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在haskell中将数字列表连接成一个整数

concatenation haskell

2

Jonno_FTW · 技术社区 · 17 年前

我正在做另一个项目- Problem 38 .

f (a,b) = a*b
conProInt x n  = map f (zip (replicate n x) ([1..n]))

prob38 = maximum [ (conProInt (x) (n)) | x <- [100..500], n <- [1..9], (sort $ nub $ (decToList $ (conProInt x n) )) == (sort $ (decToList $ (conProInt x n) )), (sort $ nub $ (decToList $ (conProInt x n))) == [1..9] ]

如:

conProInt 192 3

返回: [192,384,576]

我需要归还的是: 192384576

我四处搜索,找不到一个函数,也找不到一个我可以构造的函数来满足我的需求。我该怎么办?

编辑:

我已经更新了脚本以合并更快的连接,但它没有返回正确的结果:

f (a,b) = a*b
conProInt x n  =( combine (map f (zip (replicate n x) ([1..n]))))
prob38 = maximum [ (conProInt (x) (n)) | x <- [1..50000], n <- [2..40], (sort $ nub $ (decToList $ (conProInt x n) )) == (sort $ (decToList $ (conProInt x n) )), (sort $ nub $ (decToList $ (conProInt x n))) == [1..9] ]

我很确定泛数字测试

(sort $ nub $ (decToList $ (conProInt x n) )) == (sort $ (decToList $ (conProInt x n) )), (sort $ nub $ (decToList $ (conProInt x n))) == [1..9]

3 回复 | 直到 11 年前

1

7

thsutton 17 年前

通过 String

read $ concat $ map (show) [192,384,576]

尽管您可能需要添加类型签名:

Prelude> (read $ concat $ map (show) [192,384,576]) :: Int
192384576

2

5

Martijn 17 年前

您可以使用此函数连接数字列表:

concatNumbers :: [Int] -> String
concatNumbers = concat . map show

如果希望函数以数字形式返回串联,可以使用 read .

3

2

Michael Steele 17 年前

下面是一个如何在不转换字符串的情况下连接数字的示例。

-- foldl1' is a strict fold.  "foldl1" would also work...
import Data.List (foldl1')    

-- Combine two numbers such that their digits are concatenated.
-- op 1 23 = 123, op 0 12 = 12, op 12345 67 = 1234567
op :: Int -> Int -> Int
op a b = a * power 10 (numDigits b) + b

-- How many digits does a positive number have?
numDigits :: Int -> Int
numDigits x = length . takeWhile (>= 1) . iterate (`div` 10) $ x

-- Take a positive number and raise it to a positive power.
-- power 5 2 = 25, power 10 3 = 1000
power :: Int -> Int -> Int
power x y = foldl1' (*) . take y $ repeat x

-- Take a list of numbers, and concatenate all their digits.
combine :: [Int] -> Int
combine xs = foldl1' op xs

运行示例:

Prelude> :m +Data.List
Prelude Data.List> let power x y = foldl1' (*) . take y $ repeat x
Prelude Data.List> let numDigits = length . takeWhile (>=1) . iterate (`div` 10)
Prelude Data.List> let op a b = a * power 10 (numDigits b) + b
Prelude Data.List> let combine xs = foldl1' op xs
Prelude Data.List> combine [192, 384, 576]
192384576
Prelude Data.List>