1 / 8

Haskell

Haskell. Chapter 1, Part II. List Comprehension. List comprehensions are a way to filter, transform and combine lists Similar to mathematical set comprehensions {2 * x | x e N, X <= 10} In Haskell: [x * 2 | x <- [1..10]] “draw” our elements from the list [1..10]

zahina
Download Presentation

Haskell

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Haskell Chapter 1, Part II

  2. List Comprehension • List comprehensions are a way to filter, transform and combine lists • Similar to mathematical set comprehensions • {2 * x | x e N, X <= 10} • In Haskell: • [x * 2 | x <- [1..10]] • “draw” our elements from the list [1..10] • so x takes on each value from 1 to 10 • part before the pipe (|) is the output

  3. With a predicate • [x * 2 | x <- [50..100], x `mod` 7 == 3] • Using a predicate in this way is called filtering • Can separate predicates with a comma • [x | x <- [10..20], x /= 13, x /= 15, x /= 19] • Can draw from several lists • [x+y| x<-[1,2,3], y <- [10,100, 1000]] • result: [11,101,1001,12,102,1002,13,103,1003]

  4. More list comprehensions • Can use a temporary variable • length' xs = sum [1 | _ <- xs] • Can be used with strings (they’re lists too) • removeNonUppercasest = [c | c <- st, c `elem` ['A'..'Z']] • Nested list comprehensions • -- let xxs = [[1,3,5,2,3,1,2,4,5],[1,2,3,4,5,6,7,8,9],[1,2,4,2,1,6,3,1,3,2,3,6]] • removeOddxxs = [[x | x <- xs, even x] | xs <- xxs]

  5. Tuples • Used to store several heterogeneous elements as a single value • Tuples have a fixed size • Elements surrounded by parentheses • (1,3) • (3, ‘a’, “hello”) • (50, 50.4, “hello”, ‘b’) • tuple of size 2 is a different type from tuple of size 3 • tuples with different member elements are different types

  6. More tuples • Storing pairs is common in Haskell • Useful functions to manipulate: • fst • snd • zip [1,2,3] [4,5,6] => [(1,4),(2,5),(3,6)] • zip [1..] ["apple", "orange", "banana"] => [(1,"apple"),(2,"orange"),(3,"banana")]

  7. Tuples in list comprehensions • Generate tuples • triples = [(a,b,c) | c <- [1..10], a<-[1..10], b<-[1..10]] • Generate tuples with filter • rightTriangle = [(a,b,c) | c <- [1..10], a<-[1..c], b<-[1..a], • a^2 + b^2 == c^2]

  8. Play and Share • evenCubes[1..20] • [8,64,216,512,1000,1728,2744,4096,5832,8000] • onlyBig[200,30,50,20,120] 100 • [200,120] • noDiagonal[1..4] • [(1,2),(1,3),(1,4),(2,1),(2,3),(2,4),(3,1),(3,2),(3,4),(4,1),(4,2),(4,3)] • diagonal 10 • [(0,0),(1,1),(2,2),(3,3),(4,4),(5,5),(6,6),(7,7),(8,8),(9,9),(10,10)] • countOdd[1..30] • 15 • evenOddPairs[1..4][20, 17, 23, 42] • [(2,17),(4,17),(2,23),(4,23)] • removeDigits"abc1d23A.98" • "abcdA.“ • ends [[4,5,6],[1,2],[7,1,0]] • [6,2,0] * These are parameters

More Related