Go Arrays

External

• https://go.dev/ref/spec#Array_types

Internal

• Go Language
• Go Slice
• Go Maps

Overview

An array is a composite type that consists in a fixed-length, contiguous and numbered sequence of elements. All elements of an array have the same type. The type of the elements can be a Go built-in type, a user type or a pointer. *<type> notation is used to declare a pointer array.

The compiler knows the length of the array and the type of its elements at compile time, so it knows how much space to allocate. The length of the array is part of the type name, arrays with different lengths are of different types, even if the element type is the same. Once an array is declared, its type and length cannot be changed.

The arrays are the internal data structures used both by slices and maps, and they are rarely used directly in Go code.

Declaration

The array type includes the size, enclosed in square brackets, followed by the element type. As mentioned above, different size arrays are different types, even if they have the same element type.

var a [7]int

The elements are initialized to zero value of the type the array is composed of, upon declaration:

println(a[0]) // displays 0

Literals

The array literals are predefined values that go into an array and they are used to initialize an array. The type ([3]int) is specified as part of the array literal, so it does not have to be provided in the var syntax, but it can be provided, redundantly.

var a = [3]int{7, 17, 37}

The length of the array, provided between square brackets, must match the number of elements in the literal, otherwise the compiler will indicate compilation error.

A better, alternative representation of the array literal uses [...] to express that the size of the array will be given by the number of elements in the literal.

var a = [...]int{7, 17, 37} // the type of a is inferred to be [3]int

Initialization

The initialization with type inference, with a literal, can be used:

var a = [3]int{7, 17, 37}

Also, the short variable declaration can be used as well:

a := [3]int{7, 17, 37}

Operators

Indexing Operator []

Array elements are accessed using the indexing (subscript) operator. Indices are zero-based.

TO DISTRIBUTE

Implementation Details

Arrays are fast: data is maintain in continuous blocks of memory, which keeps them in the CPU caches longer. Index access and arithmetic is fast: the type information and the guarantee that all elements are of same type allows calculating "distance in memory" to find elements.

Arrays are Values

An array is a value in Go. It can be used in an assignment operation. The variable name denotes the entire array, thus an array can be assigned to other arrays of the same type (length and element type). When an array2 is assigned to an array1, the values of array2 are copied across to array1.

In Go, function arguments are passed by value. The fact that arrays are values also has implications on how arrays are passed between functions: by value. A copy of the entire array is made and placed on the stack, regardless of the array's size. For large arrays, this can have performance implications. That is why, passing pointers to arrays is preferred. Even better, use slices.

Declaration

Long Declaration

var a [5]int

A declaration without explicit initialization initializes the array with the type's zero value.

Long declaration with explicit initialization with an array literal:

var b [3]int = [3]int{1, 2, 3}

Go provides syntax for initializing just some of the elements with specified values, letting the compiler to initialize the rest with zero values. The syntax is:

var c [20]int = [100]int{0:10, 1:20}

In the example above, the element 0 is initialized to 10, the element 1 is initialized to 20 and the rest are initialized to their zero value (0).

Short Declaration

The short declaration:

a := [3]int{1, 2, 3}

The short declaration is invalid for arrays if the length and type is not followed by the initialization literal { ... }, the initialization literal is required.

For the format above, there's redundancy in the requirement to specify the length of the array between square brackets and the length of the literal. The compiler can infer the length of the array from the literal, and the declaration is:

a := [...]int{1, 2, 3}

The literal can be specified "vertically":

a := [...]int{
        1, 
        2, 
        3, 
   }

The extra trailing comma is required when the elements are specified on separate lines, but not when they're specified in a single line. This is to allow commenting out elements without breaking the program.

Array Operators and Functions

Indexing Operator

Indexing operator [] returns the value at the given position and can also be used to change the value at that position. Accesses are bound-checked and an attempt to access an element out of bounds produces a compile-time or runtime error:

Compile-time error:

./main.go:13: invalid array index 3 (out of bounds for 3-element array)

Runtime error:

panic: runtime error: index out of range

Array Length

len() returns the length of the array.

Iterating over Arrays

range Keyword

Multidimensional Arrays

TODO Go in Action page 84.