實驗內容

通過對PL/0詞法分析程式（GETSYM）的分析，並在此基礎上按照附錄A中給出的PL/0語言的語法描述，編寫一個PL/0語言的詞法分析程式。此程式應具有如下功能：
輸入為字串（待進行詞法分析的源程式），輸出為單詞串，即由（單詞、類別）所組成的二元組序列。
有一定檢查錯誤的能力，例如發現2A這類不能作為單詞的字串。

實驗程式碼如下：

# include<iostream>
#include <bits/stdc++.h>
using namespace std;

bool key_words(string s)//關鍵字
{
    static vector<string>
 
 keyVec = { "main", "int", "float", "double", "char",
        "if", "then","else", "switch", "case", "break", "continue", "while",
        "do", "for" ,"#include","using","namespace","std","return"};
    vector<string>::iterator result = find(keyVec.begin(), keyVec.end(),s);
    if (result != keyVec.
 
end())
        return true;
    else return false;
}
bool ident(string s)//識別符號
{
    if (!key_words(s))//識別符號不能是關鍵字
    {
        if ((s[0] >= 'a'&&s[0] <= 'z') || (s[0] >= 'A'&&s[0] <= 'Z'))//是字母
        {
            for (int i = 1; i < s.length(); i++)
            {
                if
 
 ((s[i] >= 'a'&&s[i] <= 'z') || (s[i] >= 'A'&&s[i] <= 'Z')
                    || (s[i] >= '0'&&s[i] <= '9'))
                    continue;
                else return false;
            }
            return true;
        }
        return false;
    }
    return false;
}

bool digit(string s)//整數
{
    if (s[0] >= '0'&&s[0] <= '9')
    {
        for (int i = 1; i < s.length(); ++i)
            if (s[i] >= '0'&&s[i] <= '9')
                continue;
            else return false;
        return true;
    }
    return false;
}

bool points(string s)//小數
{
    if (s[0] >= '0'&&s[0] <= '9')
    {
        for (int i = 1; i < s.length(); ++i)
            if ((s[i] >= '0'&&s[i] <= '9')||s[i]=='.')
                continue;
            else return false;
        return true;
    }
    return false;
}

bool is_operator(string s)//運算子
{
    static vector<string> opeVec = { "=","+","-","*","/","<","<=","==","!=",
        ">",">=",";","(",")","?",":",",","+=","-=" };
    vector<string>::iterator result = find(opeVec.begin(), opeVec.end(), s);
    if (result != opeVec.end())
        return true;
    else return false;
}

bool is_operator(char c)//運算子
{
    static vector<char> opeVec = { '=','+','-','*','/','<','>',';','(',')','?',':',',' };
    vector<char>::iterator result = find(opeVec.begin(), opeVec.end(), c);
    if (result != opeVec.end())
        return true;
    else return false;
}

string result(string s)//根據傳入的引數s產生對應的輸出
{
    //識別符號
    if (ident(s))
        return "(識別符號,"+s+")";

    //關鍵字
    if (key_words(s))
        return "(關鍵字,"+s+")";

    //整型常量
    if (digit(s))
        return "(整數,"+s+")";

    //小數
    if (points(s))
        return "(小數,"+s+")";

    //運算子
    static map<string, string> opeMap;
    opeMap["="] = "(等號，=）";
    opeMap["<"] = "(小於號,<)";
    opeMap["<="] = "(小於等於號,<=)";
    opeMap["+="] = "(加等於號,+=)";
    opeMap["-="] = "(減等於號,-=)";
    opeMap["=="] = "(賦值運算子，==）";
    opeMap["!="] = "(不等於號,!=)";
    opeMap[">"] = "(大於號,>)";
    opeMap[">="] = "(大於等於號,>=)";
    opeMap[";"] = "(分號,;)";
    opeMap["+"] = "(加號,+)";
    opeMap["("] = "(左括號,( )";
    opeMap["-"] = "(減號,-)";
    opeMap[")"] = "(右括號,) )";
    opeMap[">"] = "(大於號,>)";
    opeMap["*"] = "(星號,*)";
    opeMap["?"] = "(問號,?)";
    opeMap["/"] = "(除號,/)";
    opeMap[":"] = "(冒號,:)";
    opeMap[","] = "(逗號,,)";
    if (is_operator(s))
        return opeMap[s];
    return "Error";
}

int main()
{
    string file = ("C:\\test.txt");
    ifstream input(file);

    ofstream output("C:\\Result.txt",ofstream::app);
    //先將testData.txt內容拷貝到Result.txt中
    string copy;

    getline(input, copy, '\0');
    cout<< copy << endl;//測試是否正確

    //此時input已經指到了檔案尾，為了後面的讀取，需要關閉再開啟
    input.close();
    input.open(file);

    output << "原資料:\n";
    output << copy;
    output << "處理後結果:\n";

    string str;
    string words;

    cout << "處理後結果:\n";
    int l=0;//計算行數
    while (getline(input, str)) //讀取檔案每一次讀取一行,遇到EOF結束
    {
        l++;
        //從輸入流中獲取單詞，需要用到輸入流物件，即istringstream
        istringstream strCin(str);
        string s;
        while (strCin >> words)
        {
            //關鍵字肯定是單獨作為一個單詞的
            if (key_words(words))
            {
                s = result(words);
                cout <<"Line "<<l<<":"<< s << endl;
                output <<"Line "<<l<<":"<< s << endl;
                continue;
            }

            //對單詞進行掃描，肯定是識別符號，運算子，逗號分號,數字等等混合在一起的單詞
            vector<int> index = {0};
            vector<string> mulWords;//將words分解為多個單詞
            for (int i = 0; i < words.length(); i++)
            {

                //運算子有兩位的，比如"<=",">=","==","!=","-=","+="
                if ((i < words.length() - 1) && is_operator(words[i]) && is_operator(words[i + 1]))
                {
                    //但是要注意只有以上6種兩位運算子，比如+-,))就不是,但是))還是要輸出),)
                    if (string(words.begin() + i, words.begin() + i + 2) == "<=" ||
                        string(words.begin() + i, words.begin() + i + 2) == ">=" ||
                        string(words.begin() + i, words.begin() + i + 2) == "==" ||
                        string(words.begin() + i, words.begin() + i + 2) == "-=" ||
                        string(words.begin() + i, words.begin() + i + 2) == "+=" ||
                        string(words.begin() + i, words.begin() + i + 2) == "!=")
                    {
                        index.push_back(i);
                        index.push_back(i + 2);
                        ++i;
                    }
                    else if (is_operator(words[i]))
                    {
                        if (find(index.begin(), index.end(), i) == index.end())
                            index.push_back(i);
                        if (find(index.begin(), index.end(), i + 1) == index.end())
                            index.push_back(i + 1);

                    }
                }
                //逗號，運算子作為分隔
                 else if (is_operator(words[i]))
                {
                    if (find(index.begin(), index.end(), i) == index.end())
                    //比如遇到"a,b"這裡下標0和1將a分開，1到2將逗號分開，2到3將b分開
                        index.push_back(i);
                    if (find(index.begin(), index.end(), i+1) == index.end())
                        index.push_back(i + 1);
                }

            }
            for (int i = 0; i < index.size()-1; i++)
            {
                string rel;
                //比如遇到"<="，需要提取”<=“
                rel=result(string(words.begin() + index[i], words.begin() + index[i + 1]));

                output <<"Line "<<l<<":"<< rel << endl;
                cout <<"Line "<<l<<":"<< rel<<endl;
            }
        }
    }
    output << endl;
    output.close();
    input.close();
    system("pause");
    return 0;
}

實驗結果：