Haskell - 使用递归替换字符串中的 substring（在我的例子中不允许使用函数 take 和 drop）

Question

I need to Replace one line in the sentence with another for example “qwer sder” “er” “ol” “qwol sdol”. I managed to do something similar replacing one char with another

replace :: Char -> Char -> String -> String
replace _ _ "" = ""
replace x y (c:cs) = if c == x then y : replace x y cs else c : replace x y cs

But have no idea how to do that with strings (not chars).

Answer 1

A naive algorithm can make use of the isPrefixOf:: [a] -> [a] -> Bool function to check if we should replace that element.

Such function thus looks like:

replaceStr :: String -> String -> String -> String
replaceStr needle replacement = go
    where go "" = ""
          go xs@(x:xt)
              | isPrefixOf needle xs = …  -- (1)
              | otherwise = …  -- (2)
          nn = length needle

where I leave filling in the … parts as an exercise. In case the needle is a prefix of xs , one should drop nn characters from the string xs and replace these with the replacement and then recurse on the rest of the string (1). In case the needle is not a prefix of xs , then we yield x and recurse on the tail xt (2).

This algorithm can take quadratic time: if the needle is for example n characters 'A' and the "haystack" is m characters 'A' , then it will take O(m×n) time.

One can work with the Knuth-Morris-Pratt algorithm [wiki] to perform search in linear time.