Go: What is the difference between slice and array?

Array — fixed-size, value-type collection where the length is part of the type. Slice — dynamic-length reference to an underlying array, the most common collection in Go.

Arrays are assigned and returned by value, which copies all elements. This makes them safe from unintended sharing but expensive for large sizes.

1arr1 := [3]int{1, 2, 3}
2arr2 := arr1     // full copy
3arr2[0] = 99
4fmt.Println(arr1[0])  // 1 — original unchanged

Slices are the standard way to work with sequences in Go. They hold a pointer, length, and capacity. Multiple slices can share the same underlying array, and append may or may not reallocate.

1xs := []int{1, 2, 3, 4, 5}
2head := xs[:3]  // [1, 2, 3] — shares array
3head[0] = 100
4fmt.Println(xs[0])  // 100 — shared mutation
5
6xs = append(xs, 6) // may reallocate if cap exhausted

When to use:

• Array — fixed-size buffers, map keys, protocol-level constants.
• Slice — default for sequences; use everywhere for function params, returns, and data processing.

Common mistakes:

• Forgetting that append returns a new slice header — must reassign: s = append(s, v).
• Unexpected mutations through shared backing arrays when slicing.
• Passing large arrays to functions (copies everything). Use slices instead.
• Comparing slices with == — only valid against nil; use slices.Equal otherwise.