}

Makefile文件为:

dp@dp:~/flexbison % cat makefilemyjs:myast.l myast.y myast.h bison -d myast.yflex -omyast.lex.c myast.lcc -o $@ myast.tab.c myast.lex.c myastfunc.cdp@dp:~/flexbison %

运行效果如下

dp@dp:~/flexbison % ./myjs$ 12+99= 111$11*(9-3)+6/3= 68$Qdp@dp:~/flexbison %