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Lists
Declaration

Define an empty list

Assign the variable "list" to a list with no elements
haskell
let list = []

Define a static list

Define the list [One, Two, Three, Four, Five]
haskell
let a = ["One", "Two", "Three", "Four", "Five"]
Output

Join the elements of a list, separated by commas

Given the list [Apple, Banana, Carrot] produce "Apple, Banana, Carrot"
haskell
import Data.List

let join = intercalate ", " ["Apple", "Banana", "Carrot"]

Join the elements of a list, in correct english

Create a function join that takes a List and produces a string containing an english language concatenation of the list. It should work with the following examples:
join([Apple, Banana, Carrot]) = "Apple, Banana, and Carrot"
join([One, Two]) = "One and Two"
join([Lonely]) = "Lonely"
join([]) = ""
haskell
join [] = ""
join [x] = x
join [x,y] = x ++ " and " ++ y
join [x,y,z] = x ++ ", " ++ y ++ ", and " ++ z
join (x:xs) = x ++ ", " ++ join xs

Produce the combinations from two lists

Given two lists, produce the list of tuples formed by taking the combinations from the individual lists. E.g. given the letters ["a", "b", "c"] and the numbers [4, 5], produce the list: [["a", 4], ["b", 4], ["c", 4], ["a", 5], ["b", 5], ["c", 5]]
haskell
comb :: [(String, Int)]
comb = do
b <- [4,5]
a <- ["a","b","c"]
return (a,b)

main = mapM_ print comb
comb :: [(String, Int)]
comb = [(a, b) | b <- [4,5], a <- ["a","b","c"]]

main = print comb

From a List Produce a List of Duplicate Entries

Taking a list:
["andrew", "bob", "chris", "bob"]

Write the code to produce a list of duplicates in the list:
["bob"]
haskell
import Data.List

input = ["andrew", "bob", "chris", "bob"]
output = [ head l | l <- group (sort input), length l > 1]
Access

Fetch an element of a list by index

Given the list [One, Two, Three, Four, Five], fetch the third element ('Three')
haskell
let a = [1..5]
let b = a !! 2
print b

Fetch the last element of a list

Given the list [Red, Green, Blue], access the last element ('Blue')
haskell
last ["Red", "Green", "Blue"]

Find the common items in two lists

Given two lists, find the common items. E.g. given beans = ['broad', 'mung', 'black', 'red', 'white'] and colors = ['black', 'red', 'blue', 'green'], what are the bean varieties that are also color names?
haskell
import Data.List

beans = ["broad", "mung", "black", "red", "white"]
colors = ["black", "red", "blue", "green"]

main = print (intersect beans colors)

Display the unique items in a list

Display the unique items in a list, e.g. given ages = [18, 16, 17, 18, 16, 19, 14, 17, 19, 18], display the unique elements, i.e. with duplicates removed.
haskell
import Data.List

ages = [18, 16, 17, 18, 16, 19, 14, 17, 19, 18]

uniqueAges = nub ages
Modification

Remove an element from a list by index

Given the list [Apple, Banana, Carrot], remove the first element to produce the list [Banana, Carrot]
haskell
deleteNth n xs | n > 0 = take (n-1) xs ++ drop n xs

main = print $ deleteNth 1 [1..3]
fruit :: [String]
fruit = ["Apple", "Banana", "Carrot"]

main :: IO ()
main = putStrLn $ show $ tail fruit

Remove the last element of a list

haskell
ages = [1,2,3,4]

init ages

Rotate a list

Given a list ["apple", "orange", "grapes", "bananas"], rotate it by removing the first item and placing it on the end to yield ["orange", "grapes", "bananas", "apple"]
haskell
main = print $ rotate ["apple", "orange", "grapes", "bananas"]

rotate xs | length xs < 2 = xs
| otherwise = tail xs ++ [head xs]
Manipulation

Gather together corresponding elements from multiple lists

Given several lists, gather together the first element from every list, the second element from every list, and so on for all corresponding index values in the lists. E.g. for these three lists, first = ['Bruce', 'Tommy Lee', 'Bruce'], last = ['Willis', 'Jones', 'Lee'], years = [1955, 1946, 1940] the result should produce 3 actors. The middle actor should be Tommy Lee Jones.
haskell
import Prelude hiding (last)

first = ["Bruce", "Tommy Lee", "Bruce"]
last = ["Willis", "Jones", "Lee"]
years = [1955, 1946, 1940]

actors = zip3 first last years

List Combinations

Given two source lists (or sets), generate a list (or set) of all the pairs derived by combining elements from the individual lists (sets). E.g. given suites = ['H', 'D', 'C', 'S'] and faces = ['2', '3', '4', '5', '6', '7', '8', '9', '10', 'J', 'Q', 'K', 'A'], generate the deck of 52 cards, confirm the deck size and check it contains an expected card, say 'Ace of Hearts'.
haskell
import Data.List

suites = ["H", "D", "C", "S"]
faces = ["2", "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q", "K", "A"]

main = do
let cards = [(s,f) | s <- suites, f <- faces ]
print (length cards)
print $ hasCard ("H", "A") "Ace of Hearts" cards
where hasCard t name cards = (if elem t cards then "Contains "
else "Does not contain") ++ name

Perform an operation on every item of a list

Perform an operation on every item of a list, e.g.
for the list ["ox", "cat", "deer", "whale"] calculate
the list of sizes of the strings, e.g. [2, 3, 4, 5]
haskell
map length ["ox", "cat", "deer", "whale"]

Split a list of things into numbers and non-numbers

Given a list that might contain e.g. a string, an integer, a float and a date,
split the list into numbers and non-numbers.
haskell
import Data.List (partition)

type Date = String
data Things = TS String | TI Int | TD Date
deriving Show

main = do
let myList = [TI 1, TI 23, TS "Joe", TD "23/04/2009"]
print $ partition isNumber myList

where isNumber (TS _) = False
isNumber (TI _) = True
isNumber (TD _) = False
Testing

Test if a condition holds for all items of a list

Given a list, test if a certain logical condition (i.e. predicate) holds for all items of the list.
haskell

all (> 1) [2, 3, 4]

Test if a condition holds for any items of a list

Given a list, test if a certain logical condition (i.e. predicate) holds for any items of the list.
haskell
any (> 1) [1, 2, 3]