GO explicit array initialization

Question

Is there explicit array initialization (declaration and assignment) in GO or the only way is using the shorthand operator? Here is a practical example - is this two equal:

a := [3]int{1, 0, 1}

var a [3]int = [3]int{1, 0, 1}

Answer 1

They are equivalent. In general: Spec: Short variable declaration:

A short variable declaration uses the syntax:

ShortVarDecl = IdentifierList ":=" ExpressionList .

It is shorthand for a regular variable declaration with initializer expressions but no types:

"var" IdentifierList = ExpressionList .

So this line:

a := [3]int{369, 0, 963}

Is equivalent to this:

var a = [3]int{369, 0, 963}

But since the expression list is a composite literal of type [3]int, the following is the same:

var a [3]int = [3]int{369, 0, 963}

Spec: Variable declaration:

If a type is present, each variable is given that type. Otherwise, each variable is given the type of the corresponding initialization value in the assignment.

So you may use any of the following, all declare and initialize a variable of type [3]int:

a := [3]int{369, 0, 963}
b := [...]int{369, 0, 963}
var c = [3]int{369, 0, 963}
var d [3]int = [3]int{369, 0, 963}
var e [3]int = [...]int{369, 0, 963}
var f = [...]int{369, 0, 963}

Notes:

Note that in composite literals, it is valid to not list all values. Elements whose value is not explicitly specified will be the zero value of the element type. You may include an optional index before a value in the enumeration to specify the element whose value it will be.

Spec: Composite literals:

For array and slice literals the following rules apply:

• Each element has an associated integer index marking its position in the array.
• An element with a key uses the key as its index; the key must be a constant integer expression.
• An element without a key uses the previous element's index plus one. If the first element has no key, its index is zero.

Since your initial array contains a 0 which is the zero value for the element type int, you may exclude it from the literal. To create and initialize a variable to the value [3]int{369, 0, 963}, you may also do it like this:

// Value at index 1 implicitly gets 0:
g := [3]int{369, 2: 963} 
h := [...]int{369, 2: 963} 

Try all the examples on the Go Playground.

See this question for more details + practical examples: Keyed items in golang array initialization