Array(3) OCaml library Array(3)
NAME
Array - Array operations.
Module
Module Array
Documentation
Module Array
: sig end
Array operations.
The labeled version of this module can be used as described in the
StdLabels module.
type 'a t = 'a array
An alias for the type of arrays.
val length : 'a array -> int
Return the length (number of elements) of the given array.
val get : 'a array -> int -> 'a
get a n returns the element number n of array a . The first element
has number 0. The last element has number length a - 1 . You can also
write a.(n) instead of get a n .
Raises Invalid_argument if n is outside the range 0 to (length a - 1) .
val set : 'a array -> int -> 'a -> unit
set a n x modifies array a in place, replacing element number n with x
. You can also write a.(n) <- x instead of set a n x .
Raises Invalid_argument if n is outside the range 0 to length a - 1 .
val make : int -> 'a -> 'a array
make n x returns a fresh array of length n , initialized with x . All
the elements of this new array are initially physically equal to x (in
the sense of the == predicate). Consequently, if x is mutable, it is
shared among all elements of the array, and modifying x through one of
the array entries will modify all other entries at the same time.
Raises Invalid_argument if n < 0 or n > Sys.max_array_length . If the
value of x is a floating-point number, then the maximum size is only
Sys.max_array_length / 2 .
val create_float : int -> float array
create_float n returns a fresh float array of length n , with
uninitialized data.
Since 4.03
val init : int -> (int -> 'a) -> 'a array
init n f returns a fresh array of length n , with element number i
initialized to the result of f i . In other terms, init n f tabulates
the results of f applied in order to the integers 0 to n-1 .
Raises Invalid_argument if n < 0 or n > Sys.max_array_length . If the
return type of f is float , then the maximum size is only
Sys.max_array_length / 2 .
val make_matrix : int -> int -> 'a -> 'a array array
make_matrix dimx dimy e returns a two-dimensional array (an array of
arrays) with first dimension dimx and second dimension dimy . All the
elements of this new matrix are initially physically equal to e . The
element ( x,y ) of a matrix m is accessed with the notation m.(x).(y) .
Raises Invalid_argument if dimx or dimy is negative or greater than
Sys.max_array_length . If the value of e is a floating-point number,
then the maximum size is only Sys.max_array_length / 2 .
val init_matrix : int -> int -> (int -> int -> 'a) -> 'a array array
init_matrix dimx dimy f returns a two-dimensional array (an array of
arrays) with first dimension dimx and second dimension dimy , where the
element at index ( x,y ) is initialized with f x y . The element ( x,y
) of a matrix m is accessed with the notation m.(x).(y) .
Since 5.2
Raises Invalid_argument if dimx or dimy is negative or greater than
Sys.max_array_length . If the return type of f is float , then the
maximum size is only Sys.max_array_length / 2 .
val append : 'a array -> 'a array -> 'a array
append v1 v2 returns a fresh array containing the concatenation of the
arrays v1 and v2 .
Raises Invalid_argument if length v1 + length v2 > Sys.max_array_length
.
val concat : 'a array list -> 'a array
Same as Array.append , but concatenates a list of arrays.
val sub : 'a array -> int -> int -> 'a array
sub a pos len returns a fresh array of length len , containing the
elements number pos to pos + len - 1 of array a .
Raises Invalid_argument if pos and len do not designate a valid
subarray of a ; that is, if pos < 0 , or len < 0 , or pos + len >
length a .
val copy : 'a array -> 'a array
copy a returns a copy of a , that is, a fresh array containing the same
elements as a .
val fill : 'a array -> int -> int -> 'a -> unit
fill a pos len x modifies the array a in place, storing x in elements
number pos to pos + len - 1 .
Raises Invalid_argument if pos and len do not designate a valid
subarray of a .
val blit : 'a array -> int -> 'a array -> int -> int -> unit
blit src src_pos dst dst_pos len copies len elements from array src ,
starting at element number src_pos , to array dst , starting at element
number dst_pos . It works correctly even if src and dst are the same
array, and the source and destination chunks overlap.
Raises Invalid_argument if src_pos and len do not designate a valid
subarray of src , or if dst_pos and len do not designate a valid
subarray of dst .
val to_list : 'a array -> 'a list
to_list a returns the list of all the elements of a .
val of_list : 'a list -> 'a array
of_list l returns a fresh array containing the elements of l .
Raises Invalid_argument if the length of l is greater than
Sys.max_array_length .
