一、功能

输入：所给文法的源程序字符串。

输出：二元组（syn,token

或sum）构成的序列。

 其中：syn为单词种别码；

 token为存放的单词自身字符串；

sum为整型常数。

二、符号与种别码对照表

单词符号          种别码   单词符号     种别码

begin                1         ：         17

If                   2         ：=        18

Then                3         <          20

While               4         <>          21

Do                  5         <=         22

end                 6         >          23

lettet（letter|digit）*  10         >=        24

dight dight*           11        =          25

+                   13         ；         26

—                  14          (          27

*                   15           )         28

/                   16          #           0

三、词法规则

代码

#include <stdio.h>
#include <string.h>
#include<conio.h>
#include <process.h>
char prog[80],token[8],ch;
int syn,p,m,n,sum;
char *rwtab[6]={"begin","if","then","while","do","end"};

scaner();
main()
{
p=0;

printf("\n please input a string(end with ‘#‘):/n");
do{
scanf("%c",&ch);
prog[p++]=ch;
}while(ch!=‘#‘);

p=0;
do{
scaner();
switch(syn)
{case 11:printf("( %-10d%5d )\n",sum,syn);
break;
case -1:printf("you have input a wrong string\n");
getch();
exit(0);
default: printf("( %-10s%5d )\n",token,syn);
break;
}
}while(syn!=0);
getch();
}
scaner()
{
sum=0;
for(m=0;m<8;m++)token[m++]=NULL;
ch=prog[p++];
m=0;
while((ch==‘ ‘)||(ch==‘\n‘))ch=prog[p++];
if(((ch<=‘z‘)&&(ch>=‘a‘))||((ch<=‘Z‘)&&(ch>=‘A‘)))
{ while(((ch<=‘z‘)&&(ch>=‘a‘))||((ch<=‘Z‘)&&(ch>=‘A‘))||((ch>=‘0‘)&&(ch<=‘9‘)))
{token[m++]=ch;
ch=prog[p++];
}
p--;
syn=10;
for(n=0;n<6;n++)
if(strcmp(token,rwtab[n])==0)
{ syn=n+1;
break;
}
}
else if((ch>=‘0‘)&&(ch<=‘9‘))
{ while((ch>=‘0‘)&&(ch<=‘9‘))
{ sum=sum*10+ch-‘0‘;
ch=prog[p++];
p--;
syn=11;
}
else switch(ch)

{ case ‘<‘:token[m++]=ch;
ch=prog[p++];
if(ch==‘=‘)
{
syn=22;
token[m++]=ch;
}
else
{
syn=20;
p--;
}
break;
case ‘>‘:token[m++]=ch;
ch=prog[p++];
if(ch==‘=‘)
{ syn=24;
token[m++]=ch;
}
else
{ syn=23;
p--;
}
break;
case ‘+‘: token[m++]=ch;
ch=prog[p++];
if(ch==‘+‘)
{ syn=17;
token[m++]=ch;
}
else
{
syn=13;
p--;
}
break;
case ‘-‘:token[m++]=ch;
ch=prog[p++];
if(ch==‘-‘)
{ syn=29;
token[m++]=ch;
}
else
{ syn=14;
p--;
}

break;
case ‘!‘:ch=prog[p++];
if(ch==‘=‘)
{ syn=21;
token[m++]=ch;
}
else
{ syn=31;
p--; }
break;
case ‘=‘:token[m++]=ch;
ch=prog[p++];
if(ch==‘=‘)
{ syn=25;
token[m++]=ch;
}
else
{ syn=18;
p--;
}
break;
case ‘*‘: syn=15;
token[m++]=ch;
break;
case ‘/‘: syn=16;
token[m++]=ch;
break;
case ‘(‘: syn=27;
token[m++]=ch;
break;
case ‘)‘: syn=28;
token[m++]=ch;
break;
case ‘{‘: syn=5;
token[m++]=ch;
break;
case ‘}‘: syn=6;
token[m++]=ch;
break;
case ‘;‘: syn=26;
token[m++]=ch;

break;
case ‘\"‘: syn=30;
token[m++]=ch;
break;
case ‘#‘: syn=0;
token[m++]=ch;
break;
case ‘:‘:syn=17;
token[m++]=ch;
break;
default: syn=-1;
break;
}
token[m++]=‘\0‘;
}
}

词法分析程序