Go: What is the difference between slice and array?

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

Arrays have their length baked into the type, so [3]int and [4]int are different types. They are value types — assigning or passing copies all elements. Arrays are comparable and can be map keys.

1arr := [3]int{10, 20, 30}
2arr2 := arr     // full copy
3arr2[0] = 99
4fmt.Println(arr[0])  // 10 — unchanged

Slices are views into arrays with pointer, length, and capacity. They share backing memory — multiple slices can reference the same array. append may reallocate when capacity is exceeded.

1nums := []int{1, 2, 3, 4, 5}
2first3 := nums[:3]   // shares backing array
3first3[0] = 100
4fmt.Println(nums[0]) // 100 — shared mutation!
5nums = append(nums, 6)  // may reallocate

When to use:

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

Common mistakes:

• Not capturing the return of append — append returns a new slice header.
• Unexpected mutations through shared backing arrays.
• Passing large arrays to functions (copies everything).
• Comparing slices with == — only valid against nil.