Haskell Study Note Lesson 5

List

Creating Lists

Note : type homogeneous

• [1,2,3]
• [1..5]、[2,4..10]

Accessing List Elements

• index from 0
• !!：list !! index

A safe way :

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safeIndex xs i = if i < lenght xs then Just(xs !! i) else Nothing

Concatenation & Extension

• Concatenation : ++ like [1, 2, 3] ++ [4, 5, 6]
• Cons operator: : like 0 : [1, 2, 3] and we get [0, 1, 2, 3]

List Slicing

• take : take 3 [1, 2, 3, 4, 5] is [1, 2, 3]
• drop : drop 2 [1, 2, 3, 4, 5] is [3, 4, 5]
• splitAt: splitAt 2 [1, 2, 3, 4, 5] is [1, 2], [3, 4, 5], which returns a tuple

A trick :

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let xs = [1..10]
take 3 (drop 2 xs)
--- final : [3, 4, 5]

Basic Traversal

• map

  1 2	map (*2) [1, 2, 3, 4] -- get [2, 4, 6, 8]

• fliter

  1 2	filter even [1, 2, 3, 4, 5, 5] -- get [2, 4, 6]

• head / tail

  1 2 3 4

  head [1, 2, 3] -- 1 tail [1, 2, 3] -- [2, 3]

Practice : Number List Tool

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numbers :: [Int]
numbers = [3, 4, 5, 6, 7, 8]

printNumbers :: [Int] -> [Int]
printNumbers numbers = numbers

sumNumbers :: [Int] -> Int
sumNumbers numbers = sum numbers

maxNumbers :: [Int] -> Int
maxNumbers numbers = maximum numbers 

filterNumbers :: [Int] -> [Int]
filterNumbers numbers = filter even numbers

mulNumbers :: [Int] -> Int -> [Int]
mulNumbers numbers ml = map (*ml) numbers

main :: IO ()
main = do
    print (printNumbers numbers)
    print (mulNumbers numbers 2)
    print (sumNumbers numbers)
    print (maxNumbers numbers)
    print (filterNumbers numbers)