在列表中查找值的所有索引 [英] Find all indexes of a value in a List

问题描述

我正在尝试在 ArrayList 中搜索用户输入.我设法创建了一个搜索，该搜索从列表中打印出第一次出现的索引.

我在尝试获取存储该项目的其余索引时遇到麻烦.

这是到目前为止我可以打印 search 的第一个索引的代码:

  if(names.contains(search)){System.out.println(找到名称！")；System.out.println(names.indexOf(search));} 

我知道需要添加一个循环.但是我在尝试制定它时遇到了麻烦.

示例

  ArrayList< String>名称=新的ArrayList< String>();names.add(鲍勃")；names.add("Jerry")；names.add(鲍勃")；names.add("Mick")； 

说 search ="Bob" .我的预期结果将是 {0,2} .相反，我只能获取第一次出现的索引( 0 ).

 声明allIndexesOf(names，"Bob").equals(List.of(0，2));[...]私有列表< Integer>allIndexesOf(List<?>列表，对象o){//如何实现?} 

如何获取与搜索字符串匹配的所有索引?

解决方案

说明

方法 List#indexOf 仅返回找到的第一个匹配元素的索引.从其文档中:

返回指定元素在此列表中首次出现的索引；如果此列表不包含该元素，则返回-1.[...]

但是您要全部，因此还需要重复所有元素.

还请注意，不必调用 List#contains ，因为 List#indexOf 也会回答此问题，如果未找到，它将返回 -1 .实际上，在 ArrayList 中，这两个调用都非常昂贵(它们从左到右进行迭代直到找到)，因此如果它们如此昂贵，则不应使用不必要的语句.

---

解决方案

相反，只需迭代所有元素并收集匹配的元素:

  ArrayList< String>作者= ...线针= ...//收集比赛List< Integer>matchingIndices = new ArrayList<>();for(int i = 0; i< author.size(); i ++){字符串元素= author.get(i);如果(needle.equals(element)){matchingIndices.add(i);}}//打印比赛matchingIndices.forEach(System.out :: println); 

或者您可以使用 Stream API 的一些非常便捷的方法. Stream#filter (if (names.contains(search)) { System.out.println("name found!"); System.out.println(names.indexOf(search)); }

I understand that a loop needs to be added. But I am having trouble trying to formulate it.

Example

ArrayList<String> names = new ArrayList<String>();
names.add("Bob");
names.add("Jerry");
names.add("Bob"); 
names.add("Mick");

Say search = "Bob". My expected result would be {0,2}. Instead, I am only able to get the index of the first occurrence (0).

assert allIndexesOf(names, "Bob").equals(List.of(0, 2));

[...]

private List<Integer> allIndexesOf(List<?> list, Object o) {
  // How can this be implemented?
}

How can I get all indexes that match the search string?

解决方案

Explanation

The method List#indexOf only returns the index of the first found matching element. From its documentation:

Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. [...]

But you want all, therefore you also need to iterate all elements.

Also note that calling List#contains is not necessary since List#indexOf also answers this question, it returns -1 if not found. In fact in an ArrayList both calls are very expensive (they iterate from left to right until found) so you shouldn't use unnecessary statements if they are such expensive.

---

Solution

Instead just iterate all elements and collect the ones that match:

ArrayList<String> author = ...
String needle = ...

// Collect matches
List<Integer> matchingIndices = new ArrayList<>();
for (int i = 0; i < author.size(); i++) {
    String element = author.get(i);

    if (needle.equals(element)) {
        matchingIndices.add(i);
    }
}

// Print matches
matchingIndices.forEach(System.out::println);

Or you may use some of the very convenient methods of the Stream API. Stream#filter (documentation) for example:

List<Integer> matchingIndices = IntStream.range(0, author.size())
    .filter(i -> needle.equals(author.get(i))) // Only keep those indices
    .collect(Collectors.toList());

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