Difference between revisions of "Cookbook/Lists and strings"
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(→Infinite lists: creating lists) |
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Furthermore, Haskell supports some neat concepts. |
Furthermore, Haskell supports some neat concepts. |
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− | == |
+ | == Creating lists == |
⚫ | |||
− | Prelude> [1..] |
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− | </haskell> |
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+ | {| class="wikitable" |
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− | The list of all squares: |
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+ | |- |
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− | <haskell> |
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+ | ! Problem |
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− | square x = x*x |
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+ | ! Solution |
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− | squares = map square [1..] |
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+ | ! Examples |
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+ | |- |
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+ | | creating an infinite constant list |
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+ | | - |
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⚫ | |||
+ | [1..] --> [1,1,1,1,... |
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</haskell> |
</haskell> |
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+ | |- |
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− | |||
+ | | creating an infinite list |
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− | But in the end, you probably don't want to use infinite lists, but make them finite. You can do this with <hask>take</hask>: |
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+ | | - |
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− | |||
− | <haskell> |
+ | | <haskell> |
+ | [1,2..] --> [1,2,3,4,5,... |
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− | Prelude> take 10 squares |
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− | [1,4,9,16,25,36,49,64,81,100] |
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</haskell> |
</haskell> |
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+ | |} |
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== List comprehensions == |
== List comprehensions == |
Revision as of 13:48, 23 April 2009
Lists
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.
Creating lists
Problem | Solution | Examples |
---|---|---|
creating an infinite constant list | - | [1..] --> [1,1,1,1,...
|
creating an infinite list | - | [1,2..] --> [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"
|
combining many lists | concat | concat ["foo", "bar", "baz"] --> "foobarbaz"
|
Accessing sublists
Problem | Solution | Examples |
---|---|---|
accessing the first element | head | head "foo bar baz" --> 'f'
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accessing the last element | last | last "foo bar baz" --> 'z'
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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")
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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 Char
ord 'A' --> 65
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converting a numeric value to a character | chr | import Char
chr 99 --> 'c'
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Reversing a string by words or characters
Problem | Solution | Examples |
---|---|---|
reversing a string by characters | reverse | reverse "foo bar baz" --> "zab rab oof"
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reversing a string by words | words, reverse, unwords | unwords $ reverse $ words "foo bar baz" --> "baz bar foo"
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reversing a string by characters by words | words, reverse, map, unwords | unwords $ map reverse $ words "foo bar baz" --> "oof rab zab"
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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"
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converting a string to upper-case | toUpper, map | import Char
map toUpper "Foo Bar" --> "FOO BAR"
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converting a string to lower-case | toLower, map | import Char
map toLower "Foo Bar" --> "foo bar"
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Interpolation
TODO
Performance
For high performance requirements (where you would typically consider C), consider using Data.ByteString.
Unicode
TODO