:: Int -> [a] -> [a] -package:ilist

take n, applied to a list xs, returns the prefix of xs of length n, or xs itself if n >= length xs. It is an instance of the more general genericTake, in which n may be of any integral type.

Laziness

>>> take 0 undefined
[]

>>> take 2 (1 : 2 : undefined)
[1,2]

Examples

>>> take 5 "Hello World!"
"Hello"

>>> take 3 [1,2,3,4,5]
[1,2,3]

>>> take 3 [1,2]
[1,2]

>>> take 3 []
[]

>>> take (-1) [1,2]
[]

>>> take 0 [1,2]
[]

drop n xs returns the suffix of xs after the first n elements, or [] if n >= length xs. It is an instance of the more general genericDrop, in which n may be of any integral type.

Examples

>>> drop 6 "Hello World!"
"World!"

>>> drop 3 [1,2,3,4,5]
[4,5]

>>> drop 3 [1,2]
[]

>>> drop 3 []
[]

>>> drop (-1) [1,2]
[1,2]

>>> drop 0 [1,2]
[1,2]

takeExact n xs =
| n >= 0 && n <= length xs = take n xs
| otherwise                = error "some message"

dropExact n xs =
| n >= 0 && n <= length xs = drop n xs
| otherwise                = error "some message"

take n, applied to a list xs, returns the prefix of xs of length n, or xs itself if n >= length xs.

>>> take 5 "Hello World!"
"Hello"

>>> take 3 [1,2,3,4,5]
[1,2,3]

>>> take 3 [1,2]
[1,2]

>>> take 3 []
[]

>>> take (-1) [1,2]
[]

>>> take 0 [1,2]
[]

It is an instance of the more general genericTake, in which n may be of any integral type.

drop n xs returns the suffix of xs after the first n elements, or [] if n >= length xs.

>>> drop 6 "Hello World!"
"World!"

>>> drop 3 [1,2,3,4,5]
[4,5]

>>> drop 3 [1,2]
[]

>>> drop 3 []
[]

>>> drop (-1) [1,2]
[1,2]

>>> drop 0 [1,2]
[1,2]

It is an instance of the more general genericDrop, in which n may be of any integral type.

drop from the end of a list

Drop a number of elements from the end of the list.

dropEnd 3 "hello"  == "he"
dropEnd 5 "bye"    == ""
dropEnd (-1) "bye" == "bye"
\i xs -> dropEnd i xs `isPrefixOf` xs
\i xs -> length (dropEnd i xs) == max 0 (length xs - max 0 i)
\i -> take 3 (dropEnd 5 [i..]) == take 3 [i..]

Take a number of elements from the end of the list.

takeEnd 3 "hello"  == "llo"
takeEnd 5 "bye"    == "bye"
takeEnd (-1) "bye" == ""
\i xs -> takeEnd i xs `isSuffixOf` xs
\i xs -> length (takeEnd i xs) == min (max 0 i) (length xs)

dropRev n is like reverse . drop n . reverse but it is lazy enough to work for infinite lists, too.

\n xs -> dropRev n (xs::String) == reverse (drop n (reverse xs))

takeRev n is like reverse . take n . reverse but it is lazy enough to work for infinite lists, too.

\n xs -> takeRev n (xs::String) == reverse (take n (reverse xs))

keep every k-th value from the list

>>> sieve 6 ['a'..'z']
"agmsy"

rotate left

take n, applied to a list xs, returns the prefix of xs of length n, or xs itself if n > length xs:

take 5 "Hello World!" == "Hello"
take 3 [1,2,3,4,5] == [1,2,3]
take 3 [1,2] == [1,2]
take 3 [] == []
take (-1) [1,2] == []
take 0 [1,2] == []

It is an instance of the more general genericTake, in which n may be of any integral type.

drop n xs returns the suffix of xs after the first n elements, or [] if n > length xs:

drop 6 "Hello World!" == "World!"
drop 3 [1,2,3,4,5] == [4,5]
drop 3 [1,2] == []
drop 3 [] == []
drop (-1) [1,2] == [1,2]
drop 0 [1,2] == [1,2]

It is an instance of the more general genericDrop, in which n may be of any integral type.

Shuffle a list given a seed for an RNG

Take elements from the end of a list.

Cycle a list by the given count. If positive, cycle to the left. If negative, cycle to the right:

>>> cycleN 2 [1,2,3,4,5]
[3,4,5,1,2]

>>> cycleN (-2) [1,2,3,4,5]
[4,5,1,2,3]

downsample throws away every n'th sample, eg,

downsample 2 [ 1, 2, 3, 4, 5, 6 ] == [ 1, 3, 5 ]

upsampleAndHold replicates each sample n times, eg,

upsampleAndHold 3 [ 1, 2, 3 ] == [ 1, 1, 1, 2, 2, 2, 3, 3, 3 ]