function checkNumber(input, min, max, msg) {

        msg = msg + " field has invalid data: ";

        var str = input.value;
        for (var i = 0; i < str.length; i++) {
            var ch = str.substring(i, i + 1)
            if ((ch < "0" || "9" < ch) && ch != '.') {
                alert(msg);
                return false;
            }
        }
        var num = 0 + str
        if (num < min || max < num) {
            alert(msg + " Range: " + min + " to " + max );
            return false;
        }
        input.value = str;
        return true;
    }

    function computeField(input) {

        if (input.value != null && input.value.length != 0)
            input.value = "" + eval(input.value);
        computeForm(input.form);
    }

    function computeForm(form) {

        if ((form.term.value == null || form.term.value.length == 0) ||
            (form.apr.value == null || form.apr.value.length == 0) ||
            (form.loan.value == null || form.loan.value.length == 0)) {
            alert("Oops! Some fields are emtpy or not valid!");
			return;
        }

        if (!checkNumber(form.term, 1, 30, "Term of payments (years)") ||
            !checkNumber(form.apr, .001, 99, "Interest Rate") ||
            !checkNumber(form.loan, 10000, 9000000, "Amount")) {
            //form.payment.value = "Invalid";
            return;
        }

        var i = form.apr.value;
        
        if (i > 1.0) {
            i = i / 100.0;
            form.apr.value = i;
        }
        i /= 12;

        var pow = 1;
		nPmt = form.term.value * 12;
        for (var j = 0; j < nPmt; j++)
            pow = pow * (1 + i);
        monthly = (form.loan.value * pow * i) / (pow - 1)
		form.mp.value=Math.round(monthly*100)/100;
    }

    function clearForm(form) {
        form.term.value = "";
        form.apr.value = "";
        form.loan.value = "";
    }