[英]Haskell infinite list with alternating signs
I try to generate a infinite list like this: 我尝试生成一个像这样的无限列表:
[1,-2,3,-4..]
with this code: 使用此代码:
[ m | n <- [1..],m <- [n, -n]]
but this code reduplicate all numbers like this: 但是此代码将所有数字重新复制为:
[1,-1,2,-2..]
So i dont know what to do 所以我不知道该怎么办
You can group these as follows: 您可以将这些分组如下:
[[1, -2], [3, -4],..]
So we make tuples of i
where i
makes hops of two, and for such i
, we yield i
and -i-1
: 所以我们做的元组
i
在那里i
做的两跳,和这样的i
,我们得到i
和-i-1
[ m | n <- [1,3..], m <- [n, -n-1]]
then for example we can yield: 那么例如我们可以产生:
Prelude> take 10 [ m | n <- [1,3..], m <- [n, -n-1]]
[1,-2,3,-4,5,-6,7,-8,9,-10]
Or we can write it as: 或者我们可以这样写:
[1, 3..] >>= \i -> [i, -i-1]
We can also work with zipWith
: 我们还可以使用
zipWith
:
zipWith (*) [1..] $ cycle [1, -1]
Or a solution where we iterate over functions: 或我们迭代功能的解决方案:
zipWith ($) (cycle [id, negate]) [1..]
A possibility: 一个潜在可能:
> x = concatMap (\i -> [i,-i-1]) [i | i <- [0..], even i]
> take 10 x
[0,-1,2,-3,4,-5,6,-7,8,-9]
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