Problem with conversion programme

[declanmcerlane]

Okay, I'm working through Bjarne Stroustrup's Programming: Principles and Practice using C++ (really enjoying it!) and I've completed the first "Try This" exercise of Chapter 4 (Page 104). The code is as follows:

// Try This - Chapter 4: Page 104
// this programme will convert yen, euros and pounds into dollars

#include <iostream>

using namespace std;

int main()
{
  // create conversion constants
  const double yen_to_dollar = 0.012044;
  const double euro_to_dollar = 1.2932;
  const double pound_to_dollar = 1.5548;

  cout << "Please enter a value followed by a unit (y, e or p): ";
  double value;
  char unit;
  cin >> value >> unit;

  if (unit == 'y')
    cout << value << "yen == $" << value*yen_to_dollar << endl;
  else
  {
    if (unit == 'e')
      cout << value << "EUR == $" << value*euro_to_dollar << endl;
    else
    {
      if (unit  == 'p')
        cout << value << "GBP == $" << value*pound_to_dollar << endl;
      else
        cout << "Sorry, I don't recognise that unit!\n";
    }
  }
  return 0;
}

My style is not exactly like that of Bjarne's (I've read half of Accelerated C++ and done a few other web tutorials before) but I have picked up a few things I like and dislike when it comes to readability.

The code compiles without a hitch (on g++ anyway) and seems to run as intended:

Please enter a value followed by a unit (y, e or p): 100p
100GBP == $155.48

Please enter a value followed by a unit (y, e or p): 100y
100yen == $1.2044

However...

I come across a problem whenever I try to convert from Euros to Dollars:

Please enter a value followed by a unit (y, e or p): 100e
Sorry, I don't recognise that unit!

but when I space the value and char it works perfectly:

Please enter a value followed by a unit (y, e or p): 100 e
100EUR == $129.32

I wanted to know if it had anything to do with the actual mathematical value of "e" but inputs like "100ep" and "100e y" both give the result, "Sorry, I don't recognise that unit!" so that mustn't be the case.

Does anyone have any ideas as to why the lack of a simple "space" can bring this programme to its knees?

P.S.
In case anyone asks, I'm running Ubuntu 10.10 64-bit

[hamsterman]

Try entering "10e+1 y". It will become 100 yen.
Note e is not the e = 2.718...
5e+6 becomes 5*10⁶
I have no idea how to make iostream ignore that e for floats/doubles. The code would work with integers though..

[Albatross]

Non-digit characters are fair game for std::cin when it reads an integer, float, or double. I'd recommend reading the whole line minus the newline into an std::stringstream, reading out one character for your mode, and then putting the rest of it into a double via >>.

Good luck!

-Albatross

[declanmcerlane]

Cheers for the replies, guys.

@Albatross - I've heard of std::stringstream but have no idea how it works. I'm sure it'll be covered in my book eventually though. Besides, apart from that little snaggle, the programme works as intended.

I'll have another little programme posted up soon. So far it's 530+ lines (which is ridiculous!) and consists solely of if- and if-else-statements. Need to find a simpler way of expressing it but that's for another time. Thanks again!