Array

In Go an array is defined by its type, its size and shape. There is no universal array, but many typed and sized arrays. We build methods that act on arrays.

Slices are more versatile and tend to be used more in Go programs. But arrays can still be useful if we have a fixed size of data.

Example

Let us begin with a simple example that loops over an array of 3 ints. We use an array initializer to create a 3-element array. It contains the values 10, 20 and 30.

Here We use a for-loop to iterate over the array's elements. In this kind of for-loop, we specify a start, end, and an iteration.

Info An array has a length. With len() we access the count of its elements—all elements are counted.

Tip The array is indexed starting at 0. The last index is one less than the array's total length.

package main

import "fmt"

func main() {
    // Create an array of three ints.
    array := [...]int{10, 20, 30}

    // Loop over three ints and print them.
    for i := 0; i < len(array); i++ {
        fmt.Println(array[i])
    }
}10
20
30

Parameters

We can pass an array (specified by both element count and type) to a method. Arrays are values. They are copied when passed to a method.

Warning This is slow. Using slices is faster. For small arrays, passing directly maybe an effective approach.

package main

import "fmt"

func display(values [3]int) {
    fmt.Println(values[0])
    fmt.Println(values[1])
    fmt.Println(values[2])
}

func main() {
    v := [...]int{5, 10, 15}
    // Pass the entire array to a method.
    // ... This copies the array.
    display(v)
}5
10
15

Array slice, method

Usually we pass slices of arrays to methods—this avoids a copy of the elements. Here, display() receives an int array slice.

Detail We take a full-range slice of the array. The slice contains all the elements in the four-element int array.

package main

import "fmt"

func display(values []int) {
    // Loop over slice argument and display elements.
    for i:= 0; i < len(values); i++ {
        fmt.Println(values[i])
    }
}

func main() {
    // Create a four-element array.
    array := [...]int{-1, 0, 10, 100}
    // Pass a slice of the array to display.
    // ... This slice contains all elements.
    display(array[:])
}-1
0
10
100

Array, slice benchmark

Do arrays have a performance advantage over slices? Potentially, performance information could be used to optimize many programs.

Version 1 We use an array: we assign into the array's elements, and then read from the array at indexes.

Version 2 In this version of the code, we use a slice. We perform the same actions on the slice that we use on the array.

Result The array program has a consistent performance advantage in Go 1.8. Using an array can lead to a significant speedup.

Note If you remove the element assignments, there is less of a difference. So an array might be faster only if you are assigning elements.

package main

import (
    "fmt"
    "time"
)

func main() {

    // Create array and slice.
    array := [...]int{10, 20, 30}
    slice := []int{10, 20, 30}
    sum := 0

    t0 := time.Now()

    // Version 1: assign into and read array elements.
    for i := 0; i < 1000000000; i++ {
        sum = 0
        for x := 0; x < len(array); x++ {
            array[x] = 5
            sum += array[x]
        }
        if sum == 0 {
            break
        }
    }

    t1 := time.Now()

    // Version 2: assign into and read slice elements.
    for i := 0; i < 1000000000; i++ {
        sum = 0
        for x := 0; x < len(slice); x++ {
            slice[x] = 5
            sum += slice[x]
        }
        if sum == 0 {
            break
        }
    }

    t2 := time.Now()
    // Results.
    fmt.Println(t1.Sub(t0))
    fmt.Println(t2.Sub(t1))
}1.3679763s    Array: [...]int
1.6191417s    Slice: []int

Arrays versus slices

In Go programs, we typically use slices. But a slice is always based on an array. We can think of arrays as the underlying storage for slices.

Arrays are used as a building block, a foundation, of many things in languages. More complex collections can be built from arrays.