function fill_in() {

if (document.loan_form.principal.value == "") {
   document.loan_form.principal.value = 0
   }
if (document.loan_form.down.value == "") {
   document.loan_form.down.value = 0
   }
if (document.loan_form.interest.value == "") {
   document.loan_form.interest.value = 0
   }

}

function calc_loan() {

Err = "";

principal = document.loan_form.principal.value;
principal = principal.replace(/,/, "");
principal = principal * 1;
down = document.loan_form.down.value;
down = down.replace(/,/, "");
down = down * 1;
interest = document.loan_form.interest.value / 1.00;
mon_indx = document.loan_form.months.selectedIndex;

if (mon_indx == 0) {
   months = 1;
   } else if (mon_indx == 1) {
   months = 12;
   } else if (mon_indx == 2) {
   months = 18;
   } else if (mon_indx == 3) {
   months = 24;
   } else if (mon_indx == 4) {
   months = 30;
   } else if (mon_indx == 5) {
   months = 36;
   } else if (mon_indx == 6) {
   months = 48;
   } else if (mon_indx == 7) {
   months = 60;
   } else if (mon_indx == 8) {
   months = 72;
   } else if (mon_indx == 9) {
   months = 84;
   }

if (!principal) {
   Err += "     - Vehicle Cost\n";
   cost = 0;
   } else {
   cost = principal;
   }
 
if (down == "0") {
   //Do Nothing
   } else if (!down) {
   Err += "     - Down Payment\n";
   down = 0;
   }

if (interest == "0") {
   //Do Nothing
   } else if (!interest) {
   Err += "     - Interest Rate\n";
   interest = 0;
   }

Err = "";
if (Err == "" && interest == 0) {
   
   actprincipal = principal - down;
   payments = actprincipal / months;
   tmp = payments + "";
   if (!payments) {
      if (!principal) {
         principal = 0;
         }
      document.loan_form.payments.value = principal;
      } else {
      payments = tmp.split(".")

      if (!payments[1]) {
         deci = "00";
         } else {
         deci = payments[1].substr(0,2);
         }

      if (!payments[0] || !deci || payments[0] == "Infinity") {
         document.loan_form.payments.value = "0";
         } else {
         document.loan_form.payments.value = payments[0] +"."+ deci;
         }
      }
   } else if (Err == "") {

   actprincipal = principal - down;
   j = interest / 1200;
   j1 = j + 1;
   j2 = Math.pow(j1, -months);
   m = actprincipal * (j / (1 - j2));
   m2 = m + "";

   monthlyint = m2.split(".");
   m3 = (m * 100) - (monthlyint[0] * 100);
   m3 = m3 + "";
   monthlydec = m3.split("."); 
   monthlypayments = monthlyint[0] * 1 + monthlydec[0] * 1;

   if (!monthlyint[0]) {
      monthlyint[0] = "0";
      }
   if (!monthlydec[0]) {
      monthlydec[0] = "00";
      }

   dectst = monthlydec[0] + "";
   if (dectst == "0") {
      monthlydec[0] = "00";
      }

   document.loan_form.payments.value = monthlyint[0] +"."+ monthlydec[0];

   } else {

   alert("You entered invalid data for the following: \n"+ Err);

   }
}

function curr_box(n) {
   void document.loan_form.elements(n).select();
   void document.loan_form.elements(n).focus();
   }