#include <iostream.h>
#include <stdlib.h>
#include <string.h>

// ================================================================================
// Class TStringExpr is designed to contain a string with expression to evaluate.
// An element of this class allows to
// 1. read an expression from an input stream     (e.g. cin>>expr;    )
// 2. print the expression to an output stream    (e.g. cout<<expr;   )
// 3. read the next character from the string     (e.g. expr>>ch;     )
// 4. return the read character to the expression (e.g. expr.unget(); )
// 5. read an integer from the expression         {e.g. expr>>num;    }
// --------------------------------------------------------------------------------

class TStringExpr {
   private:
      string expr;
      unsigned int cur;
   public:
      // constructors
      TStringExpr() { expr=""; cur=0; };
      TStringExpr(char *str) { expr=str; cur=0; };
      TStringExpr(const string &str) { expr=str; cur=0; };
      // assignmnet operators
      string operator=(char *str);
      string operator=(const string &str);
      // some useful methods
      void putback(char ch) { if(cur>0) expr[--cur]=ch; };
      char get() { return cur<expr.length() ? expr[cur++] : 0; };
      void unget() { if(cur>0) cur--; };
      void print_wrong_char(char *fname);
      // friend operators
      friend istream& operator>>(istream &inp, TStringExpr &e);
      friend ostream& operator<<(ostream &out, const TStringExpr &e);
      friend char operator>>(TStringExpr &e, char &ch);
      friend TStringExpr& operator>>(TStringExpr &e, int &value);
};

string TStringExpr::operator=(char *str)
{
   expr = str;
   cur = 0;
   return expr;
}

string TStringExpr::operator=(const string &str)
{
   expr = str;
   cur = 0;
   return expr;
}

void TStringExpr::print_wrong_char(char *fname)
{
   cerr<<expr<<endl;
   for(unsigned int i=0;i<cur-1;i++) cerr<<" ";
   cerr<<"^"<<endl;
   cerr<<"Error in function "<<fname<<": wrong character '"<<expr[cur]<<"'"<<endl;
}


istream& operator>>(istream &inp, TStringExpr &e)
{
   char ch;
   e.expr = "";

   ch = inp.get();
   while( !inp.eof() && ch!='\n' ){
      if( ! isspace(ch) )
         e.expr += ch;
      ch = inp.get();
   }
   e.cur = 0;
   return inp;
}

ostream& operator<<(ostream &out, const TStringExpr &e)
{
   out<<e.expr;
   return out;
}

char operator>>(TStringExpr &e, char &ch)
{
   if( e.cur < e.expr.length() )
      return ch=e.expr[e.cur++];
   else return 0;
}

TStringExpr& operator>>(TStringExpr &e, int &value)
{
   char ch;
   value = 0;
   while( ch=e.get() )
      if( isdigit(ch) ) value = 10*value + (ch-'0');
      else{ e.unget(); break; }
   return e;
}

// ================================================================================