Memo List May

2017年5月18日 · 阅读需 4 分钟

Open Source Contributor

少量尚不能单独成篇的笔记

Week13

[2017-05-18 19:15:40]

## R-lang `data frame with 0 columns and 0 rows error`

Week14

[2017-05-27 12:38:29]

How do i convert String into list of integers in Haskell

将字符串型数字转成[int]

Integer -> [Int]

toDigits :: Integer -> [Int]
toDigits = map (fromIntegral . digitToInt) . show

获取随机数/随机数的list

randint :: IO Int
randint = randomRIO (1,55) -- 生成int的范围

-- 获得一个长为 10 的list
rint <- sequence (replicate 10 randint)


randomList :: IO [Int] -- 返回一个 generater
randomList = getStdGen >>= return . randomRs (1,55)

Ruby Random String

以下的shuffle均可改成shuffle(random: Random.new)以提高随机性

生成len长度的大小写字母

([*('a'..'z'),*('A'..'Z')]*len).shuffle[0,len].join

生成len长度的小写字母+数字

(36**(len-1) + rand(36**len - 36**(len-1))).to_s(36)

生成len ± 1长度的小写字母+数字

rand(36**len).to_s(36)

生成随机长度随机内容的 [[_RandomString], [_RandomString], ...]

def gen_test_case # for "Sort array by string length" Ruby Translation —— codewars
  #prng = Random.new  # 使用新得随机数生成器
  arr = [*(1..10)].
    shuffle[0, rand(1..100)]. # arr.length
    sort.
    #map{|len| ([*('a'..'z'),*('A'..'Z')]*len).shuffle[0,len].join}
    map{|len| (36**(len-1) + rand(36**len - 36**(len-1))).to_s(36)}
  [arr.shuffle(random: Random.new), arr]
  # arr 为按长度排序的 数组
end

[2017-05-23 12:03:29]

[转]MIT牛人解说数学体系

[2017-05-22 19:25:21]

Real World Haskell 中文版

修改提示符

Note: 提示符并不需要引号包起来。[ghci ver 8.0.1]

Prelude> :set prompt "ghci>"
"ghci>" :set prompt ghci>
ghci>

基本算术运算

ghci>2+2
4
ghci>2/2
1.0
ghci>7.0/2
3.5
ghci>331*2
662
ghci>123*0.5
61.5
ghci>123*1.0
123.0

前缀表达式

ghci>(+) 2 2
4
ghci>(/) 10 5
2.0

幂

ghci>2^3
8
ghci>2**3
8.0

负数

负数需要用括号括起来。单独出现时除外，此时-为一元运算符。

ghci>-1
-1
ghci>2* -1

<interactive>:18:1: error:
    Precedence parsing error
        cannot mix ‘*’ [infixl 7] and prefix `-' [infixl 6] in the same infix expression
ghci>2* (-1)
-2

Week15

[2017-06-02 22:20:48]

Haskell 可选参数(optional arguments)

1 中的第一个答案可以用，后面的跑不起来

{-# LANGUAGE FlexibleContexts #-}

multiProduct req1 opt1 opt2 opt3 = req1 * opt1' * opt2' * opt3'
  where opt1' = fromJust (fromMaybe 10 opt1)
        opt2' = fromJust (fromMaybe 20 opt2)
        opt3' = fromJust (fromMaybe 30 opt3)

multiProduct' req1 opt1 opt2 opt3 = req1 * opt1' * opt2' * opt3'
  where opt1' = if isJust opt1 then (fromJust opt1) else 10
        opt2' = if isJust opt2 then (fromJust opt2) else 20
        opt3' = if isJust opt3 then (fromJust opt3) else 30

Error

ghci> multiProduct 1

<interactive>:215:1: error:
    ? Non type-variable argument in the constraint: Num (Maybe a)
      (Use FlexibleContexts to permit this)
    ? When checking the inferred type
        it :: forall a.
              (Num (Maybe a), Num a) =>
              Maybe (Maybe a) -> Maybe (Maybe a) -> Maybe (Maybe a) -> a
ghci> multiProduct' 1

<interactive>:216:1: error:
    ? No instance for (Show (Maybe a0 -> Maybe a0 -> Maybe a0 -> a0))
        arising from a use of ‘print’
        (maybe you haven't applied a function to enough arguments?)
    ? In a stmt of an interactive GHCi command: print it

Tips

Maybe a -> a

[2017-05-29 11:12:58]

Useful Haskell functions

Week13​

Week14​

Ruby Random String​

Real World Haskell 中文版​

基本算术运算​

Week15​

Haskell 可选参数(optional arguments)​

Tips​