Cookbook/Lists and strings
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1 Lists
In Haskell, lists are what Arrays are in most other languages.
1.1 Creating 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,... |
| creating an infinite list | - | [1,2..] --> [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
