var greenball = new Image();
var orangeball = new Image();
greenball.src = "gball.gif";
orangeball.src = "oball.gif";
  function getrandom(num)
   {
    num.value = parseInt(Math.random() * 10)
    if (isNaN(num.value))
      {
       num.value = 0
      }
   }
  function newprob()
   {
    document.mathcenter.answer.value = "?"
    getrandom(document.mathcenter.first_number)
    getrandom(document.mathcenter.second_number)
   }
  function startover()
   {
    newprob()
    for (var i = 0; i< (document.images.length - 4); i++)
      {
       document.images[i].src=greenball.src;
      }
    document.mathcenter.hit.value=0
    document.mathcenter.miss.value=0
    document.mathcenter.IDX.value=18
    document.mathcenter.tenDX.value=9
   }
  function setanswer(selection)
   {
    var r
    switch (selection.value)
      {
       case "   1   " :
            r = 1
            break
       case "   2   " :
            r = 2
            break
       case "   3   " :
            r = 3
            break
       case "   4   " :
            r = 4
            break
       case "   5   " :
            r = 5
            break
       case "   6   " :
            r = 6
            break
       case "   7   " :
            r = 7
            break
       case "   8   " :
            r = 8
            break
       case "   9   " :
            r = 9
            break
       default : r = 0
      }
    if (document.mathcenter.answer.value == "?")
      {
       document.mathcenter.answer.value = r;
      }
    else
      {
       document.mathcenter.answer.value = document.mathcenter.answer.value + r
      }
    document.mathcenter.GO.focus()
   }
  function checkanswer()
   {
    if (document.mathcenter.answer.value == "?")
      {
       alert("Please enter an answer")
      }
    else
	  {
       var x, y, z
       x = parseInt(document.mathcenter.first_number.value)
       y = parseInt(document.mathcenter.second_number.value)
       z = parseInt(document.mathcenter.answer.value)
       if (document.mathcenter.sign.value == "+")
	     {
	      if (x + y == z)
	       {
                correct()
	       }
	      else
	       {
                incorrect()
	       }
	     }
	else
         {
	      if (x * y == z)
	       {
                correct()
	       }
	      else
	       {
                incorrect()
	       }
         }  
       }
   }
  function setop(operation)
   {
    document.mathcenter.sign.value = operation.value
    startover()
   }
  function increment(num)
   {
    num.value++
   }
  function decrement(num)
   {
    if (num.value != 0)
	  {
       num.value--
	  }
	else
	  {
	   num.value = 10
	  }
   }
function pointat(call)
  {
   call.src=orangeball.src
  }
function notpointat(ball)
  {
   ball.src=greenball.src
  }
function correct()
  {
 // alert("Nice Job -- Try another one!")
    newprob()
    increment(document.mathcenter.hit)
    decrement(document.mathcenter.IDX)
    if (document.mathcenter.IDX.value != 8)
      {
       document.images[parseInt(document.mathcenter.IDX.value)].src=orangeball.src 
       if ((parseInt(document.mathcenter.IDX.value) >= 9) && (parseInt(document.mathcenter.IDX.value) < 17))
         {
          document.images[parseInt(document.mathcenter.IDX.value) + 1].src=greenball.src
         }
      }
    else
      {
       if (document.mathcenter.tenDX.value != 0)
         {
          document.images[parseInt(document.mathcenter.tenDX.value)].src=greenball.src
          decrement(document.mathcenter.tenDX) 
	  document.images[parseInt(document.mathcenter.tenDX.value)].src=orangeball.src
          document.mathcenter.IDX.value=18
          document.images[9].src=greenball.src
         }
       else
         {
          var total_tries = parseInt(document.mathcenter.hit.value) + parseInt(document.mathcenter.miss.value)
          var percent = parseInt((100/total_tries) * 100)
          alert("Congratulations you achieved " + percent +"%!")
          startover()
         }
       }
   
  }
function incorrect()
  {
   alert("Sorry -- Try again!")
   document.mathcenter.answer.value = "?"
   increment(document.mathcenter.miss)
  }
function clearanswer()
  {
   document.mathcenter.answer.value = "?"
  }