问题描述
首先,我是一名JavaScript程序员,对Java8很新,并尝试新功能。
Firstly, I'm a JavaScript programmer, and fairly new to Java8 and trying the new functional feature.
由于我专注于JS编码,我实现了自己的JS用于概念验证的惰性函数库。
Since I expertise JS coding, I implemented my own JS lazy-functional library for proof of concept.
使用该库,我可以编写自然数的无限序列和Fibonacci 下方:
Using the library, I could write Infinite sequence of Natural numbers and Fibonacci as below:
JavaScript
var spacetime = require('./spacetime');
var _ = spacetime.lazy();
var natural = _(function(n) //memoized automatically
{
return n; // Natural numbers is defined as the `n`th number becomes `n`
});
var natural10 = _(natural)
.take(10)
.compute(function(x)
{
console.log(x);
});
//wrap a recursive function to memoize
// must be at the definition in the same scope
var fib = _(function(n)
{
if (n <= 1)
return 1; // as the Fib definition in Math
else
return fib(n - 2) + fib(n - 1); // as the Fib definition in Math
});
var fib10 = _(fib)
.take(10)
.compute(function(x)
{
console.log(x);
});
足够清楚。关键是我可以将Natural / Fibonacci无限序列定义为数学定义,然后使用延迟求值来计算无限序列的必需部分。
Clear enough. The point is that I can define Natural/Fibonacci infinite sequence as the math definition as it is, then later compute the required part of the infinite sequence with lazy-evaluation.
所以,现在我想知道我是否可以用Java8做同样的方式。
So, now I wonder if I can do the same manner with Java8.
对于自然序列,我在这里发布了另一个问题。
For natural sequence, I had post another Question here.
定义Natural序列的一种方法是使用Java8的 iterator
:
One of the way to define Natural sequence is to use iterator
of Java8:
Java8
IntStream natural = IntStream.iterate(0, i -> i + 1);
natural
.limit(10)
.forEach(System.out::println);
我观察 IntStream natural = IntStream.iterate(0,i - > i + 1);
是数学意义上的自然数的公平定义。
I observe IntStream natural = IntStream.iterate(0, i -> i + 1);
is a fair definition of natural numbers in math sense.
然而,我想知道是否可以将其定义为I以前做过,就是
However, I wonder if it's possible to define it as I did before, that is,
JavaScript
var natural = _(function(n) //memoized automatically
{
return n; // Natural numbers is defined as the `n`th number becomes `n`
});
因为这看起来更简洁。不幸的是,答案表明即使我们使用生成
也可能无法实现。
because this looks more concise. Unfortunately, the answers suggest it's probably not possible even we use generate
.
此外, IntStream.iterate
不适合Fibonacci序列。
In addition, IntStream.iterate
does not fit for Fibonacci sequence.
我寻求网络生成
不确定的Fibonacci序列,我发现的最佳结果是
I seek web to generate
indefinite sequence of Fibonacci, the best results I found are
Java8
private static Map<Integer,Long> memo = new HashMap<>();
static {
memo.put(0,0L); //fibonacci(0)
memo.put(1,1L); //fibonacci(1)
}
//And for the inductive step all we have to do is redefine our Fibonacci function as follows:
public static long fibonacci(int x) {
return memo.computeIfAbsent(x, n -> fibonacci(n-1) + fibonacci(n-2));
}
这不是无限序列(懒惰 Stream
in Java8)。
This is not an infinite sequence (lazy Stream
in Java8).
和
Java8
Stream.generate(new Supplier<Long>() {
private long n1 = 1;
private long n2 = 2;
@Override
public Long get() {
long fibonacci = n1;
long n3 = n2 + n1;
n1 = n2;
n2 = n3;
return fibonacci;
}
}).limit(50).forEach(System.out::println);
这是一个无限序列(懒惰流
在Java8)中,您可以说它被定义为Math。
但是我不喜欢这个实现,因为正如你所看到的,有许多内部有价值的东西可以获得序列,例如 n1
n2
n3
然后斐波纳契
,因此代码结构很复杂,你需要控制可变状态,这是反功能方式 - 与数学定义不同,可能这不是记忆。
This is an infinite sequence (lazy Stream
in Java8), and you could say it's defined as Math.However I do not like this implementation because, as you can see, there are many internal valuable to obtain the sequence such as n1
n2
n3
then fibonacci
, accordingly the code structure is complicated and you need to control mutable state which is anti-functional manner - unlike the math definition, and probably this is not memoized.
所以,这是我的问题。使用Java8 Stream
,有没有办法编写代码以简明的数学方式定义斐波纳契的无限序列,并具有类似
So, here's my question. With Java8 Stream
, is there any way to write a code to define the infinite sequence of fibonacci in concise math manner with memoization like
JavaScript
var fib = _(function(n)
{
if (n <= 1)
return 1; // as the Fib definition in Math
else
return fib(n - 2) + fib(n - 1); // as the Fib definition in Math
});
感谢您的想法。
推荐答案
您可以使用基于地图的memoized fibonacci(x)并从中创建无限流:
You can take your map-based memoized fibonacci(x) and make an infinite stream out of it like this:
LongStream fibs = IntStream.iterate(1, i->i+1).mapToLong(i -> fibonacci(i));
但是,制作无限的斐波纳契数字流的最简单方法是这样的:
But the easiest way to make an infinite stream of fibonacci numbers is like this:
LongStream fibs = Stream.iterate(
new long[]{1, 1},
f -> new long[]{f[1], f[0] + f[1]}
).mapToLong(f -> f[0]);
正如您所链接的文章指出的那样,无限实际上意味着直到长时间溢出很快。如果你想生成数百个斐波纳契数,请用BigInteger代替long:
As the article you linked to points out, "infinite" really means "until long overflows" which happens quickly. If you want to generate hundreds of fibonacci numbers, replace long with BigInteger:
Stream<BigInteger> bigFibs = Stream.iterate(
new BigInteger[]{BigInteger.ONE, BigInteger.ONE},
f -> new BigInteger[]{f[1], f[0].add(f[1])}
).map(f -> f[0]);
这篇关于Java 8中记忆的无限Fibonacci序列的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持!