Golang Array: Fixed Length, Value Semantics, vs Slices

Declaring an array

An array type is written [N]T: a length, then an element type. The length is fixed at compile time and is part of the type.

package main

import "fmt"

func main() {
	var scores [5]int // five zeros
	scores[0] = 90
	scores[4] = 75
	fmt.Println(scores, len(scores))

	days := [3]string{"Mon", "Tue", "Wed"}
	fmt.Println(days[1])

	primes := [...]int{2, 3, 5, 7, 11} // compiler counts: [5]int
	fmt.Printf("%T %v\n", primes, primes)

	// Index keys: unset positions get the zero value.
	sparse := [5]string{1: "b", 3: "d"}
	fmt.Printf("%q\n", sparse)
}

Output:

[90 0 0 0 75] 5
Tue
[5]int [2 3 5 7 11]
["" "b" "" "d" ""]

An array is always fully initialized. Elements you do not set hold the zero value of the element type: 0, "", false, nil.

Indexing and bounds

Indexes run from 0 to len(a)-1. A constant index out of range is a compile error. A variable index out of range panics at run time.

var a [3]int
a[5] = 1 // compile error: invalid argument: index 5 out of bounds [0:3]

i := 5
a[i] = 1 // panic: runtime error: index out of range [5] with length 3

Iterating

for i, v := range a visits each element in order. v is a copy of the element, so assigning to v does not change the array. Write through the index instead.

package main

import "fmt"

func main() {
	temps := [4]float64{21.5, 23.0, 19.8, 25.1}

	sum := 0.0
	for _, t := range temps {
		sum += t
	}
	fmt.Printf("average: %.2f\n", sum/float64(len(temps)))

	for i := range temps {
		temps[i] += 1 // modifies the array
	}
	fmt.Println(temps)
}

There is one more subtlety specific to arrays: range over an array value evaluates a copy of the array up front. If the body modifies later elements, v still shows the old values. Ranging over &a or a[:] avoids the copy.

Arrays are values

This is the main difference from arrays in C, Java or JavaScript. Assigning an array copies every element. Passing an array to a function copies it too.

package main

import "fmt"

func reset(a [3]int) {
	a[0] = 0 // changes the function's copy
}

func resetPtr(a *[3]int) {
	a[0] = 0 // changes the caller's array
}

func main() {
	a := [3]int{1, 2, 3}
	b := a // full copy
	b[0] = 99
	fmt.Println(a, b)

	reset(a)
	fmt.Println(a)

	resetPtr(&a)
	fmt.Println(a)
}

Output:

[1 2 3] [99 2 3]
[1 2 3]
[0 2 3]

Copying is cheap for small arrays and expensive for big ones: passing a [1_000_000]int by value copies eight megabytes. Pass a pointer (*[N]T) or a slice (a[:]) when the array is large or the function needs to change it. Indexing through an array pointer needs no explicit *: a[0] works on a *[3]int.

Comparing arrays and using them as map keys

Arrays of comparable elements support == and !=. Two arrays are equal when every element is equal. That also makes them valid map keys, which slices are not.

package main

import "fmt"

func main() {
	x := [3]int{1, 2, 3}
	y := [3]int{1, 2, 3}
	fmt.Println(x == y)

	// A grid position as a map key.
	type pos [2]int
	visited := map[pos]bool{}
	visited[pos{0, 0}] = true
	visited[pos{2, 1}] = true
	fmt.Println(visited[pos{2, 1}], visited[pos{1, 1}])
}

Arrays of different lengths are different types and cannot be compared at all: [3]int{} == [4]int{} does not compile (mismatched types [3]int and [4]int).

Multidimensional arrays

An array of arrays gives you a fixed grid. The whole grid is one contiguous block of memory.

package main

import "fmt"

func main() {
	var board [3][3]rune
	for r := range board {
		for c := range board[r] {
			board[r][c] = '.'
		}
	}
	board[1][1] = 'X'
	board[0][2] = 'O'

	for _, row := range board {
		fmt.Println(string(row[:]))
	}
}

row[:] turns the [3]rune row into a []rune slice, which string() accepts.

Arrays vs slices

In practice you will mostly meet arrays in a few places: sha256.Sum256 returns a [32]byte, lookup tables whose size never changes, and as the backing store of a slice. Slicing an array with a[:] or a[1:3] creates a slice that shares its memory; the slices page covers what that sharing means.

Converting between arrays and slices

package main

import "fmt"

func main() {
	arr := [4]int{1, 2, 3, 4}
	s := arr[:] // slice view: shares memory with arr
	s[0] = 100
	fmt.Println(arr)

	nums := []int{7, 8, 9}
	a := [3]int(nums) // Go 1.20: slice to array, copies
	nums[0] = 0
	fmt.Println(a, nums)

	p := (*[3]int)(nums) // Go 1.17: array pointer, shares memory
	p[1] = 0
	fmt.Println(nums)
}

Converting a slice to an array (Go 1.20) or an array pointer (Go 1.17) panics if the slice is shorter than the array length: panic: runtime error: cannot convert slice with length 2 to array or pointer to array with length 3.

Common mistakes

• Expecting a function to modify an array argument. It gets a copy. Pass *[N]T or a slice.
• Using an array where a slice is wanted. []int{1, 2} is a slice, [2]int{1, 2} is an array. A function taking []int will not accept a [2]int; pass a[:].
• Large arrays in a range loop. for _, v := range bigArray copies the whole array first. Range over &bigArray or a slice.