为什么不执行啊
<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var a = parseInt(document.getElementById('txt1').value);
var b = parseInt(document.getElementById('txt2').value);
var c = document.getElementById('select').value;
var f="";
switch (c){
case '+':
f = parseInt(a) + parseInt(b);
break;
case '-':
f= parseInt(a) - parseInt(b);
break;
case '*':
f= parseInt(a) * parseInt(b);
break;
case '/':
f= parseInt(a) / parseInt(b);
break;
}
f =document.getElementById('fruit').value;
}
</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>
