const int unknown = 1, constint = 2, punctuation = 3, opr = 4, variable = 5,
          keyword = 6, endoffile = 7; 

string kinds[] = { "zero", "unknown", "constint", "punctuation", "opr",
                   "variable", "keyword", "endoffile" };

struct token
{ int kind;
  string form;
  int value; };

struct LexAn
{ inputter & in;
  token t;

  LexAn(inputter & i): in(i)
  { t.kind = unknown; }

  void get()
  { t.kind = unknown;
    t.form = "";
    t.value = 0;
    char c = in.get();
    while (c == ' ' || c == '\n')
      c = in.get();

    if (isdigit(c))
    { t.form = "";
      t.value = 0;
      while (isdigit(c))
      { t.form += c;
        t.value = t.value * 10 + c - '0';
        c = in.get(); }
      in.unget();
      t.kind = constint;
      return; }

    else if (c == '{')
    { t.form = "{";
      t.kind = punctuation;
      return; }
    else if (c == '}')
    { t.form = "}";
      t.kind = punctuation;
      return; }
    // other forms of punctuation handled the same way

    else if (c == '+')
    { t.form = "+";
      t.kind = opr;
      return; }
    else if (c == '-')
    { t.form = "-";
      t.kind = opr;
      return; }
    // other operators handled the same way

    else if (isalpha(c))
    { t.form = "";
      while (isalpha(c) || isdigit(c))
      { t.form += c;
        c = in.get(); }
      in.unget();
      if (t.form == "print" || t.form == "while" || t.form == "do" ||
          t.form == "if" || t.form == "then" || t.form == "else")
        t.kind = keyword;
      else
        t.kind = variable;
      return; }

    else if (c == 'D'-64)
    { t.kind = endoffile;
      return; }

    else
    { in.error("invalid character");
      t.kind = unknown;
      return; } }
};

void print_token(token t)
{ cout << "Token, kind=" << kinds[t.kind] << ", form=\"" << t.form << "\", value=" << t.value << "\n"; }

int main()
{ inputter in("testfile.txt");
  LexAn L(in);
  while (true)
  { L.get();
    print_token(L.t);
    if (L.t.kind == endoffile)
      break; } }