英文:
Why is my while(matcher.find()) entering an infinite loop? I use it in with Streams
问题
我想要计算在地图中的 "ei" 实例次数（特别是使用流）。
public static void main(String[] args) {
List<String> L5 = List.of("Einstein", "ist", "kein", "Stein");
Pattern P = Pattern.compile("[Ee]i");
Map<String, Integer> result = L5.stream().filter(S -> P.matcher(S).find()).collect(
Collectors.toMap(i -> i, i -> {
int count = 0;
while (P.matcher(i).find()) {
count++;
}
return count;
})
);
System.out.println(result);
}
这段代码中，它使用流来转换字符串列表为一个地图，并计算 "ei" 出现的次数。
英文:
What I want: [”Einstein”, “ist”, “kein”, “Stein”] ⇒ {”Einstein”=2, “kein”=1, “Stein”=1}
Using a Stream I want to transform a list of strings to a map finding all the "ei" instances.
My Code is stuck in an infinite loop:
public static void main(String[] args) {
List<String> L5 = List.of("Einstein", "ist", "kein", "Stein");
Pattern P = Pattern.compile("[Ee]i");
Map<String, Integer> result = L5.stream().filter(S -> P.matcher(S).find()).collect(
Collectors.toMap(i -> i, i -> {
int count = 0;
while (P.matcher(i).find()) {
count++;
}
return count;
})
);
System.out.println(result);
}
I want to count the instances of "ei" in a map (specifically using streams)
答案1
得分: 2
问题出在代码的这一行：while (P.matcher(i).find())，每次循环检查条件时，都会调用P.matcher(i)并创建一个新的匹配器实例来调用find()，当然，它每次都返回true，因此变成了一个无限循环。
为了解决这个问题，首先将P.matcher(i)分配给一个变量，以便在同一个Matcher实例上调用find()。
...
Matcher matcher = P.matcher(i);
while (true) {
if (!matcher.find()) break;
count++;
}
return count;
...
一个改进是我们可以利用Matcher.results()来获取计数，如下所示：
Matcher matcher = P.matcher(i);
return (int) matcher.results().count();
英文:
The problem is on line while (P.matcher(i).find()), each time the while loop checking the condition, P.matcher(i) is invoked and a new matcher instance is created to call find(), of course it return true every time, hence it becomes infinite loop.
To solve the problem, assign P.matcher(i) to a variable first so find() is invoked on the same Matcher instance.
...
Matcher matcher = P.matcher(i);
while (true) {
if (!matcher.find()) break;
count++;
}
return count;
...
One improvement is we can make use of Matcher.results() to get the count, as below:
Matcher matcher = P.matcher(i);
return (int) matcher.results().count();
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