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.

• the index x must be an untyped constant or its core type must be an integer

var i = int(3)      // valid
var j = int8(4)     // valid
const k = 12        // valid
const l = -7        // not valid, although it does conform to the first rule
const m = "chicken" // not valid, although it does conform to the first rule

• a constant index must be non-negative and representable by a value of type int

This is the rule that makes -7 and "chicken" above invalid.

• a constant index that is untyped is given type int

So if we’re using the untyped constant k from above, it’s treated as if it were int(k).

• the index x is in range if 0 <= x < len(a), otherwise it is out of range

Given this rule:

s := []string{"foo","bar","baz","qux"}

s[i] // valid; i's value of 3 is in range
s[j] // invalid; j's value of 4 is out of range

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