求大神，为啥不管用，哪里错了？
<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var num1=document.getElementById("tex1").value;
var num2=document.getElementById("tex2").value;
var num3=document.getElementById("select").value;
var num4
switch (num3)
{
case "+":
num4=parseInt(num1)+parseInt(num2)
break;
case "-":
num4=parseInt(num1)-parseInt(num2)
break;
case "*":
num4=parseInt(num1)*parseInt(num2)
break;
default:
num4=parseInt(num1)/parseInt(num2);
}
document.getElementById("fruit").value=num4;
}
</script>
</head>
<body>
<input type='text' id='txt1' />
<select id='select'>
<option value='+'>+</option>
<option value="-">-</option>
<option value="*">*</option>
<option value="/">/</option>
</select>
<input type='text' id='txt2' />
<input type='button' value=' = ' onclick="count()"/> <!--通过 = 按钮来调用创建的函数，得到结果-->
<input type='text' id='fruit' />
</body>
</html>
