DOM练习（邓邓版）

<h4>01.图片切换</h4>
<img width = "100" src = "../img/1.jpg" id = "pic" onclick = "change()"/>
<h4>02.计算器</h4>
<input type = "text" id = "first"/>
<select id = "select">
<option value = "+">+</option>
<option value = "-">-</option>
<option value = "*">*</option>
<option value = "/">/</option>
</select>
<input type = "text" id = "second"/>
<input type = "button" value = "计算" onclick = "calculate()"/>
<input type = "text" id = "result" value = ""/>
<h4>03.复选框</h4>
<label for = "ymq">羽毛球</label><input type = "checkbox" name = "hobby" id = "ymq"/>
<label for = "yy">游泳</label><input type = "checkbox" name = "hobby" id = "yy"/>
<label for = "lq">篮球</label><input type = "checkbox" name = "hobby" id = "lq"/>
<input type = "button" value = "全选" onclick = "All()"/>
<input type = "button" value = "反选" onclick = "Other()"/>
<input type = "button" value = "取消" onclick = "Clear()"/>

var times = 1;
function change() {
    var pic = document.getElementById("pic");
    if (times == 1) {
        pic.src = "../img/2.jpg";
        times = 2;
    } else if (times == 2) {
        pic.src = "../img/3.jpg";
        times = 3;
    } else {
        pic.src = "../img/1.jpg";
        times = 1;
    }
}
function calculate() {
    var first = document.getElementById("first").value;
    var second = document.getElementById("second").value;
    var select = document.getElementById("select").value;
    var result = document.getElementById("result");
    switch (select) {
    case "+":
        result.value = (first-0) + (second-0);
        break;
    case "-":
        result.value = first - second;
        break;
    case "*":
        result.value = first * second;
        break;
    case "/":
        result.value = first / second;
        break;
    default:break;
    }
}
function All(){
    var hobby = document.getElementsByName("hobby");
    for(var i = 0; i < hobby.length; i++){
        hobby[i].checked = true;
    }
}
function Other(){
    var hobby = document.getElementsByName("hobby");
    for(var i = 0; i < hobby.length; i++){
        if(hobby[i].checked === false){
            hobby[i].checked = true;
        }else{
            hobby[i].checked = false;
        }
    }
}
function Clear(){
    var hobby = document.getElementsByName("hobby");
    for(var i = 0; i < hobby.length; i++){
        hobby[i].checked = false;
    }
}