# Module Array

    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 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 `l` (i.e. there is an `x` in `l` 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 list 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

    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.

## 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.