# Difference between revisions of "Cookbook/Lists and strings"

## 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
```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"
```

TODO

### Performance

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

TODO