main.cxx

#include <iostream>
#include <queue>
#include <cctype>

int main (int argc, char *argv[]) {
  if (argc < 2) {
    std::cout << "please provide an expression" << std::endl;
    return -1;
  }

  char *expression = argv[1];
  int expLength = strlen(argv[1]);

  std::queue<char> operands;
  std::queue<char> operators;

  for(int i=0;i<expLength;i++) {
    switch(expression[i]) {
      case '+': case '-':
        operators.push(expression[i]);
        break;
      default:
        if (isdigit(expression[i])) {
          int operand = (int)(expression[i] - '0');
          operands.push(operand);

        } else {
          std::cout << "I don't know what to do with " << expression[i] << std::endl;
          return -1;
        }
        break;
    }
  }

  int result = operands.front();
  operands.pop();

  while(!operators.empty()) {
    int nextOperand = operands.front();
    operands.pop();
    switch (operators.front()) {
      case '+':
        result += nextOperand;
        break;
      case '-':
        result -= nextOperand;
        break;
      default:
        std::cout << "This operator: " << operators.front() << " should not be here" << std::endl;
        return -1;
        break;
    }

    operators.pop();
  }

  std::cout << argv[1] << " = " << result << std::endl;

  return 0;
}