java Java在foreach中修改元素
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Java Modifying Elements in a foreach
提问by Xenoprimate
I'm learning Java on my own; and therefore the code below has no function other than for learning/testing.
我正在自己学习 Java；因此下面的代码除了学习/测试之外没有其他功能。
Essentially I'm trying to modify the elements of an Integer array (namely, halving them) whilst in a foreach loop.
本质上，我试图在 foreach 循环中修改 Integer 数组的元素（即，将它们减半）。
I should note that I'm not re-ordering, adding, or deleting elements; simply changing their values.
我应该注意，我没有重新排序、添加或删除元素；只是改变他们的价值观。
Here is my code:
这是我的代码：
Logger.describe("Now copying half of that array in to a new array, and halving each element");
Integer[] copyArray = new Integer[DEFAULT_SAMPLE_SIZE / 2];
System.arraycopy(intArray, 0, copyArray, 0, DEFAULT_SAMPLE_SIZE / 2);
for (Integer x : copyArray) x /= 2;
Logger.output(Arrays.deepToString(copyArray));
However, the original array (intArray) is this:
但是，原始数组（intArray）是这样的：
[47, 31, 71, 76, 78, 94, 66, 47, 73, 21]
And the output of copyArray is:
copyArray 的输出是：
[47, 31, 71, 76, 78]
So although the array has been halved in size, the elements (Integers) haven't also been halved in value. So what am I doing wrong?
因此，尽管数组的大小已减半，但元素（整数）的值并未减半。那我做错了什么？
Thank you
谢谢
回答by Mattias Isegran Bergander
You can't do that in a foreach loop.
你不能在 foreach 循环中做到这一点。
for (int i=0; i<copyArray.length;i++)
copyArray[i] /= 2;
Else you are not assigning it back into the array. Integerobjects are immutable by the way so can't modify them (creating new ones though).
否则你不会将它分配回数组。Integer顺便说一下，对象是不可变的，所以不能修改它们（尽管创建新的）。
Updated from comment: Beware though that there are a few things going on, autoboxing/unboxing for example, roughly:
从评论更新：请注意，虽然有一些事情正在发生，例如自动装箱/拆箱，大致如下：
copyArray[i] = Integer.valueOf(copyArray[i].intValue()/2);
回答by Autar
for (int i = 0; i< copyArray.length; i++) {
copyArray[i] = new Integer(x /2);
}
should work.
应该管用。
回答by Ravi Jain
int counter = 0;
for(int x : copyArray)
{
x /= 2;
copyArray[counter++] = x;
}
Your program just modified the value of variable x, not the values within the blocks of array copyArray
您的程序只是修改了变量x的值，而不是数组块中的值copyArray
回答by pvlissidis
I think that you can NOT use the foreach loop construct in order to modify the elements of the array you are iterating. Instead, you need to use a classic for loop like so:
我认为您不能使用 foreach 循环构造来修改您正在迭代的数组的元素。相反，您需要像这样使用经典的 for 循环：
Logger.describe("Now copying half of that array in to a new array, and halving each element");
Integer[] copyArray = new Integer[DEFAULT_SAMPLE_SIZE / 2];
System.arraycopy(intArray, 0, copyArray, 0, DEFAULT_SAMPLE_SIZE / 2);
for (int i = 0; i < copyArray.length; i++) {
copyArray[i] /= 2;
}
Logger.output(Arrays.deepToString(copyArray));
