本文主要是介绍【编译原理】一个词法分析器源码的剖析，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
一，词法分析器
作用：读取源程序的输入字符、将他们组成词素，生成并输出一个词法单元序列
二，设计原理
1）C程序语言的符号分类：关键字、标识符、常数、运算符、界符
2）词法分析器的二元输出：<单词种别，单词符号属性值>
3）正规式和状态转换图
4）程序说明：
1>main 中打开源码文件，从第一个字符流读取
2>如果第一个是字符，则交给letterprocess(str); 处理
3>如果第一个是数字，则交给numberprocess(str); 处理
4>如果第一个是数字，则交给otherprocess(str); 处理
5>注意上述过程中，File *fp每读取一个词素，fp都会移动到下一个词素。对于空格的处理：isspace（ch）检查参数c是否为空格字符，也就是判断是否为空格('')、定位字符
('\t')、CR('\r')、换行('\n')、垂直定位字符('\v')或 翻页('\f')的情况这个程序输出结果情况汇总：关键字、算术运算符、关系运算符、分割符号、特殊符号、注释符号、逻辑运算符、非法符号
三，程序源码
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <malloc.h>
#include <conio.h>
#define NULL 0
FILE *fp;
char ch;
char *keyword[34]={"auto","break","case","char","const","continue","default","do","double",
"else","enum","extern","float","for","goto","if","int","long","register",
"return","short","signed","sizeof","static","struct","switch","typedef", "printf",
"union","unsigned","void","volatile","while","main"};
char *operatornum[6]={"+","-","*","/","++","--"};
char *comparison[8]={"<","<=","=",">",">=","<>","==","!="};
char *interpunction[8]={",",";",":=",".","(",")","{","}"};
char *biaoshifu[6]={"%","$","^","&","_","#"};//特殊标识符
char *zhushifu[3]={"//","/*","*/"};//注释符
char *luoji[3]={"&&","||","!"};//逻辑运算符bool search(char searchstr[],int wordtype)//符号匹配
{int i;switch (wordtype){case 1:for(i=0;i<=33;i++){if(strcmp(keyword[i],searchstr)==0)return(true);}break;case 2:for(i=0;i<=5;i++){if(strcmp(operatornum[i],searchstr)==0)return(true);}break;case 3: for(i=0;i<=7;i++){if(strcmp(comparison[i],searchstr)==0)return(true);}break;case 4: for(i=0;i<=7;i++){if(strcmp(interpunction[i],searchstr)==0)return(true);}break;case 5: for(i=0;i<=5;i++){if(strcmp(biaoshifu[i],searchstr)==0)return(true);}break;case 6:for(i=0;i<=2;i++){if(strcmp(zhushifu[i],searchstr)==0)return(true);}break;case 7: for(i=0;i<=2;i++){if(strcmp(luoji[i],searchstr)==0)return(true);}break;}return false;
}char letterprocess (char ch)//字母处理函数
{int i=-1;char letter[20];while (isalnum(ch)!=0){letter[++i]=ch;ch=fgetc(fp);}letter[i+1]='\0';if (search(letter,1)){printf("<%s,关键字>\n",letter);//strcat(letter,"\n");//fputs('<' letter '>\n',outp);}else{printf("<%s,自定义变量>\n",letter);//strcat(letter,"\n");//fputs(letter,outp);}return(ch);
}char numberprocess(char ch)//数字处理程序
{int i=-1;char num[20];while (isdigit(ch)!=0){num[++i]=ch;ch=fgetc(fp);}if(isalpha(ch)!=0)//数字后面是字符 {while(isspace(ch)==0){num[++i]=ch;ch=fgetc(fp);}num[i+1]='\0';printf("错误！非法标识符：%s\n",num);goto u;}num[i+1]='\0';printf("<%s,数字>\n",num);u: return(ch);
}char otherprocess(char ch)//其他处理程序
{int i=-1;char other[20];if (isspace(ch)!=0){ch=fgetc(fp);goto u;}while ((isspace(ch)==0)&&(isalnum(ch)==0)){other[++i]=ch;ch=fgetc(fp);}other[i+1]='\0';if (search(other,2))printf("<%s,算数运算符>\n",other);else if (search(other,3))printf("<%s,关系运算符号>\n",other);else if (search(other,4))printf("<%s,分隔符号>\n",other);else if (search(other,5))printf("<%s,特殊标识符号>\n",other);else if (search(other,6))printf("<%s,注释符号>\n",other);else if (search(other,7))printf("<%s,逻辑运算符号>\n",other);else printf("错误！非法字符：%s\n",other);u: return (ch);
}int main ()
{char str;printf("**********************************词法分析器************************************\n");if ((fp=fopen("源程序.txt","r"))==NULL)printf("源程序无法打开！\n");else{str =fgetc(fp);//从流中读取字符 while (str!=EOF){if (isalpha(str)!=0)//如果是字符 isalpha包含在#include <cctype>str=letterprocess(str);else{if (isdigit(str)!=0)str=numberprocess(str);elsestr=otherprocess(str);}};printf("词法分析结束，谢谢使用！\n");//printf("点任意键退出!\n");}//c=getch();return 0;
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