借助表单和Javascript来制作计算器

调了很久还是不明白为什么左后不显示结果，求指导，感谢！


<html>
<head><title>计算器title>
<script>
    function f() {
        var one = document.first.one.value;
        var two = document.frist.two.value;
        var char;
        var a=document.getElementsByName("char");
        var result = 0;
        for(var i=0;i<=3;i++){
            if(a[i].checked){
                char=a[i].value;
            }
        }
        switch (char) {
            case "=":
                result = one + two;
                break;
            case "-":
                result = one - two;
                break;
            case "*":
                result = one * two;
                break;
            case "/":
                if ( one == 0) {                      
                    alert("被除数不能为0！")
                    document.second.char.focus
                }
                else {
                    result = one / two;                          
                }
                break;
            default:
                result = "选择正确的运算符";
                break;
        }
        document.third.result.value =result;
    }
script>
head>
<body>
    <form name="first"><center>
        请输入第一个操作数：<input type="text" name="one" value=""><p>

        请输入第二个操作数：<input type="text" name="two" value=""><p>
    center>form>

    <form><center>
        请选择操作
        <input type="radio" name="char" value="+" checked="">+
        <input type="radio" name="char" value="-" >-
        <input type="radio" name="char" value="*" >*
        <input type="radio" name="char" value="/" >/
    center>form>
        
    <form name="third"><center>
        <input type="button" value="=" onclick="f()"><p>
        
        结果是：
        <input type="text" name="result" value="">

    center>form>
body>
html>

first写错了 ，name换成id

img

img

 
<html>
<head>
 <title>计算器</title>
<script>
    function f() {
        var one = document.first.one.value;
        var two = document.first.two.value;
        var char;
        var a=document.getElementsByName("char");
        var result = 0;
        for(var i=0;i<=3;i++){
            if(a[i].checked){
                char=a[i].value;
            }
        }
        switch (char) {
            case '+':
                result = Number(one) + Number(two);
                break;
            case '-':
                result = one - two;
                break;
            case '*':
                result = one * two;
                break;
            case '/':
                if ( one == 0) {                      
                    alert("被除数不能为0！")
                    document.second.char.focus
                }
                else {
                    result = one / two;                          
                }
                break;
            default:
                result = "选择正确的运算符";
                break;
        }
        document.third.result.value =result;
    }
</script>
</head>
<body>
    <form name="first"><center>
        请输入第一个操作数：<input type="text" id="one" value=""><p>
 
        请输入第二个操作数：<input type="text" id="two" value=""><p>
    </center></form>
 
    <form><center>
        请选择操作
        <input type="radio" name="char" value="+" checked="">+
        <input type="radio" name="char" value="-" >-
        <input type="radio" name="char" value="*" >*
        <input type="radio" name="char" value="/" >/
    </center></form>
        
    <form name="third"><center>
        <input type="button" value="=" onclick="f()"><p>
        
        结果是：
        <input type="text" name="result" value="">
 
    </center></form>
</body>
</html>