英文:
PriorityQueue<Integer> with lambda expression
问题
我试图理解在Java中如何使用lambda函数与堆。下面的函数是用来创建一个最大堆。
PriorityQueue<Integer> pq = new PriorityQueue<>((x, y) -> y - x);
有人能告诉我(x, y) -> y - x)是什么意思吗？
有人告诉我：“这个lambda函数将以两个整数作为输入参数，将它们相互相减，然后返回算术结果。”
所以，如果我执行以下操作：
PriorityQueue<Integer> pq = new PriorityQueue<>((x, y) -> y - x);
pq.add(9);
pq.add(5);
System.out.println(pq.peek());
输出是9，因为这是一个最大堆，但是我不应该得到4作为输出吗？因为(9-5=4)？
英文:
Im trying to understand how lambda functions work with heaps in Java. the function below is to create a max heap
PriorityQueue<Integer> pq = new PriorityQueue<>((x, y) -> y - x);
Can someone tell me what (x, y) -> y - x) does?
Im told that "The lambda function will take two Integers as input parameters, subtract them from each other, and return the arithmetic result."
so if I do
PriorityQueue<Integer> pq = new PriorityQueue<>((x, y) -> y - x);
pq.add(9);
pq.add(5);
System.out.println(pq.peek());
Output is 9 since its a max heap but should I not get 4 as an output since (9-5=4)?
答案1
得分: 4
9-5=4 是 9 的优先级，而 pq.peek() 返回的是值，而不是优先级。
new PriorityQueue<>(comparator); 使用 (x, y) -> y - x 进行比较，确定下一个应该被 peek 的值。
因此，9-5=4 大于 5-9=-4。
所以值 9 将是第一个。
英文:
9-5=4 is the priority of 9, and pq.peek() returns the value, not the priority
new PriorityQueue<>(comparator); uses (x, y) -> y - x to compare and determine which value should be next to be peeked.
So 9-5=4 is greater than 5-9=-4
So the value 9 will be the first one.
答案2
得分: 1
Sure, here's the translated content:
我正在尝试理解Java中lambda函数与堆（heaps）的工作原理。
- Lambda表达式 - 是一种语言特性，类似于匿名函数，可以将其用作值来使用/传递；
- 堆 - 是一种数据结构，最初在数学中定义，在计算机科学领域广泛使用。
有人能告诉我
(x, y) -> y - x做了什么吗？
这是你的比较器，也就是一个实现了Comparator<T>接口的lambda表达式，你将其作为参数传递给了你的优先队列（Priority Queue）构造函数。
然后，PriorityQueue<T> 使用那个比较器，并且在每次从中获取（poll/peek）一个元素时都会使用 int compare(T o1, T o2) 方法。
PriorityQueue<T> 依赖于一个契约，即：
int compare(T o1, T o2) 比较它的两个参数的顺序。如果第一个参数小于、等于或大于第二个参数，则返回负整数、零或正整数。
所以，在你的代码中：
PriorityQueue<Integer> pq = new PriorityQueue<>((x, y) -> y - x);
pq.add(9);
pq.add(5);
System.out.println(pq.peek());
两个数字被入队到堆/PQ中，当你 .peek() 时，9 和 5 分别传递给了 x 和 y。由于 9-5>0，所以返回 true。
输出是9，因为这是一个最大堆，但是我不应该得到4作为输出吗，因为（9-5=4）？
9-5=4 只与比较器有关，用于决定两个元素的比较结果是否为0、正数或负数，如上所述。
英文:
>I'm trying to understand how lambda functions work with heaps in Java.
Lambda Expression has nothing to do with Heap Data Structure, per se.
-
Lambda Expression - is a language feature, acting like an anonymous function, which you can use/pass as a value;
-
Heap - is a Data Structure, originally defined in Mathematics, and nowadays heavily used in the Computer Science domains.
> Can someone tell me what (x, y) -> y - x) does?
That is your comparator, to wit, a lambda expression which implements a Comparator<T>, and which you pass as an argument into your priority queue constructor.
PriorityQueue<T> then makes use of that comparator, and utilizes int compare(T o1, T o2) method, each time you fetch (poll/peek) an element from it.
PriorityQueue<T> relies on a contract, that:
> int compare(T o1, T o2) Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.
So, in your code:
PriorityQueue<Integer> pq = new PriorityQueue<>((x, y) -> y - x);
pq.add(9);
pq.add(5);
System.out.println(pq.peek());
Two numbers are enqueued in the Heap/PQ, and when you .peek(), 9 and 5 are passed into x and y respectively. As 9-5>0 true is returned.
> Output is 9 since its a max heap but should I not get 4 as an output since (9-5=4)?
9-5=4 is only relevant for the comparator, to decide whether the comparison of two elements end up in 0, positive, or negative number, as already explained above.
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论