// JavaScript Document
<!-- Hide from old browsers
// ********************************************************************
// * Variables *
// *************

// Should we append digits to the display?  It depends on whether
// we've just hit an operation button or a number button.  This
// variable keeps track of what we're supposed to do.
var appendDigits = false

// In order to successfully calculate, we need to remember the number
// that was in the display
var prevResult = 0

// Since we're doing a form of infix calculation, we need to remember
// the operation.
var op = "="

// ********************************************************************
// * Functions *
// *************

// Function
//   clearDisplay()
// Description
//   Clears the display
function clearDisplay()
{
  document.calculator.display.value = 0
  prevResult = 0
  appendDigits = false
  op = "="
} // clear

// Function
//   doOp(op)
// Description
//   Executes the *previous* operation (e.g., when the user types
//   2 + 4 - 2, when the "-" is clicked, the "+" should be executed).
function doOp(newop)
{
  // Get the argument
  var newArg = eval(document.calculator.display.value)
  // Do the operation
  if (op == "+") {
    prevResult = prevResult + newArg
  }
  else if (op == "-") {
    prevResult = prevResult - newArg
  } 
  else if (op == "/") {
    prevResult = prevResult / newArg
  }
  else if (op == "*") {
    prevResult = prevResult * newArg
  }
  else if (op == "=") {
    prevResult = newArg
  }
  else { // something's wrong
    prevResult = newArg
  }
  // Display the result and update values
  document.calculator.display.value = prevResult
  appendDigits = false
  op = newop
} // doOp

// Function
//   digit(D)
// Description
//   Reacts to the user typing a digi
function digit(dig)
{
  if (appendDigits) {
    document.calculator.display.value += dig
  }
  else {
    document.calculator.display.value = dig
    appendDigits = true
  }
}
// End hiding -->