Note: All of this capability is implemented except fill initialization, multi-element segments, 'each' iteration, comparison, and pattern matching.

An array holds a specified number of values, all of the same type. The array's elements are co-located in memory and can be individually accessed using an integer index.

Let's illustrate with an example:

imm numbers [3] f32 = [3.1, 4.1, 5.92]

Here, numbers is an array containing three floating-point numbers as its elements. As the redundant type specification [3] f32 indicates, the length of the array is 3 and the type of its elements is f32.

An array can only exist as either a global or a local, stack-allocated value. Its length must be specified using an integer literal, making it fixed and unchangeable. If variable-sized or heap-allocated arrays are needed, use array references instead.

Initialization

To create or initialize an array, specify the values of all its elements separated by commas and enclosed within square brackets (as the above example demonstrates). When this array constructor is used to initialize a global variable, the element values must be constants. When used as part of a function's logic, element values may be specified using expressions:

imm pair = [nbr * nbr, nbr + nbr]

An alternative way to create an array is to specify it as a string:

imm hello = "Hello world!"  // type:  [12] u8

A string is encoded as an sequence of utf-8 code points. The length is the number of bytes it occupies. The type of each element is u8. For C-compatibility, the zero terminator ('\0') is placed at the end of the string, but not counted in its length.

Fill Initialization

An array can be initialized with a single fill value by using a semi-colon to separate the array's length from its fill value. This example creates a 100-byte string, where all bytes are spaces:

imm spaces = [100; ' ']

Alternatively, a closure may be specified in lieu of a fill value. The closure is repeatedly invoked to produce each of the array elements' values.

Copying

Like numeric values, array values may be passed around between variables and functions:

imm hello = "Hello"
fn main()
  mut dessert = jello(hello)

fn jello(mut saying [5] u8)
  saying = "jello"
  saying

In this example, a copy of the hello variable's array is passed to the function jello. It replaces that copy's contents with the string "jello" and returns it to main. The mutable local variable dessert now holds a copy of the array string "jello".

When arrays are passed around, we always make copies of the array (just like we do with numbers). Any changes made to a copy won't change the original array.

This may not always be the behavior that you want, though. If instead you want to pass around a reference to an array, such that any changes do change the original array, obtain and pass an array reference.

Getting the length

To get a count of how many elements are in an array, use the len method:

imm hello = "Hello"
imm length = hello.len   // =5

Indexing

A single element within an array may be accessed using an unsigned integer index value enclosed in square brackets. The specified index may be an expression. An index of 0 refers to the first element.

Indexing can be used to get or set the specified element:

mut c_array = "coal"
c_array[2] = c_array[1]    // the 3rd element is now the same as the second
// c_array now holds the array value "cool"

Array indexing is automatically bounds-checked by its known size. Any attempt to supply an invalid index results in a compile-time or run-time error.

Note: It is also possible to use the ampersand operator to borrow a reference to a specific indexed element in an array.

Indexing from the end

Sometimes it is more convenient to index an element based on its position relative to the end of the array. This can be accomplished in a verbose way:

imm nbrs = [2, 4, 6]
imm four = nbrs[my.len - 2] // =4 - the 2nd from the end

The less verbose equivalent uses the $- operator:

imm four = nbrs[$-2]

$- is equivalent to my.len -

Iterating over an array

Using indexing and the len method, one can now easily iterate over every element in an array:

imm numbers = [3.5, 4.0, 5.5, 3.0]
average = 0.0
mut i = 0
while i < numbers.len
  average += numbers[i++]
average /= numbers.len      // == 4.0

A cleaner, more concise approach uses an each block instead:

imm numbers = [3.5, 4.0, 5.5, 3.0]
average = 0.0
each nbr in numbers
    average += nbr
average /= numbers.len      // == 4.0

Multi-element copying

Multiple elements of an array may be copied into another array using a range index.

mut greetings = "Hello Jeter"
greetings[7,4] = greetings[1,4] // "Hello Jello"
greetings[6...10] = "Peter"     // "Hello Peter"
greetings[6..11] = "Helen"      // "Hello Helen"
greetings[6..] = "Jayne"        // "Hello Jayne" (end is assumed if unspecified)

As the examples show, several range operators are supported. The first number is always the index of the first element in the segment. The meaning of the number after the range operator varies:

Both array segments must have the same element type and length. Array segments can only be used for copying elements from one array to another using assignment or variable initialization. An array segment cannot be passed as an argument in a function call or used in a comparison.

Note: One can also use the ampersand operator to borrow an array reference (slice) to a specific set of elements in an array.

Multi-element fills

Using the same range operators, a single value can be repeatedly filled into a segment's elements:

mut numbers = [0, -1, -2, -3]
numbers[1 .. 3] = 2
// numbers == [0, 2, 2, -3]

The type of the fill value must be the same as the type of the array's elements.

Comparing

Two arrays may be compared, so long as they have the same element type.

To be equivalent the arrays must have the same length and every element must be the same. To make element comparison possible, the element's type must support the == method.

imm somebody = "John"
if somebody == "Jane"
  ....

Comparing for order (e.g., <) is also handled by comparing the two arrays element-by-element, stopping the first time two elements are not the same. The number of elements compared is the minimum of the lengths of the two arrays. If the element check shows them as equivalent, the lesser-sized array comes first in order.

Partial pattern match

The match statement or ~~ operator supports additional array comparison capability, particularly for partial pattern matching.

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