新手问题，求帮忙解答

[duanye252]

<SCRIPT LANGUAGE="JavaScript">
<!-- Begin
var FKeyPad = document.Keypad;
var Accum = 0;
var FlagNewNum = false;
var PendingOp = "";
function NumPressed (Num) {
if (FlagNewNum) {
FKeyPad.ReadOut.value  = Num;
FlagNewNum = false;
   }
else {
if (FKeyPad.ReadOut.value == "0")
FKeyPad.ReadOut.value = Num;
else
FKeyPad.ReadOut.value += Num;
   }
}
function Operation (Op) {
var Readout = FKeyPad.ReadOut.value;
if (FlagNewNum && PendingOp != "=");
else
{
FlagNewNum = true;
if ( '+' == PendingOp )
Accum += parseFloat(Readout);
else if ( '-' == PendingOp )
Accum -= parseFloat(Readout);
else if ( '/' == PendingOp )
Accum /= parseFloat(Readout);
else if ( '*' == PendingOp )
Accum *= parseFloat(Readout);
else
Accum = parseFloat(Readout);
FKeyPad.ReadOut.value = Accum;
PendingOp = Op;
   }
}
function Decimal () {
var curReadOut = FKeyPad.ReadOut.value;
if (FlagNewNum) {
curReadOut = "0.";
FlagNewNum = false;
   }
else
{
if (curReadOut.indexOf(".") == -1)
curReadOut += ".";
   }
FKeyPad.ReadOut.value = curReadOut;
}
function ClearEntry () {
FKeyPad.ReadOut.value = "0";
FlagNewNum = true;
}
function Clear () {
Accum = 0;
PendingOp = "";
ClearEntry();
}
function Neg () {
FKeyPad.ReadOut.value = parseFloat(FKeyPad.ReadOut.value) * -1;
}
function Percent () {
FKeyPad.ReadOut.value = (parseFloat(FKeyPad.ReadOut.value) / 100) * parseFloat(Accum);
}
// End -->
</SCRIPT>

这是一个计算器的例子，我想问if (FlagNewNum) {
FKeyPad.ReadOut.value = Num;
FlagNewNum = false;
}
这段不就是给FKeyPad.ReadOut.value赋值吗，为什么要判断FlagNewNum 呢？我把这段重新写了下

<script type="text/javascript">
	var FKeyPad = document.Keypad;
		function NumPressed (Num) {
		FKeyPad.ReadOut.value  = Num;
		if (FKeyPad.ReadOut.value == "0")
			{
				FKeyPad.ReadOut.value = Num;
			}
		else
			{
				FKeyPad.ReadOut.value = FKeyPad.ReadOut.value + Num;
			}
   
			}
</script>

比如点7就出现77再点就没反应，求详解。

[酷儿]

一个是为0的时候赋值 一个叠加赋值