求大神指点 哪错了呢！！！！
<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var txt1=document.getElementById("txt1").value;
var txt2=document.getElementById("txt2").value;
var select=document.getElementById("select").value;
var a;
switch(select){
case"+":
a=parseFloat(txt1)+parseFloat(txt2);
break;
case"-":
a=parseFloat(txt1)-parseFloat(txt2);
break;
case"*":
a=parseFloat(txt1)-parseFloat(txt2);
break;
default:
a=parseFloat(txt1)/parseFloat(txt2);
}
document.getElementById("fruit")=a;
}
</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>
