手撸词法分析器（C/C++）

一.背景

这学期开设了编译原理，要求写个基本的词法分析器。所以博主就自己写了一份代码，也比较简单基础。

二.什么是词法分析器？

简单来说就是能识别基本的符号（+，-，*，/,），关键词（for ,while,return ,int ），数字等。这么说大家应该有个基本了解，那么我们如何写代码呢，这里有个思维图。

可以大致将单词分成这几类，然后进行判断即可。

三.代码

#include <stdio.h>
#include <string>
#include < fstream >
#include <iostream>
int scan_word(char* buff);
using namespace std;
char buf[102400] = { 0 };
char key[][20] = {"if","else","for","while","do","return","break","continue",
",",";","{","}","(",")",
"+","-","*","/",
"<","<=","=",">=","< ","*=","/=","+=","-=","++",
"int", "char", "float","double","unsigned char","unsigned int","void",
"main"
};

int main()
{   
ifstream  fin;
int i =0,j=0,k=0;
    fin.open("demo.txt", ios::in);
char temp[20] = { 0 };

if (!fin.is_open())
{
cout << "读取文件失败" << endl;
return 1;
}
while ((buf[i++] = fin.get()) != EOF);
buf[i] = '\0';

while (j < i-2)
{   
start:
if (buf[j] == 0x0a || buf[j] == 0x0d || isspace(buf[j]))
 {
 j++;
goto start;
 }
//字母
else if ((buf[j] >= 65 && buf[j] <= 90) || (buf[j] >= 97 && buf[j] <= 122))
{
while ((buf[j] >= 65 && buf[j] <= 90) || (buf[j] >= 97 && buf[j] <= 122)|| ((buf[j] >= 48 && buf[j] <= 57))||buf[j]==95)
{
temp[k++] = buf[j++];

}
 
temp[k] = '\0';
k = 0;
scan_word(temp);
}
//数字
else if ((buf[j] >= 48 && buf[j] <= 57))
{
while (buf[j] >= 48 && buf[j] <= 57)
{
temp[k++] = buf[j];
j++;
}
if (buf[j] == '.')
{
temp[k++] = buf[j];
j++;
}

while (buf[j] >= 48 && buf[j] <= 57)
{
temp[k++] = buf[j];
j++;
}
temp[k] = '\0';
k = 0;
printf("\(51,\"\%s\"\)\n", temp);
//scan_word(temp);

}
 //其他符号
else 
{ 
if(buf[j]=='>'||buf[j]=='='||buf[j]=='<' || buf[j] == '+'|| buf[j] == '-' || buf[j] == '*'||buf[j] == '/')
    {
if (buf[j + 1] == '=')
{
temp[0] = buf[j++];
temp[1] = buf[j++];
temp[2] = '\0';
}
if (buf[j + 1] == '+')
{
temp[0] = buf[j++];
temp[1] = buf[j++];
temp[2] = '\0';
}
else goto end;
    }
 
else
{  
end:
temp[0] = buf[j++];
temp[1] = '\0';

}
scan_word(temp);
}

}

}
int scan_word(char *buff)
{   
for (int i = 0; i < 31; i++)
{
if (strcmp(buff, key[i]) == 0)
{
printf("\(%d,\"\%s\"\)\n",i,buff); 
return 1;
}
}

printf("\(50,\"\%s\"\)\n", buff);
return 0;
}

四.思考

这只是一个简单的词法分析器，很多功能都不完善，你们也可以自己添加代码改善一下。

原文地址：https://blog.csdn.net/qq_62553914/article/details/137501255

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