How to read a value from a text file and store that value into a temporary variable?

Question

istringstream in("4.2 + 3.4 - 4.1");
ostringstream out;

string sign;
double num;
double tempNum = 0;
double sum = 0;

while (in >> num >> sign) {
    tempNum = num;  

    if (sign == "+") {
        sum = (num + tempNum); 
    }
}
cout << sum << endl;

My Output: 8.4

Correct output: 3.5

I want to store 4.2 into tempNum and read 3.4 into num, so that I can add 8.4 and 3.4 into the sum. However, the problem that I have is whenever it reads 4.2, it assigns 4.2 into num and then it reads 3.4 and assigns 3.4 into tempNum also. How can I assign 4.2 into tempNum and leave 3.4 just in the num variable. So that I can have the sum as 4.2 + 3.4, not 4.2 + 4.2. If the question is not clear, I'd love to explain in more details.

Answer 1

There are multiple problems with the code you have. How to solve the problem as originally stated is already answered. Another problem I have mentioned in a comment. But so far none solves the problem of reading the numbers and "signs" in serial, which I will attempt to show here.

First thing you need to think about is that the input is a sequence of numbers and "signs" all after each other. While you are correct that a loop is needed you also need to think about the actual sequence... You have a number, followed by a sign followed by another number etc.

The easy way to solve this is to think of it a little differently. It's not a sequence starting with a number. Instead think of it as a lonely number, followed by an optional sequence of "signs" and numbers.

Then you could do something like

in >> sum;
while (in >> sign >> num)
{
    if (sign == "+")
    {
        sum += num;
    }
    else if (sign == "-")
    {
        sum -= num;
    }
}

The above loop will handle the case when the input is only a single number. Or arbitrary amount of numbers with + or - in between them.

This will still not be able to properly handle things like multiplication and division though. For that you need another and more complicated approach.

Answer 2

You never read from a stream a second time, just using the same first number twice. After line tempNum = num you probably meant that the new number should be read:

in >> num;

Please note also that step-by-step debug process with opened watches window is extremely good in dealing with this sort of problems.

UP : The whole code looks like this:

istringstream in("4.2 + 3.4");
ostringstream out;

string sign;
double num;
double tempNum = 0;
double sum = 0;

while (in >> num >> sign) {
    tempNum = num;  
    in >> num;
    if (sign == "+") {
        sum = (num + tempNum); 
    }
}
cout << sum << endl;

Answer 3

For the simple case, you can use the stream extraction operator without the while loop. Like so

in >> x >> sign >> y;

Here is the full example

  std::istringstream in("4.2 + 3.4");

  std::string sign;
  double x, y;

  in >> x >> sign >> y;

  std::cout << x << std::endl;
  std::cout << sign << std::endl;
  std::cout << y << std::endl;

  double sum = x + y;

  std::cout << sum << std::endl;

If the input becomes more complex try a while loop like so

  std::istringstream in("4.2 + 3.4 - 4.1");

  std::string temp;
  double op; //for + becomes 1.0 for - becomes -1.0
  double sum = 0;
  in >> sum; //Get the first value
  while (in >> temp) {
    //check for operator
    if (temp == "+") {
      op = 1.0;
    }
    else if (temp == "-") {
      op = -1.0;
    }
    else {
      double next = std::stod(temp,NULL);

      sum += op * next;
    }
  }