# 99 questions/1 to 10

### From HaskellWiki

(→Problem 8: Using a list-of-strings is closer to the spirit of this problem than list-of-chars) |
m (Avoid name clashes with prelude (using naming scheme of problems 1, 2 & 4)) |
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__NOTOC__ | __NOTOC__ | ||

− | This is part of [[H-99:_Ninety-Nine_Haskell_Problems|Ninety-Nine Haskell Problems]], based on [https:// | + | This is part of [[H-99:_Ninety-Nine_Haskell_Problems|Ninety-Nine Haskell Problems]], based on [https://sites.google.com/site/prologsite/prolog-problems Ninety-Nine Prolog Problems] and [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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== Problem 1 == | == Problem 1 == | ||

Line 87: | Line 85: | ||

<haskell> | <haskell> | ||

− | Prelude> | + | Prelude> myReverse "A man, a plan, a canal, panama!" |

"!amanap ,lanac a ,nalp a ,nam A" | "!amanap ,lanac a ,nalp a ,nam A" | ||

− | Prelude> | + | Prelude> myReverse [1,2,3,4] |

[4,3,2,1] | [4,3,2,1] | ||

</haskell> | </haskell> | ||

Line 128: | Line 126: | ||

Example in Haskell: | Example in Haskell: | ||

+ | |||

+ | We have to define a new data type, because lists in Haskell are homogeneous. | ||

+ | <haskell> | ||

+ | data NestedList a = Elem a | List [NestedList a] | ||

+ | </haskell> | ||

<haskell> | <haskell> | ||

Line 138: | Line 141: | ||

</haskell> | </haskell> | ||

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+ | |||

+ | [[99 questions/Solutions/7 | Solutions]] | ||

== Problem 8 == | == Problem 8 == | ||

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<haskell> | <haskell> | ||

− | > compress | + | > compress "aaaabccaadeeee" |

− | + | "abcade" | |

</haskell> | </haskell> | ||

Line 178: | Line 182: | ||

<haskell> | <haskell> | ||

− | *Main> pack ['a', 'a', 'a', 'a', 'b', 'c', 'c', 'a', 'a', 'd', 'e', 'e', 'e', 'e'] | + | *Main> pack ['a', 'a', 'a', 'a', 'b', 'c', 'c', 'a', |

+ | 'a', 'd', 'e', 'e', 'e', 'e'] | ||

["aaaa","b","cc","aa","d","eeee"] | ["aaaa","b","cc","aa","d","eeee"] | ||

</haskell> | </haskell> | ||

[[99 questions/Solutions/9 | Solutions]] | [[99 questions/Solutions/9 | Solutions]] | ||

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== Problem 10 == | == Problem 10 == |

## Latest revision as of 08:17, 27 February 2013

This is part of Ninety-Nine Haskell Problems, based on Ninety-Nine Prolog Problems and Ninety-Nine Lisp Problems.

## [edit] 1 Problem 1

(*) Find the last element of a list.

(Note that the Lisp transcription of this problem is incorrect.)

Example in Haskell:

Prelude> myLast [1,2,3,4] 4 Prelude> myLast ['x','y','z'] 'z'

## [edit] 2 Problem 2

(*) Find the last but one element of a list.

(Note that the Lisp transcription of this problem is incorrect.)

Example in Haskell:

Prelude> myButLast [1,2,3,4] 3 Prelude> myButLast ['a'..'z'] 'y'

## [edit] 3 Problem 3

(*) Find the K'th element of a list. The first element in the list is number 1.

Example:

* (element-at '(a b c d e) 3) c

Example in Haskell:

Prelude> elementAt [1,2,3] 2 2 Prelude> elementAt "haskell" 5 'e'

## [edit] 4 Problem 4

(*) Find the number of elements of a list.

Example in Haskell:

Prelude> myLength [123, 456, 789] 3 Prelude> myLength "Hello, world!" 13

## [edit] 5 Problem 5

(*) Reverse a list.

Example in Haskell:

Prelude> myReverse "A man, a plan, a canal, panama!" "!amanap ,lanac a ,nalp a ,nam A" Prelude> myReverse [1,2,3,4] [4,3,2,1]

## [edit] 6 Problem 6

(*) Find out whether a list is a palindrome. A palindrome can be read forward or backward; e.g. (x a m a x).

Example in Haskell:

*Main> isPalindrome [1,2,3] False *Main> isPalindrome "madamimadam" True *Main> isPalindrome [1,2,4,8,16,8,4,2,1] True

## [edit] 7 Problem 7

(**) Flatten a nested list structure.

Transform a list, possibly holding lists as elements into a `flat' list by replacing each list with its elements (recursively).

Example:

* (my-flatten '(a (b (c d) e))) (A B C D E)

Example in Haskell:

We have to define a new data type, because lists in Haskell are homogeneous.

data NestedList a = Elem a | List [NestedList a]

*Main> flatten (Elem 5) [5] *Main> flatten (List [Elem 1, List [Elem 2, List [Elem 3, Elem 4], Elem 5]]) [1,2,3,4,5] *Main> flatten (List []) []

## [edit] 8 Problem 8

(**) Eliminate consecutive duplicates of list elements.

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.

Example:

* (compress '(a a a a b c c a a d e e e e)) (A B C A D E)

Example in Haskell:

> compress "aaaabccaadeeee" "abcade"

## [edit] 9 Problem 9

(**) Pack consecutive duplicates of list elements into sublists. If a list contains repeated elements they should be placed in separate sublists.

Example:

* (pack '(a a a a b c c a a d e e e e)) ((A A A A) (B) (C C) (A A) (D) (E E E E))

Example in Haskell:

*Main> pack ['a', 'a', 'a', 'a', 'b', 'c', 'c', 'a', 'a', 'd', 'e', 'e', 'e', 'e'] ["aaaa","b","cc","aa","d","eeee"]

## [edit] 10 Problem 10

(*) Run-length encoding of a list. Use the result of problem P09 to implement the so-called run-length encoding data compression method. Consecutive duplicates of elements are encoded as lists (N E) where N is the number of duplicates of the element E.

Example:

* (encode '(a a a a b c c a a d e e e e)) ((4 A) (1 B) (2 C) (2 A) (1 D)(4 E))

Example in Haskell:

encode "aaaabccaadeeee" [(4,'a'),(1,'b'),(2,'c'),(2,'a'),(1,'d'),(4,'e')]