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 not a map:

• the index x must be of integer type or untyped; it is in range if 0 <= x < len(a), otherwise it is out of range
• a constant index must be non-negative and representable by a value of type int

For a of array type A:

• a constant index must be in range
• if x is out of range at run time, a run-time panic occurs
• a[x] is the array element at index x and the type of a[x] is the element type of A

For a of pointer to array type:

• a[x] is shorthand for (*a)[x]

For a of slice type S:

• if x is out of range at run time, a run-time panic occurs
• a[x] is the slice element at index x and the type of a[x] is the element type of S

For a of string type:

• a constant index must be in range if the string a is also constant
• if x is out of range at run time, a run-time panic occurs
• a[x] is the non-constant byte value at index x and the type of a[x] is byte
• a[x] may not be assigned to

For a of map type M:

• x's type must be assignable to the key type of M
• if the map contains an entry with key x, a[x] is the map value with key x and the type of a[x] is the value type of M
• if the map is nil or does not contain such an entry, a[x] is the zero value for the value type of M

Otherwise a[x] is illegal.

An index expression on a map a of type map[K]V used in an assignment or initialization of the special form

v, ok = a[x]
v, ok := a[x]
var v, ok = a[x]

yields an additional untyped boolean value. The value of ok is true if the key x is present in the map, and false otherwise.

Assigning to an element of a nil map causes a run-time panic.