[英]Haskell: what does this method do
In my test-exam a question was, what this method does. 在我的考试中,一个问题是,这个方法做了什么。
dos a = ([x | x <- [2..div a 2], mod ax == 0] == [])
I am new to Haskell but as far as I can say, it checks if the result of dos a = ([x | x <- [2..div a 2], mod ax == 0])
is an empty list. 我是Haskell的新手,但据我所知,它检查
dos a = ([x | x <- [2..div a 2], mod ax == 0])
是否为空列表。 Also x are all numbers of a
divided by 2 which have %number == 0. Thus this are all even numbers? 也可为x是的所有数目
a
由2具有数%== 0,因此这是所有偶数分割? It seems like it checks if the number is dividable through 2, if yes -> false, else otherwise. 它似乎检查数字是否可以通过2分割,如果是 - >假,否则。 Could anyone explain to me the semantic in detail?
任何人都可以向我详细解释语义吗?
You are close to what is going on. 你接近正在发生的事情。 There are several components to understand.
有几个组件需要了解。
First, [2 .. div a 2]
generates a list of numbers from 2 to floor(a / 2)
. 首先,
[2 .. div a 2]
生成从2到floor(a / 2)
的数字列表。
Next, mod ax == 0
filters out the values from 2 to floor(a / 2)
which divide a
(eg it finds all the factors of a
). 接着,
mod ax == 0
过滤出从2值到floor(a / 2)
其划分a
(例如它找到的所有因素a
)。 Thus, the list generated by 因此,生成的列表
[x | x <- [2 .. div a 2], mod a x == 0]
contains all the numbers that divide a
. 包含除以
a
所有数字。
Finally, the == []
checks that this list is empty (eg a
has no factors). 最后,
== []
检查此列表是否为空(例如, a
没有因素)。 So, what this function actually does is to determine whether or not a number is prime by attempting to generate its factors, which is easy to see when you use dos
as the predicate for filter: 所以,这个函数实际上做的是通过尝试生成其因子来确定数字是否为素数,当您使用
dos
作为过滤器的谓词时很容易看到:
Prelude> let dos a = ([x | x <- [2..div a 2], mod a x == 0] == [])
Prelude> :t dos
dos :: Integral t => t -> Bool
Prelude> filter dos [2 .. 100]
[2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97] -- Prime goodness
It is the basic algorithm to check if a number is prime or not. 它是检查数字是否为素数的基本算法。 It traverses over all the numbers from
2
to a/2
and checks if any of it divides a
, if the list is empty then it means it has no factors between 2
and a/2
which implies the number is prime. 它遍历从
2
到a/2
所有数字并检查其中是否有任何一个除以a
,如果列表为空则表示它没有2
和a/2
之间的因子,这意味着数字是素数。
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