List(3o) OCaml library List(3o)
NAME
List - List operations.
Module
Module List
Documentation
Module List
: sig end
List operations.
Some functions are flagged as not tail-recursive. A tail-recursive
function uses constant stack space, while a non-tail-recursive function
uses stack space proportional to the length of its list argument, which
can be a problem with very long lists. When the function takes several
list arguments, an approximate formula giving stack usage (in some un-
specified constant unit) is shown in parentheses.
The above considerations can usually be ignored if your lists are not
longer than about 10000 elements.
The labeled version of this module can be used as described in the Std-
Labels module.
type 'a t = 'a list =
| []
| (::) of 'a * 'a list
An alias for the type of lists.
val length : 'a list -> int
Return the length (number of elements) of the given list.
val compare_lengths : 'a list -> 'b list -> int
Compare the lengths of two lists. compare_lengths l1 l2 is equivalent
to compare (length l1) (length l2) , except that the computation stops
after reaching the end of the shortest list.
Since 4.05.0
val compare_length_with : 'a list -> int -> int
Compare the length of a list to an integer. compare_length_with l len
is equivalent to compare (length l) len , except that the computation
stops after at most len iterations on the list.
Since 4.05.0
val cons : 'a -> 'a list -> 'a list
cons x xs is x :: xs
Since 4.03.0 (4.05.0 in ListLabels)
val hd : 'a list -> 'a
Return the first element of the given list.
Raises Failure if the list is empty.
val tl : 'a list -> 'a list
Return the given list without its first element.
Raises Failure if the list is empty.
val nth : 'a list -> int -> 'a
Return the n -th element of the given list. The first element (head of
the list) is at position 0.
Raises Failure if the list is too short.
Raises Invalid_argument if n is negative.
val nth_opt : 'a list -> int -> 'a option
Return the n -th element of the given list. The first element (head of
the list) is at position 0. Return None if the list is too short.
Since 4.05
Raises Invalid_argument if n is negative.
val rev : 'a list -> 'a list
List reversal.
val init : int -> (int -> 'a) -> 'a list
init len f is f 0; f 1; ...; f (len-1) , evaluated left to right.
Since 4.06.0
Raises Invalid_argument if len < 0 .
val append : 'a list -> 'a list -> 'a list
Concatenate two lists. Same function as the infix operator @ . Not
tail-recursive (length of the first argument). The @ operator is not
tail-recursive either.
val rev_append : 'a list -> 'a list -> 'a list
rev_append l1 l2 reverses l1 and concatenates it with l2 . This is
equivalent to ( List.rev l1) @ l2 , but rev_append is tail-recursive
and more efficient.
val concat : 'a list list -> 'a list
Concatenate a list of lists. The elements of the argument are all con-
catenated together (in the same order) to give the result. Not
tail-recursive (length of the argument + length of the longest
sub-list).
val flatten : 'a list list -> 'a list
Same as List.concat . Not tail-recursive (length of the argument +
length of the longest sub-list).
Comparison
val equal : ('a -> 'a -> bool) -> 'a list -> 'a list -> bool
equal eq [a1; ...; an] [b1; ..; bm] holds when the two input lists have
the same length, and for each pair of elements ai , bi at the same po-
sition we have eq ai bi .
Note: the eq function may be called even if the lists have different
length. If you know your equality function is costly, you may want to
check List.compare_lengths first.
Since 4.12.0
val compare : ('a -> 'a -> int) -> 'a list -> 'a list -> int
compare cmp [a1; ...; an] [b1; ...; bm] performs a lexicographic com-
parison of the two input lists, using the same 'a -> 'a -> int inter-
face as compare :
- a1 :: l1 is smaller than a2 :: l2 (negative result) if a1 is smaller
than a2 , or if they are equal (0 result) and l1 is smaller than l2
-the empty list [] is strictly smaller than non-empty lists
Note: the cmp function will be called even if the lists have different
lengths.
Since 4.12.0
Iterators
val iter : ('a -> unit) -> 'a list -> unit
iter f [a1; ...; an] applies function f in turn to a1; ...; an . It is
equivalent to begin f a1; f a2; ...; f an; () end .
val iteri : (int -> 'a -> unit) -> 'a list -> unit
Same as List.iter , but the function is applied to the index of the el-
ement as first argument (counting from 0), and the element itself as
second argument.
Since 4.00.0
val map : ('a -> 'b) -> 'a list -> 'b list
map f [a1; ...; an] applies function f to a1, ..., an , and builds the
list [f a1; ...; f an] with the results returned by f . Not tail-recur-
sive.
val mapi : (int -> 'a -> 'b) -> 'a list -> 'b list
Same as List.map , but the function is applied to the index of the ele-
ment as first argument (counting from 0), and the element itself as
second argument. Not tail-recursive.