Iterators
val iter : ('a -> unit) -> 'a array -> unit
iter f a applies function f in turn to all the elements of a . It is
equivalent to f a.(0); f a.(1); ...; f a.(length a - 1); () .
val iteri : (int -> 'a -> unit) -> 'a array -> unit
Same as Array.iter , but the function is applied to the index of the
element as first argument, and the element itself as second argument.
val map : ('a -> 'b) -> 'a array -> 'b array
map f a applies function f to all the elements of a , and builds an
array with the results returned by f : [| f a.(0); f a.(1); ...; f
a.(length a - 1) |] .
val map_inplace : ('a -> 'a) -> 'a array -> unit
map_inplace f a applies function f to all elements of a , and updates
their values in place.
Since 5.1
val mapi : (int -> 'a -> 'b) -> 'a array -> 'b array
Same as Array.map , but the function is applied to the index of the
element as first argument, and the element itself as second argument.
val mapi_inplace : (int -> 'a -> 'a) -> 'a array -> unit
Same as Array.map_inplace , but the function is applied to the index of
the element as first argument, and the element itself as second
argument.
Since 5.1
val fold_left : ('acc -> 'a -> 'acc) -> 'acc -> 'a array -> 'acc
fold_left f init a computes f (... (f (f init a.(0)) a.(1)) ...)
a.(n-1) , where n is the length of the array a .
val fold_left_map : ('acc -> 'a -> 'acc * 'b) -> 'acc -> 'a array ->
'acc * 'b array
fold_left_map is a combination of Array.fold_left and Array.map that
threads an accumulator through calls to f .
Since 4.13
val fold_right : ('a -> 'acc -> 'acc) -> 'a array -> 'acc -> 'acc
fold_right f a init computes f a.(0) (f a.(1) ( ... (f a.(n-1) init)
...)) , where n is the length of the array a .
Iterators on two arrays
val iter2 : ('a -> 'b -> unit) -> 'a array -> 'b array -> unit
iter2 f a b applies function f to all the elements of a and b .
Since 4.03 (4.05 in ArrayLabels)
Raises Invalid_argument if the arrays are not the same size.
val map2 : ('a -> 'b -> 'c) -> 'a array -> 'b array -> 'c array
map2 f a b applies function f to all the elements of a and b , and
builds an array with the results returned by f : [| f a.(0) b.(0); ...;
f a.(length a - 1) b.(length b - 1)|] .
Since 4.03 (4.05 in ArrayLabels)
Raises Invalid_argument if the arrays are not the same size.
Array scanning
val for_all : ('a -> bool) -> 'a array -> bool
for_all f [|a1; ...; an|] checks if all elements of the array satisfy
the predicate f . That is, it returns (f a1) && (f a2) && ... && (f an)
.
Since 4.03
val exists : ('a -> bool) -> 'a array -> bool
exists f [|a1; ...; an|] checks if at least one element of the array
satisfies the predicate f . That is, it returns (f a1) || (f a2) || ...
|| (f an) .
Since 4.03
val for_all2 : ('a -> 'b -> bool) -> 'a array -> 'b array -> bool
Same as Array.for_all , but for a two-argument predicate.
Since 4.11
Raises Invalid_argument if the two arrays have different lengths.
val exists2 : ('a -> 'b -> bool) -> 'a array -> 'b array -> bool
Same as Array.exists , but for a two-argument predicate.
Since 4.11
Raises Invalid_argument if the two arrays have different lengths.
val mem : 'a -> 'a array -> bool
mem a set is true if and only if a is structurally equal to an element
of set (i.e. there is an x in set such that compare a x = 0 ).
Since 4.03
val memq : 'a -> 'a array -> bool
Same as Array.mem , but uses physical equality instead of structural
equality to compare array elements.
Since 4.03
val find_opt : ('a -> bool) -> 'a array -> 'a option
find_opt f a returns the first element of the array a that satisfies
the predicate f , or None if there is no value that satisfies f in the
array a .
Since 4.13
val find_index : ('a -> bool) -> 'a array -> int option
find_index f a returns Some i , where i is the index of the first
element of the array a that satisfies f x , if there is such an
element.
It returns None if there is no such element.
Since 5.1
val find_map : ('a -> 'b option) -> 'a array -> 'b option
find_map f a applies f to the elements of a in order, and returns the
first result of the form Some v , or None if none exist.
Since 4.13
val find_mapi : (int -> 'a -> 'b option) -> 'a array -> 'b option
Same as find_map , 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 5.1
Arrays of pairs
val split : ('a * 'b) array -> 'a array * 'b array
split [|(a1,b1); ...; (an,bn)|] is ([|a1; ...; an|], [|b1; ...; bn|]) .
Since 4.13
val combine : 'a array -> 'b array -> ('a * 'b) array
combine [|a1; ...; an|] [|b1; ...; bn|] is [|(a1,b1); ...; (an,bn)|] .
