Go: What is the difference between slice and array?

Array — fixed-length, value-type, length is part of the type. Slice — dynamic-length reference type, the standard sequence in Go.

Arrays are copied on assignment and when passed to functions. This makes them predictable but expensive for large sizes. Arrays are comparable with == and can be map keys.

1arr := [3]int{10, 20, 30}
2arr2 := arr     // full copy
3arr2[0] = 99
4fmt.Println(arr)  // [10 20 30] — unchanged
5// Arrays can be map keys
6lookup := map[[32]byte]string{sha256.Sum256(data): "found"}

Slices are views into arrays. They share backing memory, so modifications through one slice affect others. append may reallocate if capacity is exceeded.

1data := []int{1, 2, 3, 4, 5}
2mid := data[1:4]   // [2, 3, 4] — shares backing array
3mid[0] = 20
4fmt.Println(data[1])  // 20 — shared mutation
5data = append(data, 6)  // may allocate new array

When to use:

• Array — small fixed buffers, cryptographic constants, map keys.
• Slice — default for dynamic sequences; use in APIs, functions, and data processing.

Common mistakes:

• Not capturing append return value — append(s, v) returns a new header.
• Unexpected mutations via shared backing arrays.
• Passing large arrays by value (copies all elements).
• Comparing slices with == — only valid against nil; use slices.Equal.