Since 4.00.0
val rev_map : ('a -> 'b) -> 'a list -> 'b list
rev_map f l gives the same result as List.rev ( List.map f l) , but is
tail-recursive and more efficient.
val filter_map : ('a -> 'b option) -> 'a list -> 'b list
filter_map f l applies f to every element of l , filters out the None
elements and returns the list of the arguments of the Some elements.
Since 4.08.0
val concat_map : ('a -> 'b list) -> 'a list -> 'b list
concat_map f l gives the same result as List.concat ( List.map f l) .
Tail-recursive.
Since 4.10.0
val fold_left_map : ('a -> 'b -> 'a * 'c) -> 'a -> 'b list -> 'a * 'c
list
fold_left_map is a combination of fold_left and map that threads an
accumulator through calls to f .
Since 4.11.0
val fold_left : ('a -> 'b -> 'a) -> 'a -> 'b list -> 'a
fold_left f init [b1; ...; bn] is f (... (f (f init b1) b2) ...) bn .
val fold_right : ('a -> 'b -> 'b) -> 'a list -> 'b -> 'b
fold_right f [a1; ...; an] init is f a1 (f a2 (... (f an init) ...)) .
Not tail-recursive.
Iterators on two lists
val iter2 : ('a -> 'b -> unit) -> 'a list -> 'b list -> unit
iter2 f [a1; ...; an] [b1; ...; bn] calls in turn f a1 b1; ...; f an bn
.
Raises Invalid_argument if the two lists are determined to have differ-
ent lengths.
val map2 : ('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list
map2 f [a1; ...; an] [b1; ...; bn] is [f a1 b1; ...; f an bn] .
Raises Invalid_argument if the two lists are determined to have differ-
ent lengths. Not tail-recursive.
val rev_map2 : ('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list
rev_map2 f l1 l2 gives the same result as List.rev ( List.map2 f l1 l2)
, but is tail-recursive and more efficient.
val fold_left2 : ('a -> 'b -> 'c -> 'a) -> 'a -> 'b list -> 'c list ->
'a
fold_left2 f init [a1; ...; an] [b1; ...; bn] is f (... (f (f init a1
b1) a2 b2) ...) an bn .
Raises Invalid_argument if the two lists are determined to have differ-
ent lengths.
val fold_right2 : ('a -> 'b -> 'c -> 'c) -> 'a list -> 'b list -> 'c ->
'c
fold_right2 f [a1; ...; an] [b1; ...; bn] init is f a1 b1 (f a2 b2 (...
(f an bn init) ...)) .
Raises Invalid_argument if the two lists are determined to have differ-
ent lengths. Not tail-recursive.
List scanning
val for_all : ('a -> bool) -> 'a list -> bool
for_all f [a1; ...; an] checks if all elements of the list satisfy the
predicate f . That is, it returns (f a1) && (f a2) && ... && (f an) for
a non-empty list and true if the list is empty.
val exists : ('a -> bool) -> 'a list -> bool
exists f [a1; ...; an] checks if at least one element of the list sat-
isfies the predicate f . That is, it returns (f a1) || (f a2) || ... ||
(f an) for a non-empty list and false if the list is empty.
val for_all2 : ('a -> 'b -> bool) -> 'a list -> 'b list -> bool
Same as List.for_all , but for a two-argument predicate.
Raises Invalid_argument if the two lists are determined to have differ-
ent lengths.
val exists2 : ('a -> 'b -> bool) -> 'a list -> 'b list -> bool
Same as List.exists , but for a two-argument predicate.
Raises Invalid_argument if the two lists are determined to have differ-
ent lengths.
val mem : 'a -> 'a list -> bool
mem a set is true if and only if a is equal to an element of set .
val memq : 'a -> 'a list -> bool
Same as List.mem , but uses physical equality instead of structural
equality to compare list elements.
List searching
val find : ('a -> bool) -> 'a list -> 'a
find f l returns the first element of the list l that satisfies the
predicate f .
Raises Not_found if there is no value that satisfies f in the list l .
val find_opt : ('a -> bool) -> 'a list -> 'a option
find f l returns the first element of the list l that satisfies the
predicate f . Returns None if there is no value that satisfies f in
the list l .
Since 4.05
val find_map : ('a -> 'b option) -> 'a list -> 'b option
find_map f l applies f to the elements of l in order, and returns the
first result of the form Some v , or None if none exist.
Since 4.10.0
val filter : ('a -> bool) -> 'a list -> 'a list
filter f l returns all the elements of the list l that satisfy the
predicate f . The order of the elements in the input list is preserved.
val find_all : ('a -> bool) -> 'a list -> 'a list
find_all is another name for List.filter .
val filteri : (int -> 'a -> bool) -> 'a list -> 'a list
Same as List.filter , but the predicate is applied to the index of the
element as first argument (counting from 0), and the element itself as
second argument.
