Haskell中的空间程序 [英] Interspace program in Haskell
问题描述
insert :: Eq(a) => a -> a -> [a] -> [a]
insert m n [] = []
insert m n (x:xs) | m==x = n : x : insert m n xs
| otherwise = x : insert m n xs
m 的所有实例之前将值 n 插入列表中。
The insert function above is a working function that inserts a value n into a list before all instances of a value m.
我需要编写一个 interspace 函数来插入值 n 在列表中的所有值 m 和 q 之间。这是我到目前为止：
I need help writing an interspace function that inserts a value n between all values m and q in a list. This is what I have so far:
interspace :: Eq(a) => a -> a -> a->[a] -> [a]
interspace m n q[] = []
interspace m n q (x:xs)| m==x && q==(head xs) = n: x : insert m n (headxs)++interspace m n q (xs)
| otherwise = x : interspace m n q xs
推荐答案
要将值添加到列表的前面，您的 insert 函数是不必要的。 （：）就足够了。就像在 insert 中一样，我们递归地通过列表。既然我们想检查两个值是否匹配，并且会根据是否找到匹配在不同的列表上递归调用函数，最好模式匹配（x1：x2： xs）而不仅仅是（x：xs）。
Since you will only be adding values to the front of the list, your insert function is unnecessary. (:) will suffice instead. Much like in insert, we pass recursively over the list. Since we want to check if two values match at a time and will also call the function recursively on different lists based on whether or not we find a match, it's a good idea to pattern match (x1:x2:xs) rather than just (x:xs).
如果 m 匹配 x1 和 q 匹配 x2 ，我们将它放在列表的头部，并在列表的其余部分递归调用 interspace 。如果他们不行，我们在（x2：xs）上调用 interspace 。
If m matches x1 and q matches x2, we place the onto the head of the list and call interspace recursively on the rest of the list. If they do not mach, we call interspace on (x2:xs).
interspace :: Eq a => a -> a -> a-> [a] -> [a]
interspace m n q [] = []
interspace m n q [x] = [x]
interspace m n q (x1:x2:xs) | m == x1 && q == x2 = m : n : q : interspace m n q xs
| otherwise = x1 : interspace m n q (x2:xs)
用法示例：
ghci>> interspace 1 2 1 [1,1,2,1,1]
[1,2,1,2,1,2,1]
ghci>> interspace 1 2 1 [1,1,3]
[1,2,1,3]
ghci>> interspace 1 2 1 [1,2,4,2]
[1,2,4,2]
ghci>> interspace 1 2 1 [1]
[1]
ghci>> interspace 1 2 1 []
[]
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