Arrays and Slices in Go

In this section, arrays and slices are introduced.

Arrays

In Go, the size of an array is a part of the type. Therefore, arrays have a fixed size. The declaration has the following syntax:

Copy var array [size]type

You can access the data with array[index]. You can see this with a cross product:

Copy package main import ( "fmt" ) func main() { v1 := [3]float64{7, 5, 4} var v2 [3]float64 v2 = [3]float64{2, 4, 6} for v3,i := [...]float64{0, 0, 0}, 0; i < len(v3); i++ { v3[i] = v1[(i + 1) % 3] * v2[(i + 2) % 3] - v1[(i + 2) % 3] * v2[(i + 1) % 3] defer fmt.Printf("%t\n", v3) } }

Test it online (opens new window).

The compiler fits the array depending on the number of elements.

The previous example code is not well-written, but it demonstrates different aspects of arrays.

len(array) is a built-in function that gives the size of an array.

defer is used to defer the execution of last-in-first-out order until surrounding functions return.

Slices

In Go, a slice is an abstraction built on top of arrays. Slices are more flexible than arrays and are used more often than arrays because of this flexibility.

A slice does not have a fixed size. To declare a slice, use the following:

Copy var slice []type

A slice has a length (len(slice)) and a capacity (cap(slice)).

You can also use a built-in function to declare a slice: func make([]type, length, capacity) []type. This returns a slice with the given length, capacity, and type. It allocates an array, which is referred to by the returned slice.

Now create a simple slice with three vectors, and then add a vector with the built-in func append(s []T, vs ...T) [] T function:

Copy package main import "fmt" func main() { vectors := []struct { x,y,z float64 } { { 1, 2, 3 }, { 3.2, 4, 6 }, { 4, 3, 1}, } fmt.Printf("type %#T and value %v\n", vectors, vectors) vectors = append(vectors, struct{ x, y, z float64 }{ 7, 7, 7 }) fmt.Printf("type %#T and value %v\n", vectors[3:], vectors[3:]) fmt.Printf("type %#T and value %v\n", vectors[3], vectors[3]) for i, v := range vectors { fmt.Println(i, " : ", v) } numbers := make([]int, 10, 10) // create a slice with an underlying array fmt.Println(numbers) for i := range numbers { numbers[i] = i } fmt.Println(numbers) }

Test it online (opens new window).

You can use range to iterate over an array, a slice, or a map. i is the index, and v is the value of that index.

There is also a built-in func copy(dst, src []T) int to copy one slice into another and return the number of copied elements.

Maps

Maps are stored key/value pairs. The declaration is as follows:

Copy var m map[keyType]valueType

However, this creates a nil map, which is not so useful. You can read such a map but not write to it. You use make to initialize a map so you can write to it. The following is more useful:

Copy m := make(map[keyType]valueType)

Now you can work with maps:

Copy package main import "fmt" func main() { age := map[string]int {"max": 24, "tom": 28} fmt.Println("map:", age) m := make(map[string]float64) m["E"] = 2.7182818284 m["Pi"] = 3.1415926535 m["Phi"]= 1.6180339887 for key, v := range m { fmt.Printf("Key: %v, Value: %v, Value: %v \n", key, v, m[key]) } delete(m, "E") // does not return anything. It does nothing, if the key does not exist. fmt.Println("len:", len(m)) fmt.Println("map:", m) _, ok := m["E"] // does the key exists? fmt.Println("ok:", ok) }

Test it online (opens new window).

The built-in function func delete(m map[Type]Type1, key Type) deletes the element with the key from the map.

When iterating over maps, the order is not deterministic.

Further reading:

• Go slices (opens new window)

synopsis

To summarize, this section has explored:

• How the size of an array is part of the type, therefor arrays have a fixed size.
• How slices are flexible abstractions built on top of arrays.
• How maps are stored key/value pairs.