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                        /JavaScript進階篇

一則簡單的加減運算題，找錯誤

<!DOCTYPE?html>
<html>
?<head>
??<title>?事件</title>??
??<script?type="text/javascript">
???function?count(){
???????var?num1=document.getElementById("txt1").value;
???????var?num2=document.getElementById("txt2").value;
???????var?symb=document.getElementById("select").value;
???????var?res;
???????swtich(symb){
???????????case?"+":
???????????????res=parseInt(num1)+parseInt(num2);
???????????????break;
???????????case?"-":
???????????????res=parseInt(num1)-parseInt(num2);
???????????????break;
???????????case?"*":
???????????????res=parseInt(num1)*parseInt(num2);
???????????????break;
???????????case?"/":
???????????????res=parseInt(num1)/parseInt(num2);
???????????????break;
???????}
???????document.getElementById("fruit").value=res;
????//獲取第一個輸入框的值
????//獲取第二個輸入框的值
????//獲取選擇框的值
????//獲取通過下拉框來選擇的值來改變加減乘除的運算法則
????//設置結果輸入框的值?
????
???}
??</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()"/>?<!--通過?=?按鈕來調用創建的函數，得到結果-->?
???<input?type='text'?id='fruit'?/>???
?</body>
</html>

JS進階篇里的一道題，我檢查了下和標準答案只有一處不同。答案里在為number和symbol取值的時候用了取整函數，我是在switch語句里用的，其他應該都一樣。求幫忙找問題在哪，沒有輸出結果。