# Difference between revisions of "H-99: Ninety-Nine Haskell Problems"

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− | These are Haskell translations of [http://www.ic.unicamp.br/~meidanis/courses/mc336/2006s2/funcional/L-99_Ninety-Nine_Lisp_Problems.html Ninety Nine Lisp Problems]. |
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+ | __NOTOC__ |
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− | == Problem 1 == |
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+ | These are Haskell translations of [http://www.ic.unicamp.br/~meidanis/courses/mc336/2006s2/funcional/L-99_Ninety-Nine_Lisp_Problems.html Ninety-Nine Lisp Problems], |
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+ | which are themselves translations of [https://prof.ti.bfh.ch/hew1/informatik3/prolog/p-99/ Ninety-Nine Prolog Problems]. |
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− | <pre> |
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+ | If you want to work on one of these, put your name in the block so we know someone's working on it. Then, change n in your block to the appropriate problem number, and fill in the <Problem description>,<example in Haskell>,<solution in haskell> and <description of implementation> fields. Then be sure to update the status on this page to indicate that we have a solution! |
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− | (*) Find the last box of a list. |
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− | Example: |
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− | * (my-last '(a b c d)) |
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− | (D) |
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− | </pre> |
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− | This is "last" in Prelude, which is defined as: |
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+ | == The problems == |
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− | <haskell> |
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+ | These problems have been split into 11 parts, for ease of access. |
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− | last :: [a] -> a |
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− | last [x] = x |
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− | last (_:xs) = last xs |
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− | </haskell> |
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− | == Problem 2 == |
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+ | * [[99_questions/1_to_10|Questions 1 to 10]]: Lists |
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+ | * [[99_questions/11_to_20|Questions 11 to 20]]: Lists, continued |
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+ | * [[99_questions/21_to_28|Questions 21 to 28]]: Lists again |
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+ | * [[99_questions/31_to_41|Questions 31 to 41]]: Arithmetic |
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+ | * [[99_questions/46_to_50|Questions 46 to 50]]: Logic and codes |
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+ | * [[99_questions/54A_to_60|Questions 54A to 60]]: Binary trees |
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+ | * [[99_questions/61_to_69|Questions 61 to 69]]: Binary trees, continued |
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+ | * [[99_questions/70B_to_73|Questions 70B to 73]]: Multiway trees |
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+ | * [[99_questions/80_to_89|Questions 80 to 89]]: Graphs |
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+ | * [[99_questions/90_to_94|Questions 90 to 94]]: Miscellaneous problems |
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+ | * [[99_questions/95_to_99|Questions 95 to 99]]: Miscellaneous problems, continued |
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− | <pre> |
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+ | (Though the problems number from 1 to 99, there are some gaps and some additions marked with letters. There are actually only 88 problems.) |
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− | (*) Find the last but one box of a list. |
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− | Example: |
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− | * (my-but-last '(a b c d)) |
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− | (C D) |
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− | </pre> |
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− | This can be done by dropping all but the last two elements of a list: |
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+ | == Solutions == |
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− | <haskell> |
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+ | Known solutions are listed at [[99 questions/Solutions]]. Some of those we have could do with cleaning up or additional solutions. |
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− | myButLast :: [a] -> [a] |
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− | myButLast list = drop ((length list) - 2) list |
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− | </haskell> |
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− | == |
+ | == References == |

− | <pre> |
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+ | * [https://prof.ti.bfh.ch/hew1/informatik3/prolog/p-99/ P-99: Ninety-Nine Prolog Problems] contains Prolog solutions to all the problems. |
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− | (*) Find the K'th element of a list. |
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+ | * [http://www.ic.unicamp.br/~meidanis/courses/mc336/2006s2/funcional/L-99_Ninety-Nine_Lisp_Problems.html L-99: Ninety-Nine Lisp Problems] contains Lisp solutions to problems 1-11, 14, 15, 17 and 20-28. |
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− | The first element in the list is number 1. |
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+ | * [http://www.oreillynet.com/onlamp/blog/2006/12/99_problems_in_perl_6.html 99 Problems in Perl 6] has an increasing number of Perl 6 solutions (running on [http://www.pugscode.org/ Pugs]). |
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− | Example: |
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+ | * [http://www.christiankissig.de/cms/index.php/en/programming/28-ocaml/28-99-problems-in-ocaml 99 Problems in OCaml] contains Ocaml solutions to many problems. |
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− | * (element-at '(a b c d e) 3) |
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− | C |
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− | </pre> |
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− | This is (almost) the infix operator !! in Prelude, which is defined as: |
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− | |||

− | <haskell> |
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− | (!!) :: [a] -> Int -> a |
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− | (x:_) !! 0 = x |
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− | (_:xs) !! n = xs !! (n-1) |
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− | </haskell> |
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− | |||

− | Except this doesn't quite work, because !! is zero-indexed, and element-at should be one-indexed. So: |
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− | |||

− | <haskell> |
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− | elementAt :: [a] -> Int -> a |
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− | elementAt list i = list !! (i-1) |
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− | </haskell> |
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− | |||

− | == Problem 4 == |
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− | |||

− | <pre> |
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− | (*) Find the number of elements of a list. |
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− | </pre> |
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− | |||

− | This is "length" in Prelude, which is defined as: |
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− | |||

