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Cookbook/Lists and strings

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== Strings ==
+
== Lists ==
  +
  +
In Haskell, lists are what Arrays are in most other languages.
  +
  +
=== Creating simple lists ===
  +
  +
{| class="wikitable"
  +
|-
  +
! Problem
  +
! Solution
  +
! Examples
  +
|-
  +
| creating a list with given elements
  +
| -
  +
|<haskell>
  +
3 : 12 : 42 : [] --> [3,12,42]
  +
'f' : 'o' : 'o' : [] --> "foo"
  +
</haskell>
  +
|-
  +
| creating a list with stepsize 1
  +
| -
  +
|<haskell>
  +
[1..10] --> [1,2,3,4,5,6,7,8,9,10]
  +
['a'..'z'] --> "abcdefghijklmnopqrstuvwxyz"
  +
</haskell>
  +
|-
  +
| creating a list with different stepsize
  +
| -
  +
|<haskell>
  +
[1,3..10] --> [1,3,5,7,9]
  +
['a','c'..'z'] --> "acegikmoqsuwy"
  +
</haskell>
  +
|-
  +
| creating an infinite constant list
  +
| -
  +
|<haskell>
  +
[1,1..] --> [1,1,1,1,1,...
  +
</haskell>
  +
|-
  +
| creating an infinite list with stepsize 1
  +
| -
  +
| <haskell>
  +
[1..] --> [1,2,3,4,5,...
  +
</haskell>
  +
|}
  +
  +
=== List comprehensions ===
  +
  +
The list of all squares can also be written in a more comprehensive way, using list comprehensions:
  +
  +
<haskell>
  +
squares = [x*x | x <- [1..]]
  +
</haskell>
  +
  +
List comprehensions allow for constraints as well:
  +
  +
<haskell>
  +
-- multiples of 3 or 5
  +
mults = [ x | x <- [1..], mod x 3 == 0 || mod x 5 == 0 ]
  +
</haskell>
   
Since strings are lists of characters, you can use any available list function.
 
   
=== Combining strings ===
+
=== Combining lists ===
   
 
{| class="wikitable"
 
{| class="wikitable"
Line 11: Line 10:
 
! Examples
 
! Examples
 
|-
 
|-
| combining two strings
+
| combining two lists
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3A%2B%2B (++)]
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3A%2B%2B (++)]
 
|<haskell>
 
|<haskell>
 
"foo" ++ "bar" --> "foobar"
 
"foo" ++ "bar" --> "foobar"
  +
[42,43] ++ [60,61] --> [42,43,60,61]
 
</haskell>
 
</haskell>
 
|-
 
|-
| combining many strings
+
| combining many lists
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:concat concat]
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:concat concat]
 
| <haskell>
 
| <haskell>
Line 24: Line 24:
 
|}
 
|}
   
=== Accessing substrings ===
+
=== Accessing sublists ===
   
 
{| class="wikitable"
 
{| class="wikitable"
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! Examples
 
! Examples
 
|-
 
|-
| accessing the first character
+
| accessing the first element
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:head head]
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:head head]
 
|<haskell>
 
|<haskell>
Line 38: Line 38:
 
</haskell>
 
</haskell>
 
|-
 
|-
| accessing the last character
+
| accessing the last element
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Alast last]
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Alast last]
 
|<haskell>
 
|<haskell>
Line 44: Line 44:
 
</haskell>
 
</haskell>
 
|-
 
|-
| accessing the character at a given index
+
| accessing the element at a given index
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3A!! (!!)]
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3A!! (!!)]
 
|<haskell>
 
|<haskell>
Line 50: Line 50:
 
</haskell>
 
</haskell>
 
|-
 
|-
| accessing the first <code>n</code> characters
+
| accessing the first <code>n</code> elements
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:take take]
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:take take]
 
| <haskell>
 
| <haskell>
Line 56: Line 56:
 
</haskell>
 
</haskell>
 
|-
 
|-
| accessing the last <code>n</code> characters
+
| accessing the last <code>n</code> elements
| TODO
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:reverse reverse ], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:take take]
| TODO
+
| <haskell>
  +
reverse . take 3 . reverse $ "foobar" --> "bar"
  +
</haskell>
 
|-
 
|-
| accessing the <code>n</code> characters starting from index <code>m</code>
+
| accessing the <code>n</code> elements starting from index <code>m</code>
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:drop drop], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:take take]
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:drop drop], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:take take]
 
| <haskell>
 
| <haskell>
take 4 $ drop 2 "foo bar baz" --> "o ba"
+
take 4 $ drop 2 "foo bar baz" --> "o ba"
 
</haskell>
 
</haskell>
 
|}
 
|}
   
=== Splitting strings ===
+
=== Splitting lists ===
   
   
Line 78: Line 78:
 
| splitting a string into a list of words
 
| splitting a string into a list of words
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:words words]
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:words words]
| <haskell>words "foo bar\t baz\n" --> ["foo","bar","baz"]
+
| <haskell>words "foo bar\t baz\n" --> ["foo","bar","baz"]
 
</haskell>
 
</haskell>
 
|-
 
|-
| splitting a string into two parts
+
| splitting a list into two parts
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3AsplitAt splitAt]
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3AsplitAt splitAt]
 
| <haskell>splitAt 3 "foo bar baz" --> ("foo"," bar baz")
 
| <haskell>splitAt 3 "foo bar baz" --> ("foo"," bar baz")
 
</haskell>
 
</haskell>
 
|}
 
|}
  +
  +
== Strings ==
  +
  +
Since strings are lists of characters, you can use any available list function.
   
