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 <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#define _KEY_WORDEND "waiting for your expanding"
using namespace std;
typedef struct //词的结构，二元组形式（单词种别，单词自身的值）
{
int typenum; //单词种别
char * word;
}WORD;
char input[255];
char token[255] = "";
int p_input; //指针
int p_token;
char ch;
char * rwtab[] = { "begin","if","then","while","do","end","int","main",
"else","float","double","return","cout",_KEY_WORDEND };
WORD * scanner();//扫描
int main()
{
int over = 1;
WORD* oneword = new WORD;
//实现从文件读取代码段
cout << "read something from data.txt" << endl;
FILE *fp;
if((fp=freopen("data.txt","r",stdin))==NULL)
{
printf("Not found file!
");
return 0;
}
else
{
while ((scanf("%[^#]s", &input)) != EOF)
{
p_input = 0;
printf("your words:
%s
", input);
while (over < 1000 && over != -1)
{
oneword = scanner();
if (oneword->typenum < 1000)
{
if(oneword->typenum != 999)
cout << "[ "<< oneword->typenum <<" "<< oneword->word <<" ]"<< endl;
}
over = oneword->typenum;
}
scanf("%[^#]s", input);
}
}
return 0;
}
//从输入缓冲区读取一个字符到ch中
char m_getch()
{
ch = input[p_input];
p_input++;
return ch;
}
//去掉空白符号
void getbc()
{
while (ch == ' ' || ch == 10)
{
ch = input[p_input];
p_input++;
}
}
//拼接单词
void concat()
{
token[p_token] = ch;
p_token++;
token[p_token] = ' ';
}
//判断是否字母
int letter()
{
if (ch >= 'a'&&ch <= 'z' || ch >= 'A'&&ch <= 'Z')
return 1;
else
return 0;
}
//判断是否数字
int digit()
{
if (ch >= '0'&&ch <= '9')
return 1;
else
return 0;
}
//检索关键字表格
int reserve()
{
int i = 0;
while(strcmp(rwtab[i], _KEY_WORDEND))
{
if (!strcmp(rwtab[i], token))
return i + 1;
i++;
}
return 10;//如果不是关键字，则返回种别码10
}
//回退一个字符
void retract()
{
p_input--;
}
//词法扫描程序
WORD * scanner()
{
WORD * myword = new WORD;
myword->typenum = 10; //初始值
myword->word = "";
p_token = 0; //单词缓冲区指针
m_getch();
getbc();//去掉空白
if (letter())//判断读取到的首字母是字母
{
//如int
while (letter() || digit())
{
concat(); //连接
m_getch();
}
retract(); //回退一个字符
myword->typenum = reserve();//判断是否为关键字，返回种别码
myword->word = token;
return myword;
}
else if (digit()) //判断读取到的单词首字符是数字
{
while (digit()) //所有数字连接起来
{
concat();
m_getch();
}
retract();
//数字单词种别码统一为20，单词自身的值为数字本身
myword->typenum = 20;
myword->word = token;
return(myword);
}
else switch (ch)
{
case '=':
m_getch();//首字符为=,再读取下一个字符判断
if (ch == '=')
{
myword->typenum = 39;
myword->word = "==";
return(myword);
}
retract();//读取到的下个字符不是=，则要回退，直接输出=
myword->typenum = 21;
myword->word = "=";
return(myword);
break;
case '+':
myword->typenum = 22;
myword->word = "+";
return(myword);
break;
case '-':
myword->typenum = 23;
myword->word = "-";
return(myword);
break;
case '/'://读取到该符号之后，要判断下一个字符是什么符号，判断是否为注释
m_getch();//首字符为/,再读取下一个字符判断
if (ch == '*') // 说明读取到的是注释
{
m_getch();
while(ch != '*')
{
m_getch();//注释没结束之前一直读取注释，但不输出
if(ch == '*')
{
m_getch();
if(ch == '/')//注释结束
{
myword->typenum = 999;
myword->word = "注释";
return (myword);
break;
}
}
}
}
else
{
retract();//读取到的下个字符不是*，即不是注释，则要回退，直接输出/
myword->typenum = 25;
myword->word = "/";
return (myword);
break;
}
case '*':
myword->typenum = 24;
myword->word = "*";
return(myword);
break;
case '(':
myword->typenum = 26;
myword->word = "(";
return(myword);
break;
case ')':
myword->typenum = 27;
myword->word = ")";
return(myword);
break;
case '[':
myword->typenum = 28;
myword->word = "[";
return(myword);
break;
case ']':
myword->typenum = 29;
myword->word = "]";
return(myword);
break;
case '{':
myword->typenum = 30;
myword->word = "{";
return(myword);
break;
case '}':
myword->typenum = 31;
myword->word = "}";
return(myword);
break;
case ',':
myword->typenum = 32;
myword->word = ",";
return(myword);
break;
case ':':
m_getch();
if (ch == '=')
{
myword->typenum = 18;
myword->word = ":=";
return(myword);
break;
}
else
{
retract();
myword->typenum = 33;
myword->word = ":";
return(myword);
break;
}
case ';':
myword->typenum = 34;
myword->word = ";";
return(myword);
break;
case '>':
m_getch();
if (ch == '=')
{
myword->typenum = 37;
myword->word = ">=";
return(myword);
break;
}
retract();
myword->typenum = 35;
myword->word = ">";
return(myword);
break;
case '<':
m_getch();
if (ch == '=')
{
myword->typenum = 38;
myword->word = "<=";
return(myword);
break;
}
else if(ch == '<')
{
myword->typenum = 42;
myword->word = "<<";
return(myword);
break;
}
else
{
retract();
myword->typenum = 36;
myword->word = "<";
return (myword);
}
case '!':
m_getch();
if (ch == '=')
{
myword->typenum = 40;
myword->word = "!=";
return(myword);
break;
}
retract();
myword->typenum = -1;
myword->word = "ERROR";
return(myword);
break;
case ' " ':
myword->typenum = 41;
myword->word = " " ";
return(myword);
break;
case ' ':
myword->typenum = 1000;
myword->word = "OVER";
return(myword);
break;
case '#':
myword->typenum = 0;
myword->word = "#";
return (myword);
break;
default:
myword->typenum = -1;
myword->word = "ERROR";
return(myword);
break;
}
}
数据：
int main()
{
int a=2,b=5;
b/a;
b>a;
c = a+b;
cout <<c;
return 0;int
}
理解了一部分的代码，因为学习不够努力导致不能够独立完成，后续会努力学习。
参考链接：https://blog.csdn.net/just_a_new_life/article/details/80895433