                        課程
                    
                        /前端開(kāi)發(fā)
                        
                            /JavaScript
                        
                        /JavaScript進(jìn)階篇

為何加法運(yùn)算不正確

<!DOCTYPE?html>
<html>
?<head>
??<title>?事件</title>??
??<script?type="text/javascript">
????function?count(){
????var?a=document.getElementById("txt1").value;//取出id賦值給a
????var?b=document.getElementById("txt2").value;
????var?c=document.getElementById("select").value;
	var?d;
	switch(c){//選擇的符號(hào)
		case"+":
		d=a+b;
		break;
		case"-":
		d=a-b;
		break;
		case"*":
		d=a*b;
		break;
		case"/":
		if(b==0){
			alert("除數(shù)不能為0。");

			}
			else{
		d=a/b;
			}
		
		}	
		document.getElementById("fruit").value=d;
????//獲取第一個(gè)輸入框的值
	//獲取第二個(gè)輸入框的值
	//獲取選擇框的值
	//獲取通過(guò)下拉框來(lái)選擇的值來(lái)改變加減乘除的運(yùn)算法則
????//設(shè)置結(jié)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(guò)?=?按鈕來(lái)調(diào)用創(chuàng)建的函數(shù)，得到結(jié)果-->?
???<input?type='text'?id='fruit'?/>???
?</body>
</html>