求大神指教那不行
<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var but4;
var but=document.getElementById("txt1").value;
var but1=document.getElementById("txt2").value;
var but3=document.getElementById("select").value;
switch(but3){
case "+":
but4=parseFloat(txt1)+parseFloat(txt2)
break;
case "-":
but4=parseFloat(txt1)-parseFloat(txt2)
break;
case "*":
but4=parseFloat(txt1)*parseFloat(txt2)
break;
case "/"
but4=parseFloat(txt1)/parseFloat(txt2)
};
document.getElementById("fruit" ).value=but4;
}
</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=' = ' /> <!--通过 = 按钮来调用创建的函数，得到结果-->
<input type='text' id='fruit' />
</body>
</html>