Raise Invalid_argument if the two arrays have different lengths.
Since 4.13
Sorting and shuffling
val sort : ('a -> 'a -> int) -> 'a array -> unit
Sort an array 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 below for a complete
specification). For example, compare is a suitable comparison
function. After calling sort , the array is sorted in place in
increasing order. sort is guaranteed to run in constant heap space and
(at most) logarithmic stack space.
The current implementation uses Heap Sort. It runs in constant stack
space.
Specification of the comparison function: Let a be the array and cmp
the comparison function. The following must be true for all x , y , z
in a :
- cmp x y > 0 if and only if cmp y x < 0
- if cmp x y >= 0 and cmp y z >= 0 then cmp x z >= 0
When sort returns, a contains the same elements as before, reordered in
such a way that for all i and j valid indices of a :
- cmp a.(i) a.(j) >= 0 if and only if i >= j
val stable_sort : ('a -> 'a -> int) -> 'a array -> unit
Same as Array.sort , but the sorting algorithm is stable (i.e.
elements that compare equal are kept in their original order) and not
guaranteed to run in constant heap space.
The current implementation uses Merge Sort. It uses a temporary array
of length n/2 , where n is the length of the array. It is usually
faster than the current implementation of Array.sort .
val fast_sort : ('a -> 'a -> int) -> 'a array -> unit
Same as Array.sort or Array.stable_sort , whichever is faster on
typical input.
val shuffle : rand:(int -> int) -> 'a array -> unit
shuffle rand a randomly permutes a 's element using rand for
randomness. The distribution of permutations is uniform.
rand must be such that a call to rand n returns a uniformly distributed
random number in the range [ 0 ; n-1 ]. Random.int can be used for
this (do not forget to Random.self_init the generator).
Since 5.2
Arrays and Sequences
val to_seq : 'a array -> 'a Seq.t
Iterate on the array, in increasing order. Modifications of the array
during iteration will be reflected in the sequence.
Since 4.07
val to_seqi : 'a array -> (int * 'a) Seq.t
Iterate on the array, in increasing order, yielding indices along
elements. Modifications of the array during iteration will be
reflected in the sequence.
Since 4.07
val of_seq : 'a Seq.t -> 'a array
Create an array from the generator
Since 4.07
Arrays and concurrency safety
Care must be taken when concurrently accessing arrays from multiple
domains: accessing an array will never crash a program, but
unsynchronized accesses might yield surprising
(non-sequentially-consistent) results.
Atomicity
Every array operation that accesses more than one array element is not
atomic. This includes iteration, scanning, sorting, splitting and
combining arrays.
For example, consider the following program:
let size = 100_000_000
let a = Array.make size 1
let d1 = Domain.spawn (fun () ->
Array.iteri (fun i x -> a.(i) <- x + 1) a
)
let d2 = Domain.spawn (fun () ->
Array.iteri (fun i x -> a.(i) <- 2 * x + 1) a
)
let () = Domain.join d1; Domain.join d2
After executing this code, each field of the array a is either 2 , 3 ,
4 or 5 . If atomicity is required, then the user must implement their
own synchronization (for example, using Mutex.t ).
Data races
If two domains only access disjoint parts of the array, then the
observed behaviour is the equivalent to some sequential interleaving of
the operations from the two domains.
A data race is said to occur when two domains access the same array
element without synchronization and at least one of the accesses is a
write. In the absence of data races, the observed behaviour is
equivalent to some sequential interleaving of the operations from
different domains.
Whenever possible, data races should be avoided by using
synchronization to mediate the accesses to the array elements.
Indeed, in the presence of data races, programs will not crash but the
observed behaviour may not be equivalent to any sequential interleaving
of operations from different domains. Nevertheless, even in the
presence of data races, a read operation will return the value of some
prior write to that location (with a few exceptions for float arrays).
Float arrays
Float arrays have two supplementary caveats in the presence of data
races.
First, the blit operation might copy an array byte-by-byte. Data races
between such a blit operation and another operation might produce
surprising values due to tearing: partial writes interleaved with other
operations can create float values that would not exist with a
sequential execution.
For instance, at the end of
let zeros = Array.make size 0.
let max_floats = Array.make size Float.max_float
let res = Array.copy zeros
let d1 = Domain.spawn (fun () -> Array.blit zeros 0 res 0 size)
let d2 = Domain.spawn (fun () -> Array.blit max_floats 0 res 0 size)
let () = Domain.join d1; Domain.join d2
the res array might contain values that are neither 0. nor max_float .
Second, on 32-bit architectures, getting or setting a field involves
two separate memory accesses. In the presence of data races, the user
may observe tearing on any operation.
OCamldoc 2024-05-31 Array(3)