Personal tools

Cookbook/Lists and strings

From HaskellWiki

< Cookbook(Difference between revisions)
Jump to: navigation, search
(Creating lists)
m (Fixed a couple of the list shorthands)
(3 intermediate revisions by one user not shown)
Line 1: Line 1:
= Lists =
+
== Lists ==
   
 
In Haskell, lists are what Arrays are in most other languages.
 
In Haskell, lists are what Arrays are in most other languages.
   
== Creating lists ==
+
=== Creating simple lists ===
   
 
{| class="wikitable"
 
{| class="wikitable"
Line 35: Line 35:
 
| -
 
| -
 
|<haskell>
 
|<haskell>
[1..] --> [1,1,1,1,1,...
+
[1,1..] --> [1,1,1,1,1,...
 
</haskell>
 
</haskell>
 
|-
 
|-
| creating an infinite list
+
| creating an infinite list with stepsize 1
 
| -
 
| -
 
| <haskell>
 
| <haskell>
[1,2..] --> [1,2,3,4,5,...
+
[1..] --> [1,2,3,4,5,...
 
</haskell>
 
</haskell>
 
|}
 
|}
   
== List comprehensions ==
+
=== List comprehensions ===
   
 
The list of all squares can also be written in a more comprehensive way, using list comprehensions:
 
The list of all squares can also be written in a more comprehensive way, using list comprehensions:
Line 61: Line 61:
   
   
== Combining lists ==
+
=== Combining lists ===
   
 
{| class="wikitable"
 
{| class="wikitable"
Line 83: Line 83:
 
|}
 
|}
   
== Accessing sublists ==
+
=== Accessing sublists ===
   
 
{| class="wikitable"
 
{| class="wikitable"
Line 128: Line 128:
 
|}
 
|}
   
== Splitting lists ==
+
=== Splitting lists ===
   
   
Line 148: Line 148:
 
|}
 
|}
   
= Strings =
+
== Strings ==
   
 
Since strings are lists of characters, you can use any available list function.
 
Since strings are lists of characters, you can use any available list function.
   
== Multiline strings ==
+
=== Multiline strings ===
 
<haskell>
 
<haskell>
 
"foo\
 
"foo\
Line 158: Line 158:
 
</haskell>
 
</haskell>
   
== Converting between characters and values ==
+
=== Converting between characters and values ===
   
 
{| class="wikitable"
 
{| class="wikitable"
Line 181: Line 181:
 
|}
 
|}
   
== Reversing a string by words or characters ==
+
=== Reversing a string by words or characters ===
   
 
{| class="wikitable"
 
{| class="wikitable"
Line 208: Line 208:
 
|}
 
|}
   
== Converting case ==
+
=== Converting case ===
   
 
{| class="wikitable"
 
{| class="wikitable"
Line 245: Line 245:
 
|}
 
|}
   
== Interpolation ==
+
=== Interpolation ===
   
 
TODO
 
TODO
   
== Performance ==
+
=== Performance ===
   
 
For high performance requirements (where you would typically consider
 
For high performance requirements (where you would typically consider
 
C), consider using [http://hackage.haskell.org/packages/archive/bytestring/latest/doc/html/Data-ByteString.html Data.ByteString].
 
C), consider using [http://hackage.haskell.org/packages/archive/bytestring/latest/doc/html/Data-ByteString.html Data.ByteString].
   
== Unicode ==
+
=== Unicode ===
   
 
TODO
 
TODO

Revision as of 02:10, 1 February 2010

Contents

1 Lists

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

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,...

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 ]


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"

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"

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")

2 Strings

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

2.1 Multiline strings

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

2.2 Converting between characters and values

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

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"

2.4 Converting case

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

2.5 Interpolation

TODO

2.6 Performance

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

2.7 Unicode

TODO