Difference between revisions of "Cookbook/Lists and strings"
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− | == |
+ | == Lists == |
+ | |||
+ | In Haskell, lists are what Arrays are in most other languages. |
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+ | |||
+ | === Creating simple lists === |
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+ | |||
+ | {| class="wikitable" |
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⚫ | |||
+ | ! Problem |
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+ | ! Solution |
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+ | ! Examples |
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+ | |- |
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+ | | creating a list with given elements |
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⚫ | |||
⚫ | |||
+ | 3 : 12 : 42 : [] --> [3,12,42] |
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⚫ | |||
⚫ | |||
+ | |- |
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+ | | creating a list with stepsize 1 |
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⚫ | |||
+ | |<haskell> |
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+ | [1..10] --> [1,2,3,4,5,6,7,8,9,10] |
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+ | ['a'..'z'] --> "abcdefghijklmnopqrstuvwxyz" |
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+ | </haskell> |
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+ | |- |
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+ | | creating a list with different stepsize |
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+ | | - |
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+ | |<haskell> |
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+ | [1,3..10] --> [1,3,5,7,9] |
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+ | ['a','c'..'z'] --> "acegikmoqsuwy" |
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+ | </haskell> |
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+ | |- |
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+ | | creating an infinite constant list |
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+ | | - |
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+ | |<haskell> |
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+ | [1,1..] --> [1,1,1,1,1,... |
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+ | </haskell> |
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+ | |- |
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+ | | creating an infinite list with stepsize 1 |
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+ | | - |
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+ | | <haskell> |
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+ | [1..] --> [1,2,3,4,5,... |
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+ | </haskell> |
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+ | |} |
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+ | |||
+ | === List comprehensions === |
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+ | |||
+ | The list of all squares can also be written in a more comprehensive way, using list comprehensions: |
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+ | |||
+ | <haskell> |
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+ | squares = [x*x | x <- [1..]] |
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+ | </haskell> |
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+ | |||
+ | List comprehensions allow for constraints as well: |
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+ | |||
+ | <haskell> |
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+ | -- multiples of 3 or 5 |
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+ | mults = [ x | x <- [1..], mod x 3 == 0 || mod x 5 == 0 ] |
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+ | </haskell> |
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⚫ | |||
− | === Combining |
+ | === Combining lists === |
{| class="wikitable" |
{| class="wikitable" |
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Line 11: | Line 69: | ||
! Examples |
! Examples |
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|- |
|- |
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− | | combining two |
+ | | 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-4.12.0.0/Prelude.html#v%3A%2B%2B (++)] |
|<haskell> |
|<haskell> |
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"foo" ++ "bar" --> "foobar" |
"foo" ++ "bar" --> "foobar" |
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+ | [42,43] ++ [60,61] --> [42,43,60,61] |
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</haskell> |
</haskell> |
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|- |
|- |
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− | | combining many |
+ | | 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-4.12.0.0/Prelude.html#v:concat concat] |
| <haskell> |
| <haskell> |
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concat ["foo", "bar", "baz"] --> "foobarbaz" |
concat ["foo", "bar", "baz"] --> "foobarbaz" |
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|} |
|} |
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− | === Accessing |
+ | === Accessing sublists === |
{| class="wikitable" |
{| class="wikitable" |
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! Examples |
! Examples |
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|- |
|- |
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− | | accessing the first |
+ | | 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-4.12.0.0/Prelude.html#v:head head] |
|<haskell> |
|<haskell> |
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head "foo bar baz" --> 'f' |
head "foo bar baz" --> 'f' |
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</haskell> |
</haskell> |
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|- |
|- |
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− | | accessing the last |
+ | | 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-4.12.0.0/Prelude.html#v%3Alast last] |
|<haskell> |
|<haskell> |
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last "foo bar baz" --> 'z' |
last "foo bar baz" --> 'z' |
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</haskell> |
</haskell> |
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|- |
|- |
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− | | accessing the |
+ | | 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-4.12.0.0/Prelude.html#v%3A!! (!!)] |
|<haskell> |
|<haskell> |
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"foo bar baz" !! 4 --> 'b' |
"foo bar baz" !! 4 --> 'b' |
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</haskell> |
</haskell> |
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|- |
|- |
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− | | accessing the first <code>n</code> |
+ | | 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-4.12.0.0/Prelude.html#v:take take] |
| <haskell> |
| <haskell> |
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take 3 "foo bar baz" --> "foo" |
take 3 "foo bar baz" --> "foo" |
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</haskell> |
</haskell> |
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|- |
|- |
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− | | accessing the last <code>n</code> |
+ | | accessing the last <code>n</code> elements |
+ | | [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:reverse reverse ], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:take take] |
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⚫ | |||
+ | | <haskell> |
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⚫ | |||
+ | reverse . take 3 . reverse $ "foobar" --> "bar" |
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+ | </haskell> |
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|- |
|- |
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− | | accessing the <code>n</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-4.12.0.0/Prelude.html#v:drop drop], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:take take] |
| <haskell> |
| <haskell> |
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− | take 4 $ drop 2 "foo bar baz" --> "o ba" |
+ | take 4 $ drop 2 "foo bar baz" --> "o ba" |
</haskell> |
</haskell> |
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|} |
|} |
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− | === Splitting |
+ | === Splitting lists === |
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|- |
|- |
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| splitting a string into a list of words |
| splitting a string into a list of words |
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− | | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:words words] |
+ | | [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:words words] |
− | | <haskell>words "foo bar\t baz\n" |
+ | | <haskell>words "foo bar\t baz\n" --> ["foo","bar","baz"] |
</haskell> |
</haskell> |
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|- |
|- |
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− | | splitting a |
