php 为什么我的数组中的某些值未定义
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/19807953/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Why are some values in my array undefined
提问by Xtreme
I have a for loop in php that adds a number of checkboxes on my page that look like this
我在 php 中有一个 for 循环，它在我的页面上添加了许多看起来像这样的复选框
<input type="checkbox" name="checkbox[]">
I want to use javascript to check which is checked and add value in an Array
我想使用 javascript 来检查哪个被检查并在数组中添加值
var cboxes = document.getElementsByName('checkbox[]');
var len = cboxes.length;
var imageArray = new Array();
for (var i = 0; i < len; i++) {
if (cboxes[i].checked) {
imageArray[i] = cboxes[i].value;
}
}
If I have 50 boxes and click the checkbox number 2,4 and 6, loops through my array, I get the result.
如果我有 50 个框并单击复选框编号 2,4 和 6，循环遍历我的数组，我会得到结果。
for(var i = 0; i < imageArray.length; i++){
gallery.innerHTML += imageArray[i] + "<br>";
}
--
——
undefined
Correct value
undefined
Correct value
undefined
Correct value
If I check number 1, 2, 3 I get the result
如果我检查数字 1、2、3，我会得到结果
Correct value
Correct value
Correct value
Why do I get undefined when I skip a checkbox? How do I fix it
为什么跳过复选框时会出现未定义？我如何解决它
回答by lonesomeday
This is because you are adding extra elements to an array. Take this code, for instance:
这是因为您要向数组添加额外的元素。以这段代码为例：
var a = []; // empty array
a[1] = 'foo'; // don't set a[0]
console.log(a.length); // gives 2
Javascript will always "fill in" gaps in the list of array keys. If you miss some out, Javascript will fill in the blanks with undefined.
Javascript 将始终“填充”数组键列表中的空白。如果您错过了一些，Javascript 将用undefined.
Rather than adding the element to your array by the key name, just pushit onto the end of the array:
不是通过键名将元素添加到数组中，而是将其添加到数组push的末尾：
imageArray.push(cboxes[i].value);
回答by Anthony Grist
You get undefined because you're skipping indexes in imageArray. If the first checkbox isn't checked, it won't put anything in index 0, then because the second checkbox is checked, the first entry is placed into index 1.
你得到 undefined 因为你跳过了imageArray. 如果第一个复选框没有被选中，它不会在索引 0 中放置任何东西，然后因为第二个复选框被选中，第一个条目被放入索引 1。
When you iterate it doesn't skip those missed indexes if there isan index with a value after it, they just don't have a set value so it comes back as undefined.
当你迭代它不会跳过那些错过的指标，如果有是用后一个值的索引，他们只是没有一个设定值，所以它回来的undefined。
You could change this line:
你可以改变这一行：
imageArray[i] = cboxes[i].value;
to:
到：
imageArray.push(cboxes[i].value);
That way it won'tskip indexes when there are unchecked checkboxes.
这样，当有未选中的复选框时，它不会跳过索引。
回答by James Donnelly
It's because you're setting the value of imageArray[i]onlywhen the corresponding checkbox is checked. If you check checkboxes 2, 4 and 6, you're essentially doing this:
这是因为您imageArray[i]仅在选中相应复选框时才设置 的值。如果您选中复选框 2、4 和 6，您实际上是在执行以下操作：
imageArray[1] = cboxes[1].value;
imageArray[3] = cboxes[3].value;
imageArray[5] = cboxes[5].value;
imageArray[0], [2]and [4]are never being set, and thus are undefined.
imageArray[0]，[2]并且[4]永远不会被设置，因此是undefined。
To fix this, either make use of push()to push the values into the imageArray, or simply set dummy values for non-matching keys:
要解决此问题，请使用push()将值推送到 中imageArray，或者简单地为不匹配的键设置虚拟值：
for (var i = 0; i < len; i++) {
if (cboxes[i].checked) {
imageArray[i] = cboxes[i].value;
} else {
imageArray[i] = "";
}
}
The result:
结果：
imageArray[0] = "";
imageArray[1] = cboxes[1].value;
imageArray[2] = "";
imageArray[3] = cboxes[3].value;
imageArray[4] = "";
imageArray[5] = cboxes[5].value;
Alternatively using push():
或者使用push()：
for (var i = 0; i < len; i++) {
if (cboxes[i].checked) {
imageArray.push(cboxes[i].value);
}
}
The result:
结果：
imageArray[0] = cboxes[1].value;
imageArray[1] = cboxes[3].value;
imageArray[2] = cboxes[5].value;
回答by rab
try this
尝试这个
var cboxes = document.getElementsByName('checkbox[]');
var imageArray =[];
for (var i = 0, len = cboxes.length ; i < len; i++) {
if (cboxes[i].checked) {
imageArray.push(cboxes[i].value );
}
}
回答by Deepika Janiyani
You are setting the imageArray with respect to the variable i,
您正在设置关于变量 i 的 imageArray，
the loop is executing each time and hence it is setting undefined value for those elements in the array that are not set,
循环每次都在执行，因此它为数组中未设置的元素设置了未定义的值，
you should use a different loop variable, and increase its value only on successful condition.
您应该使用不同的循环变量，并且仅在成功的情况下才增加其值。
Try modifying loop as below.
尝试修改循环如下。
var j=0;
for (var i = 0; i < len; i++) {
if (cboxes[i].checked == true) {
imageArray[j] = cboxes[i].value;
j++;
}
}
