Q: With Go 1.22 or later, what does this print, given the goroutines are awaited?for i := 0; i < 3; i++ { go func() { fmt.Print(i) }() }

0, 1 and 2 in some order Since Go 1.22 each iteration has its own i, so each closure sees its own value. Before 1.22 they shared one variable and often printed 3s. Goroutine order is not guaranteed.

Q: What happens?var m map[string]int fmt.Println(m["a"]) m["a"] = 1

It prints 0, then panics on the write Reading a nil map returns the zero value. Writing panics with assignment to entry in nil map. Create it with make(map[string]int).

Q: What does this print?a := []int{1, 2, 3} b := a[:2] b = append(b, 9) fmt.Println(a)

[1 2 9] b shares a's array and has capacity 3, so append writes into a[2] without reallocating. Use a full slice expression a[:2:2] or copy to avoid the aliasing.

Q: What does this print?type MyErr struct{} func (*MyErr) Error() string { return "x" } var p *MyErr = nil var err error = p fmt.Println(err != nil)

true An interface is nil only when both its type and value are nil. err holds type *MyErr with a nil value, so it is not nil. Return a literal nil for no error.

Q: What does this print?for i := 0; i < 3; i++ { defer fmt.Print(i) }

210 Deferred calls run last in, first out when the function returns, and their arguments are evaluated when defer runs.

Q: What does this print when the function returns?x := 1 defer fmt.Println(x) x = 2

1 Arguments to a deferred call are evaluated immediately. To see the final value, defer a closure: defer func() { fmt.Println(x) }().

Q: What happens when main runs this?ch := make(chan int) ch <- 1 fmt.Println(<-ch)

fatal error: all goroutines are asleep, deadlock A send on an unbuffered channel blocks until a receiver is ready. The only goroutine is blocked on the send, so the runtime reports a deadlock. Use a buffer of 1 or receive in another goroutine.

Q: What does this print?ch := make(chan int, 1) close(ch) v, ok := <-ch fmt.Println(v, ok)

0 false Receiving from a closed, drained channel returns the zero value immediately with ok false. Sending on a closed channel is what panics.

Q: What does this print?type Counter struct{ n int } func (c Counter) Inc() { c.n++ } c := Counter{} c.Inc() fmt.Println(c.n)

0 A value receiver gets a copy, so the increment is lost. Use func (c *Counter) Inc() to modify the caller's value.

Q: In Go, what does len("héllo") return? (é is the single code point U+00E9.)

6 In Go, len on a string counts bytes. Go source is UTF-8, and é (U+00E9) takes 2 bytes. Use utf8.RuneCountInString to count characters.

Question 1

With Go 1.22 or later, what does this print, given the goroutines are awaited?for i := 0; i < 3; i++ {
 go func() { fmt.Print(i) }()
}

Accepted Answer

0, 1 and 2 in some order Since Go 1.22 each iteration has its own i, so each closure sees its own value. Before 1.22 they shared one variable and often printed 3s. Goroutine order is not guaranteed.

Question 2

What happens?var m map[string]int
fmt.Println(m["a"])
m["a"] = 1

Accepted Answer

It prints 0, then panics on the write Reading a nil map returns the zero value. Writing panics with assignment to entry in nil map. Create it with make(map[string]int).

Question 3

What does this print?a := []int{1, 2, 3}
b := a[:2]
b = append(b, 9)
fmt.Println(a)

Accepted Answer

[1 2 9] b shares a's array and has capacity 3, so append writes into a[2] without reallocating. Use a full slice expression a[:2:2] or copy to avoid the aliasing.

Question 4

What does this print?type MyErr struct{}
func (*MyErr) Error() string { return "x" }

var p *MyErr = nil
var err error = p
fmt.Println(err != nil)

Accepted Answer

true An interface is nil only when both its type and value are nil. err holds type *MyErr with a nil value, so it is not nil. Return a literal nil for no error.

Question 5

What does this print?for i := 0; i < 3; i++ {
 defer fmt.Print(i)
}

Accepted Answer

210 Deferred calls run last in, first out when the function returns, and their arguments are evaluated when defer runs.

Question 6

What does this print when the function returns?x := 1
defer fmt.Println(x)
x = 2

Accepted Answer

1 Arguments to a deferred call are evaluated immediately. To see the final value, defer a closure: defer func() { fmt.Println(x) }().

Question 7

What happens when main runs this?ch := make(chan int)
ch <- 1
fmt.Println(<-ch)

Accepted Answer

fatal error: all goroutines are asleep, deadlock A send on an unbuffered channel blocks until a receiver is ready. The only goroutine is blocked on the send, so the runtime reports a deadlock. Use a buffer of 1 or receive in another goroutine.

Question 8

What does this print?ch := make(chan int, 1)
close(ch)
v, ok := <-ch
fmt.Println(v, ok)

Accepted Answer

0 false Receiving from a closed, drained channel returns the zero value immediately with ok false. Sending on a closed channel is what panics.

Question 9

What does this print?type Counter struct{ n int }
func (c Counter) Inc() { c.n++ }

c := Counter{}
c.Inc()
fmt.Println(c.n)

Accepted Answer

0 A value receiver gets a copy, so the increment is lost. Use func (c *Counter) Inc() to modify the caller's value.

Question 10

In Go, what does len("héllo") return? (é is the single code point U+00E9.)

Accepted Answer

6 In Go, len on a string counts bytes. Go source is UTF-8, and é (U+00E9) takes 2 bytes. Use utf8.RuneCountInString to count characters.

Question 11

Several goroutines write to a plain Go map with no locking. What can happen?

Accepted Answer

The runtime may crash with fatal error: concurrent map writes Built in maps are not safe for concurrent writes, and the runtime detects some races and aborts. Guard the map with a sync.Mutex, or use sync.Map for its specific use cases.

Question 12

What is the bug?var wg sync.WaitGroup
for _, j := range jobs {
 go func() {
 wg.Add(1)
 defer wg.Done()
 run(j)
 }()
}
wg.Wait()

Accepted Answer

Wait can return before Add runs, so jobs may not be awaited Add must happen before the goroutine starts, otherwise Wait may see a zero counter and return early. Call wg.Add(1) before go, or wg.Add(len(jobs)) once. Since Go 1.22 j is per iteration.

Question 13

In a Go select statement, two cases are ready at the same time. Which runs?

Accepted Answer

One chosen pseudo randomly When several cases can proceed, Go picks one by uniform pseudo random selection, which prevents starvation. Do not rely on case order for priority.

Question 14

In Go, a handler starts a goroutine that sends its result on an unbuffered channel, and returns early on context timeout. What goes wrong?

Accepted Answer

The goroutine blocks on the send forever, leaking Nobody receives after the timeout, so the send blocks forever. Make the channel buffered with capacity 1, or select on ctx.Done() in the sender. Cancellation is cooperative.

Question 15

In Go, map[string]int, func(), []int and [2]int: name the only type here that can be used as a map key.

Accepted Answer

[2]int Map keys must be comparable. Arrays of comparable types are, while slices, maps and functions are not.

Go interview questions

More programming language topics

Practise these on your commute