`take' gives the first n elements of a list (or less if the list is shorter). `tails' gives a list of all suffixes of a list.
Signed-off-by: Matthias Görgens <[email protected]> stdext/listext.ml | 18 ++++++++++++++++++ stdext/listext.mli | 5 +++++ 2 files changed, 23 insertions(+), 0 deletions(-)
# HG changeset patch # User Matthias Görgens <[email protected]> # Date 1265126416 0 # Node ID c3c6244bb7d6387caed5c200ead5b20fab27485f # Parent 93721338057fdff1465e54be45bd282fd1af0de0 Adds take and tails to our extended list module `take' gives the first n elements of a list (or less if the list is shorter). `tails' gives a list of all suffixes of a list. Signed-off-by: Matthias Görgens <[email protected]> diff -r 93721338057f -r c3c6244bb7d6 stdext/listext.ml --- a/stdext/listext.ml +++ b/stdext/listext.ml @@ -11,6 +11,7 @@ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU Lesser General Public License for more details. *) +open Fun module List = struct include List (** Turn a list into a set *) @@ -174,4 +175,21 @@ (* Like the Lisp cons *) let cons a b = a :: b +(* Could use fold_left to get the same value, but that would necessarily go through the whole list everytime, instead of the first n items, only. *) +(* ToDo: This is complicated enough to warrant a test. *) +(* Is it wise to fail silently on negative values? (They are treated as zero, here.) + Pro: Would mask fewer bugs. + Con: Less robust. +*) +let take n list = + let rec helper i acc list = + if i <= 0 || list = [] + then acc + else helper (i-1) (List.hd list :: acc) (List.tl list) + in List.rev $ helper n [] list + +(* Thanks to sharing we only use linear space. (Roughly double the space needed for the spine of the original list) *) +let rec tails = function + | [] -> [[]] + | (_::xs) as l -> l :: tails xs end diff -r 93721338057f -r c3c6244bb7d6 stdext/listext.mli --- a/stdext/listext.mli +++ b/stdext/listext.mli @@ -172,4 +172,9 @@ (* Like Lisp cons*) val cons : 'a -> 'a list -> 'a list + (* take n list: Return the first n elements of list (or less if list is shorter).*) + val take : int -> 'a list -> 'a list + + val tails : 'a list -> ('a list) list + end
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