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Teacher's comments：switch中判断能否用更加直观的方式？
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<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8" /><meta http-equiv="X-UA-Compatible" content="IE=edge" /><meta name="viewport" content="width=device-width, initial-scale=1.0" /><title>异步计数器</title><style>form input {width: 80px;}</style></head><body><form id="cal" action="calculate.php" onsubmit="return false"><input type="number" name="num1" /><select name="opt" id=""><option value="1">+</option><option value="2">-</option><option value="3">*</option><option value="4">/</option><option value="5">%</option></select><input type="number" name="num2" />= <span>?</span><button>计算</button></form><script>const btn = document.querySelector("button");btn.addEventListener("click", calClick);async function calClick() {const num1 = document.forms.cal.num1.value;const num2 = document.forms.cal.num2.value;const opt = document.forms.cal.opt.value;const response = await fetch("calculate.php?num1=" + num1 + "&opt=" + opt + "&num2=" + num2);const comments = await response.json();const res = document.querySelector("form span");res.textContent = comments;}</script></body></html>
<?php$num1=$_GET['num1'];$num2=$_GET['num2'];$opt=$_GET['opt'];$res=0;switch($opt){case "1":$res = $num1 + $num2;break;case "2":$res = $num1 - $num2;break;case "3":$res = $num1 * $num2;break;case "4":if($num2==0){$res="除数不能是0";break;}$res = $num1 / $num2;break;case "5":if($num2==0){$res="除数不能是0";break;}$res = $num1 % $num2;break;}echo json_encode($res);