− | <haskell> |
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− | length :: [a] -> Int |
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− | length [] = 0 |
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− | length (_:l) = 1 + length l |
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− | </haskell> |
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− | |||

− | == Problem 5 == |
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− | |||

− | <pre> |
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− | (*) Reverse a list. |
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− | </pre> |
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− | |||

− | This is "reverse" in Prelude, which is defined as: |
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− | |||

− | <haskell> |
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− | reverse :: [a] -> [a] |
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− | reverse = foldl (flip (:)) [] |
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− | </haskell> |
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− | |||

− | The standard definition is concise, but not very readable. Another way to define reverse is: |
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− | |||

− | <haskell> |
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− | reverse :: [a] -> [a] |
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− | reverse [] = [] |
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− | reverse (x:xs) = reverse xs ++ [x] |
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− | </haskell> |
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− | |||

− | == Problem 6 == |
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− | |||

− | <pre> |
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− | (*) Find out whether a list is a palindrome. |
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− | A palindrome can be read forward or backward; e.g. (x a m a x). |
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− | </pre> |
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− | |||

− | This is trivial, because we can use reverse: |
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− | |||

− | <haskell> |
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− | isPalindrome :: (Eq a) => [a] -> Bool |
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− | isPalindrome xs = xs == (reverse xs) |
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− | </haskell> |
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− | |||

− | == Problem 7 == |
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− | |||

− | <pre> |
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− | (**) Flatten a nested list structure. |
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− | Transform a list, possibly holding lists as elements into a `flat' list by replacing each list with its elements (recursively). |
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− | |||

− | Example: |
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− | * (my-flatten '(a (b (c d) e))) |
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− | (A B C D E) |
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− | </pre> |
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− | |||

− | This is tricky, because lists in Haskell are homogeneous. [1, [2, [3, 4], 5]] |
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− | is a type error. We have to devise some way of represent a list that may (or |
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− | may not) be nested: |
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− | |||

− | <haskell> |
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− | data NestedList a = Elem a | List [NestedList a] |
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− | |||

− | flatten :: NestedList a -> [a] |
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− | flatten (Elem x) = [x] |
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− | flatten (List []) = [] |
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− | flatten (List (x:xs)) = flatten x ++ flatten (List xs) |
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− | </haskell> |
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− | |||

− | Our NestedList datatype is either a single element of some type (Elem a), or a |
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− | list of NestedLists of the same type. (List [NestedList a]). Let's try it out in ghci: |
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− | |||

− | <pre> |
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− | *Main> flatten (Elem 5) |
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− | [5] |
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− | *Main> flatten (List [Elem 1, List [Elem 2, List [Elem 3, Elem 4], Elem 5]]) |
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− | [1,2,3,4,5] |
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− | *Main> flatten (List []) |
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− | [] |
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− | </pre> |
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− | |||

− | == Problem 8 == |
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− | <pre> |
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− | (**) Eliminate consecutive duplicates of list elements. |
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− | If a list contains repeated elements they should be replaced with a single copy of the element. The order of the elements should not be changed. |
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− | |||

− | Example: |
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− | * (compress '(a a a a b c c a a d e e e e)) |
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− | (A B C A D E) |
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− | </pre> |
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− | |||

− | Example in Haskell: |
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− | compress ['a','a','a','a','b','c','c','a','a','d','e','e','e','e'] |
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− | ['a','b','c','a','d','e'] |
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− | |||

− | <haskell> |
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− | compress = map head . group |
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− | </haskell> |
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− | |||

− | We simply group equal values together (group), then take the head of each. |
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− | |||

[[Category:Tutorials]] |
[[Category:Tutorials]] |
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+ | [[Category:Code]] |

## Latest revision as of 20:39, 22 November 2013

These are Haskell translations of Ninety-Nine Lisp Problems,
which are themselves translations of Ninety-Nine Prolog Problems.

If you want to work on one of these, put your name in the block so we know someone's working on it. Then, change n in your block to the appropriate problem number, and fill in the <Problem description>,<example in Haskell>,<solution in haskell> and <description of implementation> fields. Then be sure to update the status on this page to indicate that we have a solution!

## The problems

These problems have been split into 11 parts, for ease of access.

- Questions 1 to 10: Lists
- Questions 11 to 20: Lists, continued
- Questions 21 to 28: Lists again
- Questions 31 to 41: Arithmetic
- Questions 46 to 50: Logic and codes
- Questions 54A to 60: Binary trees
- Questions 61 to 69: Binary trees, continued
- Questions 70B to 73: Multiway trees
- Questions 80 to 89: Graphs
- Questions 90 to 94: Miscellaneous problems
- Questions 95 to 99: Miscellaneous problems, continued

(Though the problems number from 1 to 99, there are some gaps and some additions marked with letters. There are actually only 88 problems.)

## Solutions

Known solutions are listed at 99 questions/Solutions. Some of those we have could do with cleaning up or additional solutions.

## References

- P-99: Ninety-Nine Prolog Problems contains Prolog solutions to all the problems.
- L-99: Ninety-Nine Lisp Problems contains Lisp solutions to problems 1-11, 14, 15, 17 and 20-28.
- 99 Problems in Perl 6 has an increasing number of Perl 6 solutions (running on Pugs).
- 99 Problems in OCaml contains Ocaml solutions to many problems.