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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>calculator</title>
<style type="text/css">
.box{
width: 400px;height: 400px;
background-color: #E6E6E6;
margin:50px auto;
text-align: center;
}
table{
margin: 0 auto;
}
td {border: 1px solid E8E8E8;}
input,select,button{width: 100%;height: 100%;border: none;}
button:hover {background-color: coral;cursor: pointer;}
</style>
</head>
<body>
<div class="box">
<h1>计算器</h1>
<form >
<table>
<tr>
<td><input type="text" name="c1" placeholder="第一个数"></td>
<td>
<select name="option">
<option value="Null">请选择</option>
<option value="add">+</option>
<option value="sub">-</option>
<option value="mul">*</option>
<option value="div">/</option>
</select>
</td>
<td><input type="text" name="c2" placeholder="第二个数"></td>
<td><button type="button">计算</button></td>
</tr>
<tr>
<td colspan="2"><h2>计算结果</h2></td>
<td colspan="2"><h2 id="placeholder"></h2></td>
</tr>
</table>
</form>
</div>
<script type="text/javascript">
var c1=document.getElementsByName('c1')[0]
var c2=document.getElementsByName('c2')[0]
var c=document.getElementsByName('option')[0]
var btn =document.getElementsByTagName('button')[0]
var placeholder=document.getElementById('placeholder')
btn.onclick =function(){
if (c1.value.lenth==0){
alert('不能为空')
c1.focus()
return false
} else if (isNaN(c1.value)){
alert('非数字')
c1.focus()
} else if (c2.value.lenth==0){
alert('不能为空')
c2.focus()
return false
} else if (isNaN(c2.value)){
alert('非数字')
c2.focus()
} else {
var data1 = parseFloat(c1.value)
var data2 = parseFloat(c2.value)
}
var option =c.value
var temp= 0
var flag =' '
var result = ''
switch (option){
case 'Null':
alert('请选择操作符')
c.foucs()
return false
case 'add':
flag= '+'
temp=data1+data2
break
case 'sub':
flag= '-'
temp=data1-data2
break
case 'mul':
flag= '*'
temp=data1*data2
break
case 'div':
flag='/'
temp= data1/data2
break
}
placeholder.innerHTML = '<span style="color:coral">'+data1+' '+ flag+' '+ data2+' = '+temp +'</span>'
}
placeholder.innerHTML = str
</script>
</body>
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