Arrays & Slices

Arrays — Fixed Size

Go arrays have a fixed size that's part of the type. [3]int and [5]int are different types. Arrays are value types — assigning or passing copies the entire array. In practice, you rarely use arrays directly; slices are much more common.

// Declare and initialize
var a [5]int                    // [0 0 0 0 0] (zero values)
b := [3]string{"go", "is", "fun"}
c := [...]int{10, 20, 30}      // Size inferred from elements

fmt.Println(a)
fmt.Println(b)
fmt.Println(len(c)) // 3

[0 0 0 0 0]
[go is fun]
3

Slices — Dynamic, Powerful

Slices are Go's workhorse data structure. They're dynamic-length views into underlying arrays. A slice has three components: a pointer to the array, a length (how many elements), and a capacity (how many elements the underlying array can hold).

// Create slices
s := []int{1, 2, 3, 4, 5}         // Slice literal (no size = slice, not array)
s2 := make([]int, 5)               // make(type, length) - [0 0 0 0 0]
s3 := make([]int, 0, 10)           // make(type, length, capacity)

fmt.Printf("s:  len=%d cap=%d %v\n", len(s), cap(s), s)
fmt.Printf("s3: len=%d cap=%d %v\n", len(s3), cap(s3), s3)

s:  len=5 cap=5 [1 2 3 4 5]
s3: len=0 cap=10 []

append — Growing Slices

s := []int{1, 2, 3}
s = append(s, 4)           // Add one element
s = append(s, 5, 6, 7)     // Add multiple
fmt.Println(s) // [1 2 3 4 5 6 7]

// Append another slice with ...
more := []int{8, 9}
s = append(s, more...)
fmt.Println(s) // [1 2 3 4 5 6 7 8 9]

When append exceeds the capacity, Go allocates a new, larger underlying array and copies elements. This is amortized O(1) but can cause unexpected behavior if you hold references to the old slice.

Slicing — Creating Sub-slices

s := []int{0, 1, 2, 3, 4, 5}
fmt.Println(s[1:4])  // [1 2 3] (index 1 to 3, exclusive end)
fmt.Println(s[:3])   // [0 1 2] (first 3)
fmt.Println(s[3:])   // [3 4 5] (from index 3)
fmt.Println(s[:])    // [0 1 2 3 4 5] (copy of entire slice)

original := []int{1, 2, 3, 4, 5}
independent := make([]int, 3)
copy(independent, original[1:4]) // independent copy of [2 3 4]

Practice Exercises