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老师批语：
```html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>简单计算器</title>
</head>
<body>
<div>
<input type="text" size="5" id="num1" value="" onchange="fun(this.id)"/>
<select id="ysf">
<option value="+">+</option>
<option value="-">-</option>
<option value="*">*</option>
<option value="/">/</option>
<option value="%">%</option>
</select>
<input type="text" size="5" id="num2" value="" onchange="fun(this.id)"/>
<input type="button" value="计算" id="btn"/>
</div>
<p id="res"></p>
<script>
// 设置两个输入框只能输入数字
function fun(id) {
var patrn = /^\d+(.\d+)?$/;//正则表达式匹配规则
var num = document.getElementById(id);
if (!patrn.exec(num.value)) {
alert(“只能输入数字！”);
num.value = “”;
}
}
//获取按钮注册事件
document.getElementById(‘btn’).onclick=function(){
var num1 = document.getElementById(‘num1’);
var num2 = document.getElementById(‘num2’);
var ysf = document.getElementById(‘ysf’);
var res = document.getElementById(‘res’);
switch (ysf.value){
case “+”:
res = parseFloat(num1) + parseFloat(num2.value);
break;
case “-“:
res = parseFloat(num1.value) - parseFloat(num2.value);
break;
case ““:
res = parseFloat(num1.value) parseFloat(num2.value);
break;
case “/“:
res = parseFloat(num1.value) / parseFloat(num2.value);
break;
case “%”:
res = parseFloat(num1.value) % parseFloat(num2.value);
break;
}
res.value = res
document.getElementById(‘res’).innerHTML=num1.value +ysf.value+num2.value+”=”+res;
};
</script>
</body>
</html>
```
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