# 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) |
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myButLast :: [a] -> [a] |
myButLast :: [a] -> [a] |
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myButLast list = drop ((length list) - 2) list |
myButLast list = drop ((length list) - 2) list |
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+ | </haskell> |
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+ | |||

+ | == Problem 3 == |
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+ | |||

+ | <pre> |
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+ | (*) Find the K'th element of a list. |
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+ | The first element in the list is number 1. |
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+ | Example: |
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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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+ | |||

+ | == 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> |
</haskell> |
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## 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)
```