Go: What is the difference between slice and array?

Array — fixed-length, value-type, length is part of the type definition. Slice — dynamic reference type backed by an array, the standard Go collection.

Arrays are rarely used directly because their fixed size is restrictive. They're mainly useful as building blocks for slices and as map keys (since arrays are comparable).

1arr := [3]int{1, 2, 3}
2// Can be map keys
3lookup := map[[32]byte]string{
4    sha256.Sum256([]byte("hello")): "greeting",
5}

Slices wrap arrays with a pointer, length, and capacity. They share backing memory — mutations through one slice affect others referencing the same array.

1data := []int{10, 20, 30, 40}
2sub := data[1:3]  // [20, 30] — shares backing array
3sub[0] = 200      // mutates data[1] too
4fmt.Println(data[1])  // 200
5
6// append may or may not reallocate
7data = append(data, 50)

When to use:

• Array — small fixed buffers, map keys, cryptographic constants.
• Slice — all dynamic sequences; use in function params and returns.

Common mistakes:

• Not capturing append return value.
• Unexpected mutations through shared backing arrays.
• Passing large arrays by value.
• Comparing slices with == — only valid against nil.