// Prog: simple_calculator_l.cpp
// evaluate arithmetic expressions involving +, -
// over double operands, with left-to-right evaluation order

// Syntax in BNF:
// ---------------
// sum = val | val "+" sum | val "-" sum

#include<iostream>
#include<istream>
#include<sstream>
#include<cassert>

char lookahead (std::istream& is)
{
  is >> std::ws;         // skip whitespaces
  if (is.eof())
    return 0;            // end of stream
  else
    return is.peek();    // next character in is
}

double val (std::istream& is)
{
  double v;
  is >> v;
  assert (!is.fail());
  return v;
}

// sum = val | val "+" sum | val "-" sum.
double sum (double v, char sign, std::istream& is)
{
  if (sign == '+')
    v += val (is);
  else if (sign == '-')
    v -= val (is);
  char c = lookahead (is);
  if (c == '+' || c == '-')
    return sum (v, c, is >> c);
  else
    return v;
}

int main()
{
  std::stringstream input ("3-4-5");                // -6
  std::cout << sum (0, '+', input) << "\n";
  return 0;
}