Difference between revisions of "Cookbook/Lists and strings"

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(Combining lists)
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! 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 (++)]
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| 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>

Revision as of 13:45, 23 April 2009


In Haskell, lists are what Arrays are in most other languages. Haskell has all of the general list manipulation functions, see also Data.List.

head [1,2,3]      --> 1
tail [1,2,3]      --> [2,3]
length [1,2,3]    --> 3
init [1,2,3]      --> [1,2]
last [1,2,3]      --> 3

Furthermore, Haskell supports some neat concepts.

Infinite lists

Prelude> [1..]

The list of all squares:

square x = x*x
squares = map square [1..]

But in the end, you probably don't want to use infinite lists, but make them finite. You can do this with take:

Prelude> take 10 squares

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 ]

Combining lists

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

Accessing sublists

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

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 string into two parts splitAt
splitAt 3 "foo bar baz"    --> ("foo"," bar baz")


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

Multiline strings

\bar"               --> "foobar"

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'

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"

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"




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