// functions are used for monthly payment calculator
function floor(number){
   return Math.floor(number*Math.pow(10,2))/Math.pow(10,2);
}

function res(){
   var f = document.calc;
   var rate = f.interest.value / 1200;
   var prim = 1;
   var mprim = 1 + rate;
  
   for (i=0; i<f.years.value * 12; i++) {
      prim = prim * mprim;
   }
   
   // check Loan Amount
   var checkAmount = f.amount.value;

   if (!isInteger(checkAmount)) {
      
      alert("Please enter a valid Loan Amount.");
      f.amount.focus();
      return false;
   }
   
   // check Interest Rate

   if (!isDigit(rate)) {
      alert("Please enter a valid Interest Rate.");
      f.interest.focus();
      return false;
   }
   
   // check Loan Term
   var checkYears = f.years.value;

   if (!isInteger(checkYears)) {
      alert("Please enter a valid Loan Term.");
      f.years.focus();
      return false;
   }


   if (f.amount.value=="0"||f.amount.value==null||f.amount.value=="") {
      alert("Please enter Loan Amount");
      f.amount.focus();
      return false;
   }
   else if (f.interest.value=="0"||f.interest.value==null||f.interest.value=="") {
      alert("Please enter Interest Rate.");
      f.interest.focus();
      return false;
   }
   else if (f.years.value=="0"||f.years.value==null||f.years.value=="") {
      alert("Please enter Loan Term.");
      f.years.focus();
      return false;
   }

   else f.total.value = floor(f.amount.value * rate / ( 1 - (1/prim)));   
   return true;
}

function isDigit (c){   
   return ((c >= "0") && (c <= "9"))
}

function isInteger (s) {   
   
   var i;
   
   for (i = 0; i < s.length; i++){   
      var c = s.charAt(i);
      
      if (!isDigit(c)) return false;
    }
    
   return true;
}