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Arrays & Slices  ›  Capstone Project

Converting Between Slices and Arrays

Slice to Array Conversion 14 minSlicing an array with arr[:] gives a slice that shares the array's memory, while converting a slice to an array type copies its first elements and panics if the slice is too short.

Converting Between Slices and Arrays

tally ranks words in a slice, because the number of distinct words is unknown. The podium, though, is a fixed [3]string. Moving data between the two is simple, but one direction shares memory and the other copies — and mixing them up changes what your program prints.

The rule

Array to slice shares; slice to array copies, and needs enough elements.

Array → slice: arr[:]. Slicing an array gives a slice whose underlying array is that array. No copy is made:

podium := [3]string{"the", "cat", "sat"}
view := podium[:]            // len 3, cap 3, same memory
view[0] = "THE"
fmt.Println(podium[0])       // THE

This is how you hand an array to the many functions that take slices, such as strings.Join(podium[:], ", ") or sort.Strings(podium[:]), which then sorts the array in place.

Slice → array: [N]T(s) (Go 1.20 and later). It creates a new array and copies the first N elements:

ranked := []string{"the", "cat", "sat", "mat"}
top := [3]string(ranked)   // [the cat sat], a copy
ranked[0] = "a"
fmt.Println(top[0])        // the

Extra elements are ignored, but if the slice has fewer than N elements the conversion panics at run time. Guard it:

if len(ranked) >= 3 {
	top = [3]string(ranked)
}

Why would you want an array at all, once you have a slice? Arrays are comparable and can be map keys, so [3]string can key a map of three-word phrases, and a fixed size documents intent in a function signature.

Two relatives are worth knowing:

  • (*[3]string)(ranked) converts to an array pointer, which shares memory instead of copying (same length rule).
  • copy(top[:], ranked) copies as many elements as fit and never panics; it returns how many it copied.

The shortcut

For every conversion, ask two questions: "does it copy or share?" and "can it be too short?" arr[:]: shares, never too short. [N]T(s): copies, panics if len(s) < N.

A file with only two distinct wordsStep 1 of 4

  1. The data

    ranked is []string{"hello", "world"}, len 2.

Drills

Answer fast — trust the rule.

quick drill

arr := [3]int{1, 2, 3}; s := arr[:]; s[1] = 20. What is arr?

quick drill

s := []int{1, 2, 3, 4}; a := [2]int(s). What is a?

quick drill

s := []int{1, 2}; a := [3]int(s). What happens?

quick drill

s := []int{1, 2, 3}; a := [3]int(s); s[0] = 9. What is a[0]?

quick drill

var a [3]int; n := copy(a[:], []int{5, 6}). What is n?

Remember

  • Array to slice shares; slice to array copies, and needs enough elements.
  • Shortcut: ask "copy or share?" and "can it be too short?"
  • Trap: [N]T(s) panics on a short slice; guard with a length check or use copy.

Practice — level up

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Write your answer, then compare it with a rubric and a model answer.

ExplainPodium in, podium out

tally sorts its words into a slice ranked, then fills the podium with podium := [3]string(ranked), and later passes podium[:] to a helper that upper-cases each entry. Explain what each conversion does to memory, what happens when a file has only two distinct words, and whether the helper changes podium.