Haskell：使用递归比较两个字符串并写出位置和长度

Question

I'm new to programming and just started with recursive programming in Haskell. I could use some help getting started with a problem I want to solve.

I'm looking for a simple way of solving following problem:

compareReverseStrings :: String -> [(Position, Length)]

Examples:

      compareReverseStrings "ABCDEEEEEFG" = [(5,3)]


      compareReverseStrings "1234444567444447" = [(4,3), (11,3)]

(In this case the funtion compares "ABCDEEEFG" and "GFEEEEEDCBA" in example 1)

(the function compares "1234444567444447" and "7444447654444321" in example 2)

Let me know if I'm being unclear :) Thank you in advance!

Answer 1

Note that the comparison is independent of the reversal; that is, you can implement your function as the composition of two separate functions

compareReverseStrings str = compareStrings str (reverseString str)
-- Using the Applicative instance of functions, 
-- compareReverseStrings = compareStrings <*> reverseString

Then you can implement the two functions separately. ( reverseString should be trivial; it's already implemented as reverse , but you can write your own definition for practice if you like).

compareStrings :: String -> String -> [(Position, Length)]
compareStrings x y = ...
-- Hint: consider what you might do with the result of zip3 [1..] x y
-- What if the letters in the first element are the same?
-- What if they are different?

reverseString :: String -> String
reverseString str = ...

In general, this is how you solve all problems in programming: break your problem down into smaller pieces, solve the subproblems, then combine the results to get the solution to your original problem.