Since 4.11.0
val partition : ('a -> bool) -> 'a list -> 'a list * 'a list
partition f l returns a pair of lists (l1, l2) , where l1 is the list
of all the elements of l that satisfy the predicate f , and l2 is the
list of all the elements of l that do not satisfy f . The order of the
elements in the input list is preserved.
val partition_map : ('a -> ('b, 'c) Either.t) -> 'a list -> 'b list *
'c list
partition_map f l returns a pair of lists (l1, l2) such that, for each
element x of the input list l :
-if f x is Left y1 , then y1 is in l1 , and
-if f x is Right y2 , then y2 is in l2 .
The output elements are included in l1 and l2 in the same relative or-
der as the corresponding input elements in l .
In particular, partition_map (fun x -> if f x then Left x else Right x)
l is equivalent to partition f l .
Since 4.12.0
Association lists
val assoc : 'a -> ('a * 'b) list -> 'b
assoc a l returns the value associated with key a in the list of pairs
l . That is, assoc a [ ...; (a,b); ...] = b if (a,b) is the leftmost
binding of a in list l .
Raises Not_found if there is no value associated with a in the list l .
val assoc_opt : 'a -> ('a * 'b) list -> 'b option
assoc_opt a l returns the value associated with key a in the list of
pairs l . That is, assoc_opt a [ ...; (a,b); ...] = Some b if (a,b) is
the leftmost binding of a in list l . Returns None if there is no
value associated with a in the list l .
Since 4.05
val assq : 'a -> ('a * 'b) list -> 'b
Same as List.assoc , but uses physical equality instead of structural
equality to compare keys.
val assq_opt : 'a -> ('a * 'b) list -> 'b option
Same as List.assoc_opt , but uses physical equality instead of struc-
tural equality to compare keys.
Since 4.05.0
val mem_assoc : 'a -> ('a * 'b) list -> bool
Same as List.assoc , but simply return true if a binding exists, and
false if no bindings exist for the given key.
val mem_assq : 'a -> ('a * 'b) list -> bool
Same as List.mem_assoc , but uses physical equality instead of struc-
tural equality to compare keys.
val remove_assoc : 'a -> ('a * 'b) list -> ('a * 'b) list
remove_assoc a l returns the list of pairs l without the first pair
with key a , if any. Not tail-recursive.
val remove_assq : 'a -> ('a * 'b) list -> ('a * 'b) list
Same as List.remove_assoc , but uses physical equality instead of
structural equality to compare keys. Not tail-recursive.
Lists of pairs
val split : ('a * 'b) list -> 'a list * 'b list
Transform a list of pairs into a pair of lists: split [(a1,b1); ...;
(an,bn)] is ([a1; ...; an], [b1; ...; bn]) . Not tail-recursive.
val combine : 'a list -> 'b list -> ('a * 'b) list
Transform a pair of lists into a list of pairs: combine [a1; ...; an]
[b1; ...; bn] is [(a1,b1); ...; (an,bn)] .
Raises Invalid_argument if the two lists have different lengths. Not
tail-recursive.
Sorting
val sort : ('a -> 'a -> int) -> 'a list -> 'a list
Sort a list in increasing order according to a comparison function. The
comparison function must return 0 if its arguments compare as equal, a
positive integer if the first is greater, and a negative integer if the
first is smaller (see Array.sort for a complete specification). For ex-
ample, compare is a suitable comparison function. The resulting list
is sorted in increasing order. List.sort is guaranteed to run in con-
stant heap space (in addition to the size of the result list) and loga-
rithmic stack space.
The current implementation uses Merge Sort. It runs in constant heap
space and logarithmic stack space.
val stable_sort : ('a -> 'a -> int) -> 'a list -> 'a list
Same as List.sort , but the sorting algorithm is guaranteed to be sta-
ble (i.e. elements that compare equal are kept in their original or-
der).
The current implementation uses Merge Sort. It runs in constant heap
space and logarithmic stack space.
val fast_sort : ('a -> 'a -> int) -> 'a list -> 'a list
Same as List.sort or List.stable_sort , whichever is faster on typical
input.
val sort_uniq : ('a -> 'a -> int) -> 'a list -> 'a list
Same as List.sort , but also remove duplicates.
Since 4.02.0 (4.03.0 in ListLabels)
val merge : ('a -> 'a -> int) -> 'a list -> 'a list -> 'a list
Merge two lists: Assuming that l1 and l2 are sorted according to the
comparison function cmp , merge cmp l1 l2 will return a sorted list
containing all the elements of l1 and l2 . If several elements compare
equal, the elements of l1 will be before the elements of l2 . Not
tail-recursive (sum of the lengths of the arguments).
Lists and Sequences
val to_seq : 'a list -> 'a Seq.t
Iterate on the list.
Since 4.07
val of_seq : 'a Seq.t -> 'a list
Create a list from a sequence.
Since 4.07
OCamldoc 2023-02-12 List(3o)
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