Index expressions for arrays - Boldly Go

Blog / Go Language Spec

I hope you can join me again today on my live stream, when I’ll be continuing my series about deploying a simple Go app to Kubernetes.

Last week we started with index expressions. Today we look at the additional rules that apply specifically to index expressions for arrays.

Index expressions

… For a of array type A:

a constant index must be in range

Since the length of an array is known at compile time, it is a compilation error to use a constant index that is out of range.

var a [5]string
a[12] // index 12 out of bounds [0:5]
const i = 7
a[i]  // index 7 out of bounds [0:5]

if x is out of range at run time, a run-time panic occurs

When the index is determined at runtime, the compler won’t complain, but you will get a run-time panic if you try to use an out-of-bounds index:

var a [5]string
i := 7
a[i] // panic: runtime error: index out of range [7] with length 5

a[x] is the array element at index x and the type of a[x] is the element type of A

var a [5]string
s = a[3] // s's type is `string`, because that's the element type of a

For a of pointer to array type:

a[x] is shorthand for (*a)[x]

As a nifty short-hand, we can reference elements of array pointers the same way we do array values:

var a = &[5]string{}
i := a[3]    // These are
j := (*a)[3] // equivalent

Quotes from The Go Programming Language Specification Version of August 2, 2023

Share this

Direct to your inbox, daily. I respect your privacy .

Unsure? Browse the archive .

Related Content

Index expressions for type parameters

Index expressions … For a of type parameter type P: So just a reminder what this means: We’re talking about index expressions for different data types. And we’re going through different interpretations of the expression a[x]. In this example, a is of a “type parameter” type, represented by P. And as a further reminder, a type parameter looks something like [X any] or [Y ~int]. So for this example, we’re assuming that we have [P <.

Index expressions

Index expressions A primary expression of the form a[x] denotes the element of the array, pointer to array, slice, string or map a indexed by x. The value x is called the index or map key, respectively. The following rules apply: If a is neither a map nor a type parameter: Alright, so we’re looking at the rules that apply to any of the following types: array, pointer to array, slice, or string—except for type parameters of these types, which have special rules we’ll get to.

Clear

Clear The built-in function clear takes an argument of map, slice, or type parameter type, and deletes or zeroes out all elements [[Go 1.21(https://go.dev/ref/spec#Go_1.21)]]. Call Argument type Result clear(m) map[K]T deletes all entries, resulting in an empty map (len(m) == 0) clear(s) []T sets all elements up to the length of s to the zero value of T clear(t) type parameter see below clear is a relatively recent addition to Go, added just over a year ago.