從一個檔案中讀取原始碼，執行後可以直接看到結果，也可在指定檔案中檢視結果。

這個詞法分析器只實現了部分關鍵字、字元等的識別，可在Key[][]陣列自行新增，種別碼的判別與新增用case更加方便，可寫下試試。

#include <iostream>
#include <fstream>
#include <ctype.h>
#include <cstdlib>
#include <cstring>
using namespace std;
char key[6][10]={"begin","if","then","while","do","end"};
ifstream in;
ofstream out;
void deal(char *ch)
{
    char str[110];
    int l,i,j;
    memset(str,0,sizeof(str));
    for(i=0;i<strlen(ch);)
    {
        if(ch[i]==' '||ch[i]=='\n'||ch[i]=='\t')//處理空格，製表符，換行符 
        {
            i++;
        }
        else if(isalpha(ch[i]))//處理遇到的字元 
        {
            l=0;
            str[l++]=ch[i];//暫存字元 
            i++;
            while(isdigit(ch[i])|| isalpha(ch[i]))//一直搜尋到最後一個字元或數字 
            {
                str[l++]=ch[i];
                i++;
            }
            str[l]='\0';//封裝為字串 
            for(j=0;j<6;j++)//檢查是否與給定關鍵字匹配 
            {
                if(strcmp(str,key[j])==0)//若匹配則為對應關鍵字 
                {
                    cout<<"("<<j+1<<","<<str<<")"<<endl;//輸出到dos視窗 
                    out<<"("<<j+1<<","<<str<<")"<<endl;//輸入到檔案 
                    break; 
                }
            }
            if(j==6)//若不匹配則為一般字元 
            {
                cout<<"(10,"<<str<<")"<<endl;
                out<<"(10,"<<str<<")"<<endl;
            }
        }
        else if(isdigit(ch[i]))//若遇到數字 
        {
            l=0;
            str[l++]=ch[i];//暫存遇到的數字 
            i++;
            while(isdigit(ch[i]))//一直搜尋到數字結尾 
            {
                str[l++]=ch[i];
                i++;
            }
            str[l]='\0';
            cout<<"(11,"<<str<<")"<<endl;
            out<<"(11,"<<str<<")"<<endl;
        }
        else//判斷是否為運算子，界符符號 
        {
            if(ch[i]=='+')
            {
                cout<<"(13,+)"<<endl;
                out<<"(13,+)"<<endl;
            }
            else if(ch[i]=='-')
            {
                cout<<"(14,-)"<<endl;
                out<<"(14,-)"<<endl;
            }
            else if(ch[i]=='*')
            {
                cout<<"(15,*)"<<endl;
                out<<"(15,*)"<<endl;
            }
            else if(ch[i]=='/')
            {
                cout<<"(16,/)"<<endl;
                out<<"(16,/)"<<endl;
            }
            else if(ch[i]==':')
            {
                if((i+1)<strlen(ch)&&ch[i+1]=='=')
                {
                    cout<<"(18,:=)"<<endl;
                    out<<"(18,:=)"<<endl;
                    i++;
                }
                else
                {
                    cout<<"(17,:)"<<endl;
                    out<<"(17,:)"<<endl;
                }
            }
            else if(ch[i]=='<')
            {
                if((i+1)<strlen(ch)&&ch[i+1]=='>')
                {
                    cout<<"(21,<>)"<<endl;
                    out<<"(21,<>)"<<endl;
                    i++;
                }
                else if((i+1)<strlen(ch)&&ch[i+1]=='=')
                {
                    cout<<"(22,<=)"<<endl;
                    out<<"(22,<=)"<<endl;
                    i++;
                }
                else
                {
                    cout<<"(20,<)"<<endl;
                    out<<"(20,<)"<<endl;
                }
            }
            else if(ch[i]=='>')
            {
                if((i+1)<strlen(ch)&&ch[i+1]=='=')
                {
                    cout<<"(24,>=)"<<endl;
                    out<<"(24,>=)"<<endl;
                    i++;
                }
                else
                {
                    cout<<"(23,>)"<<endl;
                    out<<"(23,>)"<<endl;
                }
            }
            else if(ch[i]=='=')
            {
                cout<<"(25,=)"<<endl;
                out<<"(25,=)"<<endl;
            }
            else if(ch[i]==';')
            {
                cout<<"(26,;)"<<endl;
                out<<"(26,;)"<<endl;
            }
            else if(ch[i]=='(')
            {
                cout<<"(27,()"<<endl;
                out<<"(27,()"<<endl;
            }
            else if(ch[i]==')')
            {
                cout<<"(28,))"<<endl;
                out<<"(28,))"<<endl;
            }
            else if(ch[i]=='#')
            {
                cout<<"(0,#)"<<endl;
                out<<"(0,#)"<<endl;
            }
            else
            {
                cout<<"出錯"<<endl; 
                out<<"出錯"<<endl;
                exit(0);
            }
            i++;
        }
    }
}
int main()
{
    char ch[110],s;
    /*int sum=0;
    do{
       s=cin.get();
       ch[sum++]=s;
    }while(s!='#');
    ch[sum]='\0';
    deal(ch);
    */
    
    char in_name[50],out_name[50];//讀入檔案 輸出檔案 
    cout<<"指定讀入檔名："<<endl;
    cin>>in_name;
    in.open(in_name);
    if(in.fail())
    {
        cout<<"讀入檔案開啟失敗"<<endl;
        exit(0);
    } 
    cout<<"指定輸出檔名:"<<endl;
    cin>>out_name;
    out.open(out_name);
    if(out.fail())
    {
        cout<<"寫入檔案開啟失敗"<<endl;
        exit(0); 
    } 
    while(in.good()&&!in.eof())//如果檔案正常且沒有到達檔案結束 
    {
        memset(ch,0,110);
        in.getline(ch,110);
         deal(ch);
    }
    system("pause");
    return 0;
}