英文:
How do I loop through a list of lists in order to manipulate the contents of the inner lists?
问题
我想循环遍历一个列表的列表，并访问内部列表项，以便有条件地向列表中添加项目。我想象中的代码大致如下：
for 每个子列表 in 主列表：
如果 子列表[4] = "x" 并且 子列表[5] = "y"，
子列表.添加("z")
我在添加到列表或比较字符串方面没有问题，只是不知道如何从循环遍历主列表转到检查子列表中的项目。
英文:
I'd like to loop through a list of lists and access the inner-list items in order to append an item to the list conditionally. I'm imagining something along the lines of
for each sublist in mainlist,
if sublist[4] = "x" and sublist[5] = "y",
sublist.add("z")
I have no issue adding to the list or comparing the strings, I just don't know how to get from looping through the main list to checking the item's in the sublists.
答案1
得分: 0
以下是一个非常简单的示例，用于在Java中循环遍历一个列表的列表：
import java.util.ArrayList;
import java.util.List;
public class Main {
public static void main(String[] args) {
List<ArrayList<String>> mainList = new ArrayList<ArrayList<String>>();
ArrayList<String> subList1 = new ArrayList<String>();
subList1.add("A");
subList1.add("B");
mainList.add(subList1);
ArrayList<String> subList2 = new ArrayList<String>();
subList2.add("C");
subList2.add("D");
mainList.add(subList2);
for (int i = 0; i < mainList.size(); i++) {
for (int j = 0; j < mainList.get(i).size(); j++) {
System.out.print(mainList.get(i).get(j) + " ");
}
System.out.println();
}
// Using for each
System.out.println("Using for-each");
mainList.forEach((list) -> {list.forEach((alphabet)->System.out.println(alphabet)); });
}
}
输出示例：
A B
C D
Using for-each
A
B
C
D
现在您可以放置您想要的条件。
英文:
Here is a very simple example to loop through a List of lists in Java :
import java.util.ArrayList;
import java.util.List;
public class Main {
public static void main(String[] args) {
List<ArrayList<String>> mainList = new ArrayList<ArrayList<String>>();
ArrayList<String> subList1 = new ArrayList<String>();
subList1.add("A");
subList1.add("B");
mainList.add(subList1);
ArrayList<String> subList2 = new ArrayList<String>();
subList2.add("C");
subList2.add("D");
mainList.add(subList2);
for (int i = 0; i < mainList.size(); i++) {
for (int j = 0; j < mainList.get(i).size(); j++) {
System.out.print(mainList.get(i).get(j) + " ");
}
System.out.println();
}
// Using for each
System.out.println("Using for-each");
mainList.forEach((list) -> {list.forEach((alphabet)->System.out.println(alphabet)); });
}
}
Sample Output :
A B
C D
Using for-each
A
B
C
D
Now you can place your desired conditions.
答案2
得分: 0
根据问题中发布的代码，它正按照所需方式进行操作。mainlist是一个列表的列表，sublist是字符串的列表，在子列表中添加z会更新主列表，因为您是在引用上进行更新。
这个Java版本可能是：
for (List<String> subList : mainList) {
if (null != sublist && !sublist.isEmpty() && sublist.get(4).equals("x") &&
sublist.get(5).equals("y")) {
sublist.add("z");
}
}
英文:
As per the code posted in question, it is exactly doing what is required. The mainlist being a list of lists, sublist being a list of strings, adding z to the sublist updates it in the main list since you are updating on the reference.
The java version for this could be:
for (List <String> subList: mainList) {
if (null != sublist && !sublist.isEmpty() && sublist.get(4).equals("x") &&
sublist.get(5).equals("y")) {
sublist.add("z");
}
}
答案3
得分: 0
请尝试以下方式：
public static void main(String[] args) {
List<List<String>> parentList = new ArrayList();
List<String> listA = new ArrayList<String>();
listA.add("listA-item1");
listA.add("listA-item2");
listA.add("listA-item3");
List<String> listB = new ArrayList<String>();
listB.add("listB-item1");
listB.add("listB-item2");
listB.add("listB-item3");
parentList.add(listA);
parentList.add(listB);
for (List<String> list : new ArrayList<>(parentList)) {
for (String s : new ArrayList<>(list)) {
list.remove(s);
s += " appended thing";
list.add(s);
}
}
listA.forEach(System.out::println);
listB.forEach(System.out::println);
}
输出结果为：
listA-item1 appended thing
listA-item2 appended thing
listA-item3 appended thing
listB-item1 appended thing
listB-item2 appended thing
listB-item3 appended thing
请注意，在foreach语句中我创建了新的ArrayList：
for (List<String> list : new ArrayList<>(parentList)) {
for (String s : new ArrayList<>(list)) {
如果不这样做，会出现java.util.ConcurrentModificationException异常。
英文:
Try this way:
public static void main(String[] args) {
List<List<String>> parentList = new ArrayList();
List<String> listA = new ArrayList<String>();
listA.add("listA-item1");
listA.add("listA-item2");
listA.add("listA-item3");
List<String> listB = new ArrayList<String>();
listB.add("listB-item1");
listB.add("listB-item2");
listB.add("listB-item3");
parentList.add(listA);
parentList.add(listB);
for (List<String> list : new ArrayList<>(parentList)) {
for (String s : new ArrayList<>(list)) {
list.remove(s);
s += " appended thing";
list.add(s);
}
}
listA.forEach(System.out::println);
listB.forEach(System.out::println);
}
and the output is :
listA-item1 appended thing
listA-item2 appended thing
listA-item3 appended thing
listB-item1 appended thing
listB-item2 appended thing
listB-item3 appended thing
Just take attention to the fact that I am creating new ArralyList in foreach statement :
for (List<String> list : new ArrayList<>(parentList)) {
for (String s : new ArrayList<>(list)) {
if you do not do that you will get java.util.ConcurrentModificationException
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