用自定义过滤器快速排序 [英] Quicksort with custom filter
问题描述
我需要制作快速排序,但需要使用自定义筛选器。
在编译过程中,我在过滤器(> = x)xs
中得到一个错误。
- 使用两个过滤器进行排序
quicksort(x:xs)=(quicksort lesser)++ [x] ++(quicksort greater)
其中lesser = filter(< x)xs
greater = filter(> = x)xs
- 使用自定义过滤器
csort f(x:xs)=(csort f lessser)++ [x] ++(csort f更大)
其中lesser = [e | e < - xs,f x e]
greater = [e | e < - xs,不是$ fxe]
有什么不对?
有两个问题我觉得可能会让你烦恼。
首先,加载一个把你的定义文件放到ghci中,我试试看 ghci> csort(> = x)[2,1,3]
< interactive>:1:10:不在范围内:'x'
您编写它的方式 f
需要两个参数。事实上 x
不在这里。因此,自定义过滤器函数应该简单地为(> =)
。
ghci中> csort(> =)[2,1,3]
***例外:blahblah:函数csort中的非穷举模式
现在真正的问题是:非穷举模式。这是什么意思?你已经写了一个关于如何用至少一个元素对列表进行排序的定义。但是,如何排序没有元素的列表?很简单,你忽略了自定义过滤器功能,并简单地生成一个空列表。由于空列表中没有元素,因此它已被排序。
csort _ [] = []
一旦我将该行添加到源文件中,它突然生效。模式 []
表达了模式(x:xs)
,并且这两个模式一起是详尽无遗的list是空的,或者它至少有一个元素)。
ghci> csort(> =)[2,1,3]
[1,2,3]
ghci> csort(<)[2,1,3]
[3,2,1]
ghci> quickCheck(\ xs - > csort(<)xs ==(reverse $ csort(>)xs))
+++ OK,通过了100次测试。
这是我的sort.hs文件:
csort _ [] = []
csort f(x:xs)= csort f lessser ++ [x] ++ csort f greater
where lesser = [e | e < - xs,f x e]
greater = [e | e < - xs,而不是$ f x e]
我不知道为什么您仍然有错误;这对我来说非常合适。
I need to make quicksort but with a custom filter.
During compilation I get an error on filter (>=x) xs
.
--sort with two filters
quicksort (x:xs) = (quicksort lesser) ++[x] ++ (quicksort greater)
where lesser = filter (<x) xs
greater = filter (>=x) xs
--sort with custom filter
csort f (x:xs) = (csort f lesser) ++ [x] ++ (csort f greater)
where lesser = [e | e <- xs, f x e]
greater = [e | e <- xs, not $ f x e]
What is wrong?
There are two problems I think might be troubling you.
First of all, loading a file with your definitions into ghci, I try
ghci> csort (>= x) [2,1,3]
<interactive>:1:10: Not in scope: 'x'
The way you wrote it, f
takes two parameters. And indeed x
is not in scope here. So the custom filter function should be simply (>=)
.
ghci> csort (>=) [2,1,3]
***Exception: blahblah: Non-exhaustive patterns in function csort
Now the real problem: non-exhaustive patterns. What does this mean? You've written a definition of how to sort a list with at least one element. But how do you sort a list with no elements? Simple, you ignore the custom filter function, and simply produce an empty list. Since an empty list has no elements, it is already "sorted".
csort _ [] = []
Once I added that line to the source file, it suddenly worked. The pattern []
compliments the pattern (x:xs)
, and those two patterns, together, are exhaustive (a list is either empty, or it has at least one element).
ghci> csort (>=) [2,1,3]
[1,2,3]
ghci> csort (<) [2,1,3]
[3,2,1]
ghci> quickCheck (\xs -> csort (<) xs == (reverse $ csort (>) xs))
+++ OK, passed 100 tests.
Here's my sort.hs file:
csort _ [] = []
csort f (x:xs) = csort f lesser ++ [x] ++ csort f greater
where lesser = [e | e <- xs, f x e]
greater = [e | e <- xs, not $ f x e]
I have no idea why you would still have errors; this works perfectly well for me.
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