+ | | 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-4.12.0.0/Prelude.html#v%3AsplitAt splitAt] |
| <haskell>splitAt 3 "foo bar baz" --> ("foo"," bar baz") |
| <haskell>splitAt 3 "foo bar baz" --> ("foo"," bar baz") |
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</haskell> |
</haskell> |
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|} |
|} |
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+ | |||
+ | == Strings == |
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+ | |||
⚫ | |||
=== Multiline strings === |
=== Multiline strings === |
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|- |
|- |
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| converting a character to a numeric value |
| converting a character to a numeric value |
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− | | [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v:ord ord] |
+ | | [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Data-Char.html#v:ord ord] |
|<haskell> |
|<haskell> |
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− | import Char |
+ | import Data.Char |
ord 'A' --> 65 |
ord 'A' --> 65 |
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</haskell> |
</haskell> |
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|- |
|- |
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| converting a numeric value to a character |
| converting a numeric value to a character |
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− | | [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3Achr chr] |
+ | | [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Data-Char.html#v%3Achr chr] |
| <haskell> |
| <haskell> |
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− | import Char |
+ | import Data.Char |
chr 99 --> 'c' |
chr 99 --> 'c' |
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</haskell> |
</haskell> |
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|- |
|- |
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| reversing a string by characters |
| reversing a string by characters |
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− | | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:reverse reverse] |
+ | | [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:reverse reverse] |
|<haskell> |
|<haskell> |
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reverse "foo bar baz" --> "zab rab oof" |
reverse "foo bar baz" --> "zab rab oof" |
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|- |
|- |
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| reversing a string by words |
| reversing a string by words |
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− | | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Awords words], [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%3Aunwords unwords] |
+ | | [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v%3Awords words], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:reverse reverse], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v%3Aunwords unwords] |
| <haskell> |
| <haskell> |
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unwords $ reverse $ words "foo bar baz" --> "baz bar foo" |
unwords $ reverse $ words "foo bar baz" --> "baz bar foo" |
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|- |
|- |
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| reversing a string by characters by words |
| reversing a string by characters by words |
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− | | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Awords words], [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:map map], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Aunwords unwords] |
+ | | [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v%3Awords words], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:reverse reverse], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:map map], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v%3Aunwords unwords] |
| <haskell> |
| <haskell> |
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unwords $ map reverse $ words "foo bar baz" --> "oof rab zab" |
unwords $ map reverse $ words "foo bar baz" --> "oof rab zab" |
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|- |
|- |
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| converting a character to upper-case |
| converting a character to upper-case |
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− | | [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoUpper toUpper] |
+ | | [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Data-Char.html#v%3AtoUpper toUpper] |
|<haskell> |
|<haskell> |
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− | import Char |
+ | import Data.Char |
− | toUpper 'a' --> |
+ | toUpper 'a' --> 'A' |
+ | </haskell> |
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+ | |- |
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⚫ | |||
⚫ | |||
+ | | <haskell> |
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⚫ | |||
+ | toLower 'A' --> 'a' |
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</haskell> |
</haskell> |
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|- |
|- |
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| converting a string to upper-case |
| converting a string to upper-case |
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− | | [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] |
+ | | [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Data-Char.html#v%3AtoUpper toUpper], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:map map] |
|<haskell> |
|<haskell> |
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− | import Char |
+ | import Data.Char |
map toUpper "Foo Bar" --> "FOO BAR" |
map toUpper "Foo Bar" --> "FOO BAR" |
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⚫ | |||
⚫ | |||
⚫ | |||
⚫ | |||
⚫ | |||
⚫ | |||
⚫ | |||
</haskell> |
</haskell> |
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|- |
|- |
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| converting a string to lower-case |
| converting a string to lower-case |
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− | | [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-4.12.0.0/Data-Char.html#v%3AtoLower toLower], [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Prelude.html#v:map map] |
| <haskell> |
| <haskell> |
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− | import Char |
+ | import Data.Char |
map toLower "Foo Bar" --> "foo bar" |
map toLower "Foo Bar" --> "foo bar" |
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</haskell> |
</haskell> |
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=== Performance === |
=== Performance === |
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+ | Text handles character strings with better performance than Strings; it should be the prefered data type for UTF-8 encoded strings. |
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− | For high performance requirements (where you would typically consider |
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+ | |||
− | + | If observe that Text does not give sufficient performance, consider [http://hackage.haskell.org/packages/archive/bytestring/latest/doc/html/Data-ByteString.html Data.ByteString], which is essentially a byte array. It can contain UTF-8 characters, but handle with care! . |
|
=== Unicode === |
=== Unicode === |
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+ | Current GHC (later than 6) encodes Strings and Text in UTF-8. This may change the behavior of some of the functions explained above when applied to characters beyond the traditional ASCII characters. Remember that not every character in UTF-8 encoding is one byte! |
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− | TODO |
Latest revision as of 21:02, 6 January 2019
Lists
In Haskell, lists are what Arrays are in most other languages.
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,...
|
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"
[42,43] ++ [60,61] --> [42,43,60,61]
|
combining many lists | concat | concat ["foo", "bar", "baz"] --> "foobarbaz"
|
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"
|
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")
|
Strings
Since strings are lists of characters, you can use any available list function.
Multiline strings
"foo\
\bar" --> "foobar"
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'
|
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 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"
|
Interpolation
TODO
Performance
Text handles character strings with better performance than Strings; it should be the prefered data type for UTF-8 encoded strings.
If observe that Text does not give sufficient performance, consider Data.ByteString, which is essentially a byte array. It can contain UTF-8 characters, but handle with care! .
Unicode
Current GHC (later than 6) encodes Strings and Text in UTF-8. This may change the behavior of some of the functions explained above when applied to characters beyond the traditional ASCII characters. Remember that not every character in UTF-8 encoding is one byte!