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

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(added chessguy from #haskell's solution to problem 2)
(Added 3 & 4 from prelude, and a trivial definition of 5)
Line 32: Line 32:
 
myButLast :: [a] -> [a]
 
myButLast :: [a] -> [a]
 
myButLast list = drop ((length list) - 2) list
 
myButLast list = drop ((length list) - 2) list
  +
</haskell>
  +
  +
== Problem 3 ==
  +
  +
<pre>
  +
(*) 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
  +
</pre>
  +
  +
This is (almost) the infix operator !! in Prelude, which is defined as:
  +
  +
<haskell>
  +
(!!) :: [a] -> Int -> a
  +
(x:_) !! 0 = x
  +
(_:xs) !! n = xs !! (n-1)
  +
</haskell>
  +
  +
Except this doesn't quite work, because !! is zero-indexed, and element-at should be one-indexed. So:
  +
  +
<haskell>
  +
elementAt :: [a] -> Int -> a
  +
elementAt list i = list !! (i-1)
  +
</haskell>
  +
  +
== Problem 4 ==
  +
  +
<pre>
  +
(*) Find the number of elements of a list.
  +
</pre>
  +
  +
This is "length" in Prelude, which is defined as:
  +
  +
<haskell>
  +
length :: [a] -> Int
  +
length [] = 0
  +
length (_:l) = 1 + length l
  +
</haskell>
  +
  +
== Problem 5 ==
  +
  +
<pre>
  +
(*) Reverse a list.
  +
</pre>
  +
  +
This is "reverse" in Prelude, which is defined as:
  +
  +
<haskell>
  +
reverse :: [a] -> [a]
  +
reverse = foldl (flip (:)) []
  +
</haskell>
  +
  +
== Problem 6 ==
  +
  +
<pre>
  +
(*) Find out whether a list is a palindrome.
  +
A palindrome can be read forward or backward; e.g. (x a m a x).
  +
</pre>
  +
  +
This is trivial, because we can use reverse:
  +
  +
<haskell>
  +
isPalindrome :: (Eq a) => [a] -> Bool
  +
isPalindrome xs = xs == (reverse xs)
 
</haskell>
 
</haskell>
   

Revision as of 04:36, 12 December 2006

These are Haskell translations of Ninety Nine Lisp Problems.

Problem 1

(*) Find the last box of a list.
Example:
* (my-last '(a b c d))
(D)

This is "last" in Prelude, which is defined as:

last :: [a] -> a
last [x] = x
last (_:xs) = last xs

Problem 2

(*) Find the last but one box of a list.
Example:
* (my-but-last '(a b c d))
(C D)

This can be done by dropping all but the last two elements of a list:

myButLast :: [a] -> [a]
myButLast list = drop ((length list) - 2) list

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

This is (almost) the infix operator !! in Prelude, which is defined as:

(!!)                :: [a] -> Int -> a
(x:_)  !! 0         =  x
(_:xs) !! n         =  xs !! (n-1)

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

elementAt :: [a] -> Int -> a
elementAt list i = list !! (i-1)

Problem 4

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

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

length           :: [a] -> Int
length []        =  0
length (_:l)     =  1 + length l

Problem 5

(*) Reverse a list.

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

reverse          :: [a] -> [a]
reverse          =  foldl (flip (:)) []

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).

This is trivial, because we can use reverse:

isPalindrome :: (Eq a) => [a] -> Bool
isPalindrome xs = xs == (reverse xs)