0929作业

1词法分析器的功能 1）输入源程序 2）从左至右地对源程序进行扫描，按照语言的词法规则识别各类单词，并产生相应单词的属性字

2.

3.用文法描述词法规则

<字母> A a|b|c|……y|z

<数字> 
A→1|2|3|4|5|6|7|8|9
S→A|SA|SA0 
<整数常数>
A→1|2|3|4|5|6|7|8|9
S→A|SA|SA0 
<标识符>
A→a|b|c|……y|z
B→0|1|2|3|4|5|6|7|8|9
S→A|SB
<关键字>
S→if|else|while|do|for|int|char|……
<运算符>
S→+|-|x|/|=|#|<|>|<=|>=|：=
<界符>
S→(|)|,|;|.

#include <stdio.h>
#include <string.h>  

char prog[80],token[8],ch;
int syn,p,m,n,sum;
char *rwtab[6]={"begin","if","then","while","do","end"};  

void scaner(void);  

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();
                    return 0;
                break;  

                default:
                printf("( %-10s%5d )\n",token,syn);
                break;
            }
        }while(syn!=0);
    //getch();
    return 0;
 }  

void scaner(void)
{
    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‘;
}