词法分析程序(Lexical Analyzer)要求:
- 从左至右扫描构成源程序的字符流
- 识别出有词法意义的单词(Lexemes)
- 返回单词记录(单词类别,单词本身)
- 滤掉空格
- 跳过注释
- 发现词法错误
程序结构:
输入:字符流(什么输入方式,什么数据结构保存)
处理:
–遍历(什么遍历方式)
–词法规则
输出:单词流(什么输出形式)
–二元组
单词类别:
1.标识符(10)
2.无符号数(11)
3.保留字(一词一码)
4.运算符(一词一码)
5.界符(一词一码)
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 |
l(l|d)* |
10 |
>= |
24 |
dd* |
11 |
= |
25 |
+ |
13 |
; |
26 |
- |
14 |
( |
27 |
* |
15 |
) |
28 |
/ |
16 |
# |
0 |
代码:
#include<stdio.h>
#include <iostream>
#include<string.h>
using namespace std;
char *reserved[6] = {"begin","if","then","while","do","end"};
char input[80],output[8];
char ch;
int syn,p,i;
int m = 0,n,row,sum = 0,count=0;
void scanner(){
for( i = 0 ; i < 8 ; i++ ){
output[i] = NULL;
}
ch = input[p++];
while( ch == ' ' ){
ch = input[p] ;
p ++ ;
}
if( ( ch >= 'a' && ch <= 'z' ) || ( ch >= 'A' && ch <= 'Z' ) ){
m = 0 ;
while( ( ch >= 'a' && ch <= 'z' ) || ( ch >= 'A' && ch <= 'Z' ) || ( ch >= '0' && ch <= '9' ) ){
output[m++] = ch ;
ch = input[p++];
}
output[m++] = ' ';
p -- ;
syn = 10 ;
for( n = 0 ; n < 6 ; n ++ ){
if( strcmp(output,reserved[n]) == 0 ){
syn = n + 1;
break;
}
}
}else if( ( ch >= '0' && ch <= '9' ) ){
sum = 0 ;
while( ch >= '0' && ch <= '9' ){
sum=sum*10+ch-'0' ;
ch = input[p++] ;
}
p -- ;
syn = 11;
}
else if(ch == '/'){
ch = input[p++] ;
if(ch == '/'){
while(ch != '
'){
ch = input[p++] ;
}
scanner();
}
else if(ch == '*'){
ch = input[p++];
int flag=0;
while((ch == '*' && input[p] != '/')||(ch != '*' && input[p] == '/') || (ch != '*' && input[p] != '/')){
ch = input[p++] ;
if(p==count-2 && (ch != '*' && input[p] != '/')){
flag =1;
syn = 100;
break;
}
}
p++;
if(flag==0){
scanner();
}
}else{
p = p - 2 ;
ch = input[p++] ;
output[0] = ch ;
syn = 16 ;
}
}else switch(ch){
case '+':
output[0] = ch ;
syn = 13;
break;
case '-':
output[0] = ch ;
syn = 14;
break;
case '*':
output[0] = ch ;
syn = 15 ;
break;
case '/':
output[0] = ch ;
syn = 16 ;
break;
case ':':
i = 0;
output[i++] = ch ;
ch = input[p++];
if( ch == '=' ){
output[i++] = ch;
syn = 18 ;
}else{
syn = 17;
p-- ;
}
break;
case '<':
i = 0 ;
output[i++] = ch ;
ch = input[p++] ;
if( ch == '=' ){
output[i++] = ch ;
syn = 21 ;
}else if( ch == '>' ){
output[i++] = ch ;
syn = 22 ;
}else{
syn = 20 ;
p-- ;
}
break;
case '>':
i = 0;
output[i++] = ch ;
ch = input[p++] ;
if( ch == '=' ){
output[i++] = ch;
syn=24;
}else{
syn=23;
p--;
}
break;
case '=':
output[0] = ch ;
syn = 25 ;
break;
case ';':
output[0] = ch ;
syn = 26 ;
break;
case '(':
output[0] = ch ;
syn = 27 ;
break;
case ')':
output[0] = ch ;
syn = 28 ;
break;
case '#':
output[0] = ch ;
syn = 0;
break;
case '
':
syn = 99 ;
break;
default:
syn = -1 ;
break;
}
}
int main() {
p = 0;
printf("请输入程序:");
do{
ch = getchar();
input[p++] = ch;
}while(ch != '#');
count = p;
p = 0;
printf(" 单词符号 种别码
");
do{
scanner();
switch(syn){
case -1:
printf("在第%d行有错误!",row+1);
break;
case 11:
printf(" %d %d
",sum,syn);
break;
case 99:
row+=1;
break;
case 100:
printf("注释到尾结束!
");
break;
default:
printf(" %s %d
",output,syn);
break;
}
}while(syn != 0);
return 0;
}