37 Go interview questions on arrays, slices, maps & strings, with detailed answers and code.
A slice is a small struct of three words, {ptr *T, len int, cap int} . It holds a pointer to a backing array, the number of elements you can see ( len ), and how many…
func add(s []int) { s = append(s, 4) s[0] = 100 } func main() { s := make([]int, 3, 4) add(s) fmt.Println(s, len(s), cap(s)) fmt.Println(s[:4]) t := []int{1, 2, 3} //…
When len + n > cap , append calls runtime.growslice . Since Go 1.20 the rule ( nextslicecap ) is: If the needed length is more than twice the old cap, use the needed…
x := make([]int, 0, 5) x = append(x, 1, 2, 3) y := append(x, 4) z := append(x, 5) fmt.Println(x, y, z) Output: [1 2 3] [1 2 3 5] [1 2 3 5] x has len 3 and cap 5. Both…
a[low:high:max] makes a slice with len = high-low and cap = max-low . The constraint is 0 <= low <= high <= max <= cap(a) . A plain a[low:high] keeps all the capacity up…
var s []int is nil : ptr is nil, len and cap are 0. s := []int{} or make([]int, 0) is empty but non-nil : ptr points at a zero-size allocation (often runtime.zerobase ).…
copy(dst, src) copies min(len(dst), len(src)) elements and returns that count. It uses len, not cap . So copying into make([]T, 0, n) copies nothing, which is a common…
An array's length is part of its type ( [4]int and [5]int are different types). An array is a value : assigning it, passing it, or ranging over it copies every element.…
arr := [3]int{1, 2, 3} for i, v := range arr { arr[2] = 100 if i == 2 { fmt.Println("array:", v) } } sl := []int{1, 2, 3} for i, v := range sl { sl[2] = 100 if i == 2 {…
A subslice keeps the entire backing array reachable, not just the part you can see. If you read a 10 MB file and keep data[:16] in a long-lived cache, all 10 MB stays…
There are three common patterns: Order-preserving delete: O(n), using append or copy. Unordered delete: O(1), swap with the last element. In-place filter: O(n), no…
Slices, maps, and functions are not comparable . You can compare them only to nil . The language designers left out == for slices on purpose, because it is ambiguous:…
make([][]int, n) creates only the outer slice of n nil rows. Writing grid[0][0] panics until you allocate each row. There are two approaches. // 1) Row-per-allocation:…
If a function is declared as func Filter[E any](s []E) []E , then passing a named slice type like type IDs []int returns plain []int . The caller loses the named type…
Go 1.21 added slices and maps to the standard library. Go 1.23 added iterator-returning functions based on iter.Seq / iter.Seq2 and range-over-func. s := []int{3, 1, 4,…
The classic runtime map ( runtime.hmap ) is a hash table with 2^B buckets : Each bucket ( bmap ) holds 8 entries . It starts with tophash [8]uint8 , the top 8 bits of…
Go 1.24 replaced the bucket-chaining map with an implementation based on Swiss Tables (Google's Abseil design), in internal/runtime/maps . The design has several parts:…
The spec says iteration order is unspecified . The runtime goes further and deliberately randomizes it: each range starts at a random position (in the classic map, a…
Yes, within one goroutine. The spec defines the behaviour: Deleting an entry that has not been reached yet means it will not be produced. Deleting the current key is…
Built-in maps are not safe for concurrent use if at least one goroutine writes. Concurrent reads alone are fine. The runtime has a cheap best-effort check (a hashWriting…
The sync.Map docs name two cases where it wins: The entry for a key is written once and read many times , as in caches that only grow. Many goroutines read, write, and…
Map elements are not addressable . m["k"] gives you a copy of the value, and assigning to a field of a temporary copy would do nothing. So the compiler rejects it:…
Because the runtime moves map entries . When a map grows (classic evacuation, or a Swiss-table split), entries are rehashed and copied to new memory. A pointer from…
Keys must be comparable : booleans, numbers, strings, pointers, channels, interfaces, and arrays or structs made only of comparable types. Slices, maps, and functions…
No. delete marks slots as empty (or as tombstones in Swiss tables), but the bucket or group memory is never given back. A map that once held 1M entries keeps its…
clear (Go 1.21) has different semantics depending on the type: Map: deletes every entry, so len becomes 0. It is the only way to remove NaN keys, and it is faster than…
A map variable is a pointer to a runtime header . Passing it copies the pointer, so inserts and deletes inside the callee are visible to the caller, without needing *map…
A string is a two-word header, {ptr *byte, len int} , pointing at read-only bytes. Immutability means strings can be shared freely: substrings and assignments copy no…
s := "héllo" fmt.Println(len(s), utf8.RuneCountInString(s)) fmt.Println(s[1], string(s[1])) for i, r := range s { fmt.Printf("%d:%c ", i, r) } fmt.Println()…
Because strings are immutable, s += x in a loop allocates a new string and copies everything built so far each time, which is O(n²) . The options compare like this: +…
Go 1.20 added the supported primitives unsafe.String , unsafe.StringData and unsafe.SliceData . They replace the old reflect.StringHeader / SliceHeader hacks, which are…
Go has no built-in set type. The idiom is map[T]struct{} . The empty struct takes zero bytes , so values cost nothing, and the type makes the intent clear. map[T]bool…
var ptrs []*int for _, v := range []int{1, 2, 3} { ptrs = append(ptrs, &v) } var funcs []func() for i := 0; i < 3; i++ { funcs = append(funcs, func() { fmt.Print(i, " ")…
base := make([]int, 0, 4) m := map[string][]int{} m["a"] = append(base, 1) m["b"] = append(base, 2) fmt.Println(m["a"], m["b"]) m["a"] = append(m["a"], 10)…
func main() { var p *[3]int fmt.Println(len(p)) for i := range p { fmt.Print(i, " ") } fmt.Println() for i := range 3 { // Go 1.22 range-over-int fmt.Print(i, " ") }…
Since Go 1.23 a for range loop can take a function of type func(yield func(K, V) bool) (that is, iter.Seq2[K, V] ), func(yield func(V) bool) ( iter.Seq[V] ) or…
unique.Make[T comparable](v T) Handle[T] canonicalizes (interns) values. All calls with equal values return handles that point to the same single copy. Comparing two…