 
=== Multiline strings ===
 
=== Multiline strings ===
Line 104: Line 108:
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v:ord ord]
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v:ord ord]
 
|<haskell>
 
|<haskell>
import Char
+
import Data.Char
 
ord 'A' --> 65
 
ord 'A' --> 65
 
</haskell>
 
</haskell>
Line 111: Line 115:
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3Achr chr]
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3Achr chr]
 
| <haskell>
 
| <haskell>
import Char
+
import Data.Char
 
chr 99 --> 'c'
 
chr 99 --> 'c'
 
</haskell>
 
</haskell>
Line 154: Line 158:
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoUpper toUpper]
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoUpper toUpper]
 
|<haskell>
 
|<haskell>
import Char
+
import Data.Char
 
toUpper 'a' --> "A"
 
toUpper 'a' --> "A"
</haskell>
 
|-
 
| converting a string to upper-case
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoUpper toUpper], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:map map]
 
|<haskell>
 
import Char
 
map toUpper "Foo Bar" --> "FOO BAR"
 
 
</haskell>
 
</haskell>
 
|-
 
|-
Line 168: Line 165:
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoLower toLower]
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoLower toLower]
 
| <haskell>
 
| <haskell>
import Char
+
import Data.Char
 
toLower 'A' --> "a"
 
toLower 'A' --> "a"
  +
</haskell>
  +
|-
  +
| converting a string to upper-case
  +
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoUpper toUpper], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:map map]
  +
|<haskell>
  +
import Data.Char
  +
map toUpper "Foo Bar" --> "FOO BAR"
 
</haskell>
 
</haskell>
 
|-
 
|-
Line 175: Line 179:
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoLower toLower], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:map map]
 
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoLower toLower], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:map map]
 
| <haskell>
 
| <haskell>
import Char
+
import Data.Char
 
map toLower "Foo Bar" --> "foo bar"
 
map toLower "Foo Bar" --> "foo bar"
 
</haskell>
 
</haskell>

Latest revision as of 02:44, 7 August 2013

Contents

[edit] 1 Lists

In Haskell, lists are what Arrays are in most other languages.

[edit] 1.1 Creating simple lists

Problem Solution Examples
creating a list with given elements -
3 : 12 : 42 : []        --> [3,12,42]
'f' : 'o' : 'o' : []    --> "foo"
creating a list with stepsize 1 -
[1..10]                 --> [1,2,3,4,5,6,7,8,9,10]
['a'..'z']              --> "abcdefghijklmnopqrstuvwxyz"
creating a list with different stepsize -
[1,3..10]               --> [1,3,5,7,9]
['a','c'..'z']          --> "acegikmoqsuwy"
creating an infinite constant list -
[1,1..]                   --> [1,1,1,1,1,...
creating an infinite list with stepsize 1 -
[1..]                 --> [1,2,3,4,5,...

[edit] 1.2 List comprehensions

The list of all squares can also be written in a more comprehensive way, using list comprehensions:

squares = [x*x | x <- [1..]]

List comprehensions allow for constraints as well:

-- multiples of 3 or 5
mults = [ x | x <- [1..], mod x 3 == 0 || mod x 5 == 0 ]


[edit] 1.3 Combining lists

Problem Solution Examples
combining two lists (++)
"foo" ++ "bar"                  --> "foobar"
[42,43] ++ [60,61]              --> [42,43,60,61]
combining many lists concat
concat ["foo", "bar", "baz"]    --> "foobarbaz"

[edit] 1.4 Accessing sublists

Problem Solution Examples
accessing the first element head
head "foo bar baz"      --> 'f'
accessing the last element last
last "foo bar baz"      --> 'z'
accessing the element at a given index (!!)
"foo bar baz" !! 4      --> 'b'
accessing the first n elements take
take 3 "foo bar baz"    --> "foo"
accessing the last n elements reverse , take
reverse . take 3 . reverse $ "foobar"    --> "bar"
accessing the n elements starting from index m drop, take
take 4 $ drop 2 "foo bar baz"            --> "o ba"

[edit] 1.5 Splitting lists

Problem Solution Examples
splitting a string into a list of words words
words "foo bar\t baz\n"    --> ["foo","bar","baz"]
splitting a list into two parts splitAt
splitAt 3 "foo bar baz"    --> ("foo"," bar baz")

[edit] 2 Strings

Since strings are lists of characters, you can use any available list function.

[edit] 2.1 Multiline strings

"foo\
\bar"               --> "foobar"

[edit] 2.2 Converting between characters and values

Problem Solution Examples
converting a character to a numeric value ord
import Data.Char
ord 'A'    --> 65
converting a numeric value to a character chr
import Data.Char
chr 99     --> 'c'

[edit] 2.3 Reversing a string by words or characters

Problem Solution Examples
reversing a string by characters reverse
reverse "foo bar baz"                        --> "zab rab oof"
reversing a string by words words, reverse, unwords
unwords $ reverse $ words "foo bar baz"      --> "baz bar foo"
reversing a string by characters by words words, reverse, map, unwords
unwords $ map reverse $ words "foo bar baz"  --> "oof rab zab"

[edit] 2.4 Converting case

Problem Solution Examples
converting a character to upper-case toUpper
import Data.Char
toUpper 'a'            --> "A"
converting a character to lower-case toLower
import Data.Char
toLower 'A'            --> "a"
converting a string to upper-case toUpper, map
import Data.Char
map toUpper "Foo Bar"  --> "FOO BAR"
converting a string to lower-case toLower, map
import Data.Char
map toLower "Foo Bar"  --> "foo bar"

[edit] 2.5 Interpolation

TODO

[edit] 2.6 Performance

For high performance requirements (where you would typically consider C), consider using Data.ByteString.

[edit] 2.7 Unicode

TODO