Difference between revisions of "Cookbook/Lists and strings"

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m (Fix quote characters in toUpper/toLower examples)
m (Fixed broken links)
 
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|-
 
|-
 
| combining two lists
 
| 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>
 
"foo" ++ "bar" --> "foobar"
 
"foo" ++ "bar" --> "foobar"
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|-
 
|-
 
| combining many lists
 
| 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>
 
concat ["foo", "bar", "baz"] --> "foobarbaz"
 
concat ["foo", "bar", "baz"] --> "foobarbaz"
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|-
 
|-
 
| accessing the first element
 
| 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>
 
head "foo bar baz" --> 'f'
 
head "foo bar baz" --> 'f'
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|-
 
|-
 
| accessing the last element
 
| 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>
 
last "foo bar baz" --> 'z'
 
last "foo bar baz" --> 'z'
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|-
 
|-
 
| accessing the element 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-4.12.0.0/Prelude.html#v%3A!! (!!)]
 
|<haskell>
 
|<haskell>
 
"foo bar baz" !! 4 --> 'b'
 
"foo bar baz" !! 4 --> 'b'
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|-
 
|-
 
| accessing the first <code>n</code> elements
 
| 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>
 
take 3 "foo bar baz" --> "foo"
 
take 3 "foo bar baz" --> "foo"
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|-
 
|-
 
| accessing the last <code>n</code> elements
 
| accessing the last <code>n</code> elements
| [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]
+
| [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]
 
| <haskell>
 
| <haskell>
 
reverse . take 3 . reverse $ "foobar" --> "bar"
 
reverse . take 3 . reverse $ "foobar" --> "bar"
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|-
 
|-
 
| accessing the <code>n</code> elements 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-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>
 
take 4 $ drop 2 "foo bar baz" --> "o ba"
 
take 4 $ drop 2 "foo bar baz" --> "o ba"
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|-
 
|-
 
| 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-4.12.0.0/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 list 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-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")
 
</haskell>
 
</haskell>
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|-
 
|-
 
| converting a character to a numeric value
 
| converting a character to a numeric value
| [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>
 
import Data.Char
 
import Data.Char
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|-
 
|-
 
| converting a numeric value to a character
 
| converting a numeric value to a character
| [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>
 
import Data.Char
 
import Data.Char
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|-
 
|-
 
| reversing a string by characters
 
| reversing a string by characters
| [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>
 
reverse "foo bar baz" --> "zab rab oof"
 
reverse "foo bar baz" --> "zab rab oof"
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|-
 
|-
 
| reversing a string by words
 
| reversing a string by words
| [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>
 
unwords $ reverse $ words "foo bar baz" --> "baz bar foo"
 
unwords $ reverse $ words "foo bar baz" --> "baz bar foo"
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|-
 
|-
 
| reversing a string by characters by words
 
| reversing a string by characters by words
| [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>
 
unwords $ map reverse $ words "foo bar baz" --> "oof rab zab"
 
unwords $ map reverse $ words "foo bar baz" --> "oof rab zab"
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|-
 
|-
 
| converting a character to upper-case
 
| converting a character 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-4.12.0.0/Data-Char.html#v%3AtoUpper toUpper]
 
|<haskell>
 
|<haskell>
 
import Data.Char
 
import Data.Char
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|-
 
|-
 
| converting a character to lower-case
 
| converting a character to lower-case
| [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoLower toLower]
+
| [http://haskell.org/ghc/docs/latest/html/libraries/base-4.12.0.0/Data-Char.html#v%3AtoLower toLower]
 
| <haskell>
 
| <haskell>
 
import Data.Char
 
import Data.Char
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|-
 
|-
 
| converting a string to upper-case
 
| 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]
+
| [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>
 
import Data.Char
 
import Data.Char
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|-
 
|-
 
| converting a string to lower-case
 
| converting a string to lower-case
| [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>
 
import Data.Char
 
import Data.Char

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!