//
// Program:  stock_value
//
// Purpose:  track value of stock investment as price per share changes.
//
// Input (from standard input; program will prompt for it):
//   number of shares (integer)
//   initial price per share (floating-point)
//   sequence of updated prices, terminated by control-D
//
// Output (on standard output):
//   initial value (number of shares * price per share)
//   for each updated price, new value, and difference between old
//     value and new value
//      
#include <iostream.h>
#include <iomanip.h>		// has setiosflags(), setprecision(2)

// Function prototypes -- see comments with function definitions below.
void printPrompt(void);
void printMoney(double d);

// Main program.
int main(void)
{
  int shares;
  double price;
  double oldval;		// total value at previous price
  double newval;		// total value at current price

  cout << "How many shares?\n";
  cin >> shares;
  cout << "Initial price per share?\n";
  cin >> price;
  oldval = shares * price;
  cout << "Initial value = ";
  printMoney(oldval);
  cout << endl;
  printPrompt();
  while (cin >> price) {
    // "Loop invariant":  oldval is the total value at the previous
    //   price.  True every time through the loop.
    newval = shares * price;
    cout << "New value = ";
    printMoney(newval);
    cout << endl;
    cout << "You made ";
    printMoney(newval - oldval);
    cout << " today!\n";
    oldval = newval;
    printPrompt();
  }
  return 0;
}

// Function to print prompt for next price.
// Post:  user has been prompted for next price, with instructions
//          on how to signal end of input.
void printPrompt(void) {
  cout << "Enter new price per share, or control-D to end:\n";
  return;
}

// Function to print a double as "money".
// Post:  d is written to standard output, with exactly two decimal
//          places and preceded by a dollar sign.
void printMoney(double d) {
  cout << setiosflags(ios::showpoint | ios::fixed) 
       << setprecision(2)
       << d;
  return;
}