「学习总结-Haskell-2」Haskell重要数据结构:List
Haskell 重要数据结构 - List
Table of Contents
- 1 Haskell 基本知识
- 2 Haskell 重要数据结构 - List
- 2.1 List基本知识
- 2.2 List 相关函数
- 2.3 利用List特性编写程序
1 Haskell 基本知识
2 Haskell 重要数据结构 - List
2.1 List基本知识
- List 表示
例1 基本元素为整数的List
ghci>let tList = [1,2,3,4] ghci>tList [1,2,3,4]
例2 基本元素为List的List
ghci>[[1,2],[3],[4,5,6]] [[1,2],[3],[4,5,6]]
例3 基本元素为字符的List,即字符串
ghci>['h','e','l','l','o']
"hello"
例4 List列举表示
ghci>[1..10]
[1,2,3,4,5,6,7,8,9,10]
ghci>['a'..'e']
"abcde"
ghci>[2,4..10]
[2,4,6,8,10]
ghci>[5..2]
[]
ghci>[5,4..2]
[5,4,3,2]
例5 List的描述表示
ghci>[x | x <- [1..10], x > 3, x < 7] [4,5,6] ghci>[x + y | x <- [1,2,3], y <- [4,5]] [5,6,6,7,7,8]
例6 List是什么 List就是相同类型的元素粘在一起
ghci>3:[4,5] [3,4,5] ghci>3:4:[5] [3,4,5] ghci>3:4:5:[] [3,4,5]
- List基本操作
- List 连接
ghci>[1,2,3] ++ [4,5] [1,2,3,4,5]
- List 元素的访问
ghci>[4,2,1] !! 2
1
ghci>[4,2,1] !! 1
2
ghci>[4,2,1] !! 0
4
ghci>"hi,jack" !! 2
','
- List 的比较
List的比较方式是按字典顺序
ghci>[1,2] > [2,3] False ghci>[1,2] > [1,1] True ghci>[1,2,3] > [1,2] True ghci>[1,2,-1] > [1,2] True ghci>[1,2] > [1,2,3] False
2.2 List 相关函数
- head
ghci>head [3,1,4] 3
- tail
ghci>tail [3,1,4] [1,4]
- init
ghci>init [3,1,4] [3,1]
- last
ghci>last [3,1,4] 4
- length
ghci>length [3,1,4] 3
- null
ghci>null [3,1,4] False ghci>null [] True
- reverse
ghci>reverse [1,2,3,4] [4,3,2,1]
- take
ghci>take 2 [3,1,5] [3,1] ghci>take 1 [3,1,5] [3] ghci>take 0 [3,1,5] []
- drop
ghci>drop 2 [3,1,5] [5] ghci>drop 1 [3,1,5] [1,5] ghci>drop 0 [3,1,5] [3,1,5]
- maximum
ghci>maximum [1,5,2,3] 5
- sum
ghci>sum [1,2,3] 6 ghci>sum [1..10] 55
- product
ghci>product [4,2,5] 40
- elem
ghci>3 `elem` [6,7,8] False ghci>6 `elem` [6,7,8] True ghci>7 `elem` [6,7,8] True ghci>elem 3 [6,7,8] False ghci>elem 6 [6,7,8] True ghci>elem 7 [6,7,8] True
- cycle
ghci>cycle [5,2,8] [5,2,8,5,2,8,5,2,8,5,2,8,...] --无穷重复下去 ghci>take 5 (cycle [5,2,8]) [5,2,8,5,2] ghci>take 10 (cycle [5,2,8]) [5,2,8,5,2,8,5,2,8,5]
- repeat
ghci>repeat 3 3,3,3,... ghci>take 5 (repeat 3) [3,3,3,3,3]
- replicate
ghci>replicate 2 10 [10,10] ghci>replicate 3 10 [10,10,10]
2.3 利用List特性编写程序
- 利用List描述表示方法
例1
boomBang xs = [ if x < 3 then "BOOM" else "BANG" | x <- xs ]
ghci>:l listProg.hs [1 of 1] Compiling Main ( listProg.hs, interpreted ) Ok, modules loaded: Main. ghci>boomBang [1..5] ["BOOM","BOOM","BANG","BANG","BANG"] ghci>boomBang [0..4] ["BOOM","BOOM","BOOM","BANG","BANG"]
例2
cartesianproduc xs ys = [ [x,y] | x <- xs, y <- ys]
ghci>cartesianproduc [3,2] [1,5,6] [[3,1],[3,5],[3,6],[2,1],[2,5],[2,6]]
例3
removeNonUppercase xs = [ x | x <- xs, x `elem` ['A'..'Z'] ]
ghci>removeUppercase "SDFAasdfRQf" "SDFARQ"
- List 与 模式匹配(Pattern Matching)
例1
head' [] = error "empty list can not be head" head' (x:_) = x
ghci>head' []
*** Exception: empty list can not be head
ghci>head' "dafs"
'd'
ghci>head' [3,4,1]
3
例2
tell [] = "This is a empty list" tell (x:[]) = "This list have only one element:" ++ show x tell (x:y:[]) = "This list have two elemtn: " ++ show x ++ " and " ++ show y tell (x:y:_) = "This is a long list"
ghci>tell [] "This is a empty list" ghci>tell [3] "This list have only one element:3" ghci>tell [3,4] "This list have two elemtn: 3 and 4" ghci>tell [3,4,5,6] "This is a long list" ghci>tell "sdfa" "This is a long list" ghci>tell "sd" "This list have two elemtn: 's' and 'd'"
例3
abbrev firstName lastName = [f] ++ "." ++ [l] where (f:_) = firstName (l:_) = lastName
ghci>abbrev "Tom" "Hacks" "T.H" ghci>abbrev "Alan" "Kay" "A.K"
例4
describeList ls = "This list is " ++ case ls of [] -> "empty" [x] -> "a singleton list" xs -> "a long list"
ghci>describeList [] "This list is empty" ghci>describeList "a" "This list is a singleton list" ghci>describeList "hello" "This list is a long list"
- List 与 递归 (Recursion)
例1
maxElem [] = error "Empty List have no max element" maxElem [x] = x maxElem (x:xs) = max x (maxElem xs)
ghci>maxElem [] *** Exception: Empty List have no max element ghci>maxElem [1] 3 ghci>maxElem [3,2,6,1] 6
例2
replicate' n x | n <= 0 = [] | otherwise = x : replicate' (n-1) x
ghci>replicate' 0 2 [] ghci>replicate' 5 2 [2,2,2,2,2]
例3
take' n _ | n <= 0 = [] take' _ [] = [] take' n (x:xs) = x : take' (n-1) xs
ghci>take' (-3) [1,2] [] ghci>take' 4 [1..10] [1,2,3,4]
例4
reverse' [] = [] reverse' (x:xs) = reverse' xs ++ [x]
ghci>reverse' [] [] ghci>reverse' [1..10] [10,9,8,7,6,5,4,3,2,1]
例5
elem' x [] = False elem' x (y:xs) | x == y = True | otherwise = elem' x xs
ghci>elem' 5 [2,5,7] True ghci>elem' 5 [2,7] False
例6 quicksort
quickSort [] = [] quickSort (x:xs) = let smallerPart = [a | a <- xs, a <= x] largerPart = [a | a <- xs, a > x] in quickSort smallerPart ++ [x] ++ quickSort largerPart
注意不能写成[a | a <= x, a <-xs]的形式，会提示a不在域内， 先写a <-xs , 才能决定 a <= x 是否成立。
ghci>quickSort [2,6,1,8,4] [1,2,4,6,8] ghci>quickSort [] []