Go: What is the difference between slice and array?

Array — fixed-length, value-type container whose length is part of its type. Slice — a flexible, dynamically-sized view into an underlying array.

Arrays are rarely used directly in Go because their fixed size makes them cumbersome for most tasks. However, they are useful as building blocks for slices and as keys in maps (since arrays are comparable).

1arr := [3]int{10, 20, 30}   // compile-time size
2arr2 := [3]int{10, 20, 30}  // same type
3arr3 := [4]int{1, 2, 3, 4}  // different type
4fmt.Println(arr == arr2)     // true

Slices wrap an array with three components: a pointer to the first element, a length, and a capacity. This makes them efficient for passing to functions — you share the underlying data without copying.

1nums := []int{1, 2, 3, 4, 5}
2sub := nums[1:3]   // [2, 3] — shares backing array
3sub[0] = 99       // modifies nums[1] too
4fmt.Println(nums)  // [1 99 3 4 5]
5
6// append may or may not reallocate
7nums = append(nums, 6)

When to use:

• Array — small fixed buffers, map keys, cryptographic constants.
• Slice — default choice for collections; use in function signatures, APIs, and data processing.

Common mistakes:

• Assuming append always modifies the original slice — it may return a new pointer if capacity is exceeded.
• Sharing a backing array via slicing and mutating unexpectedly.
• Using len(arr) == len(slice) to compare — they may have different backing arrays.