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Arrays

By · Reviewed & last updated · Part of Go Tutorial

Go arrays are fixed-length and pass by value. Slices (the dynamically-resizable wrapper around them) are what you actually use 99% of the time, but understanding arrays matters — especially for performance and embedded uses.

Fixed-size arrays + when they help

EXAMPLE
package main

import "fmt"

func main() {
    // 1) Declaration
    var a [5]int                       // zero-valued
    b := [5]int{1, 2, 3, 4, 5}         // literal
    c := [...]int{1, 2, 3}             // length inferred
    d := [5]int{2: 99}                 // {0, 0, 99, 0, 0}

    fmt.Println(a, b, c, d, len(b))

    // 2) Arrays are VALUES — passing copies the whole thing
    e := b
    e[0] = 999
    fmt.Println(b[0], e[0])            // 1 999  (no aliasing)

    // 3) Element access + iteration
    for i, v := range b {
        fmt.Println(i, v)
    }

    // 4) Arrays as map keys / struct fields work (no slice issues)
    type Vec3 = [3]float64
    var origin Vec3                     // {0, 0, 0}
    var translate Vec3 = [3]float64{1, 2, 3}

    counts := map[[2]int]int{}
    counts[[2]int{3, 4}]++

    // 5) Convert to / from a slice
    s := b[:]                           // slice viewing the whole array
    s[0] = 100
    fmt.Println(b[0])                   // 100 — slice and array share memory

    // 6) When to actually use an array (not a slice)
    //    • Inside a struct, when the field is a known fixed size
    //    • As a map key (slices CAN'T be map keys)
    //    • Hot-path inner loops where the size is constant
    //    • Cryptographic / binary protocols (fixed-length tokens, MAC outputs)
}

Why it matters

Reach for [N]T only when N is genuinely fixed at compile time. For everything else — growth, dynamic indexing, passing around — use a slice ([]T).

Tip: Tweak the snippet with Try it Yourself », then sit the quiz at the bottom of the page.

Example

Example
var nums [3]int = [3]int{1, 2, 3}
fmt.Println(nums[0], len(nums))
Try it Yourself »

Discussion

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