Software Development: Go Puzzles

Here are a few puzzles intended to help you round out your knowledge of the Go programming language.

Background

In 1983 I had read my copy of K&R from cover to cover (twice I think) and believed I knew everything there was to know about C. I thought K&R (The C Programming Language by Kernighan and Ritchie) was the only book on C that existed but one day I came across another - the C Puzzle Book by Alan Feuer. I bought it and soon discovered that I knew very little about C - such as the myriad possibilities for undefined behavior.

Luckily Go has greatly limited the prospects of undefined behavior. However, I still think there is room for a selection of puzzles to test your knowledge of the language. Most of puzzles below are the result of discovering something I didn't know in my reading of the excellent "Go Programming Language" by Donovan and Kernighan.

The questions are in no particular order but I tried to start with easy ones. Don't worry if you get most of them wrong as some are pretty obscure - I'd get a few wrong myself even though I wrote them all!

The Questions

For brevity most of the puzzles only provide a snippet of Go code. In which case, you can assume that the code is wrapped in a main() function with all relevant imports added.

m := map[string]int{"": 0}
mcopy := m
mcopy["1"] = 1
mcopy["0"] = 0
mcopy[""] = 1

print(len(m))

What does this print?
a. 1
b. 2
c. 3
d. 4
s := []int{1, 2, 3, 4}
copy(s, []int{42})
fmt.Println(s)

What does this print?
a. [42]
b. [42 1 2 3 4]
c. [1 2 3 4 42]
d. [42 2 3 4]
defer func() {
defer func() {
fmt.Print(recover())
}()
fmt.Print(recover())
panic("DEF")
}()
panic("ABC")

What does this print?
a. ABC
b. DEF
c. ABCDEF
d. <nil>
c := 'c'
print(strings.IndexFunc("abcdefg", func(r rune) bool {
return r == c
}))

What does this print?
a. -1
b. 2
c. 3
d. compile error: undefined: c
s := "世界"
fmt.Println(len(s), len([]byte(s)))

What does this print?
a. 2 2
b. 6 2
c. 6 6
d. 2 6
func init() {
i = make(chan int, 2)
i <- 1
}

var i chan int

func init() {
i <- 2
}

func main() {
fmt.Println(<-i)
}

What does this print?
a. 1
b. 2
c. prints nothing and never terminates
d. compilation error: init redeclared in this block
const (
a = iota
b
c
d
)
fmt.Println(a+b+c+d)

What does this print?
a. 0
b. 4
c. 6
d. 10
a := [...]int{41, 42, 43, 44, 45}
s := a[1:2]
copy(s, []int{1, 2, 3})
fmt.Println(a[2])

What does this print?
a. 2
b. 3
c. 42
d. 43
m := map[int]int{1: 1, 2: 2}
for k := range m {
if k == 1 {
delete(m, k)
}
fmt.Print(k, len(m), " ")
}

What does this print?
a. 1 1
b. 1 1 2 1
c. 2 2 1 1
d. either b or c
a := []int{0, 1, 2, 3, 4}
b := a[0:2:4]
fmt.Println(cap(b) - len(b))

What does this print?
a. 0
b. 2
c. 3
d. 4
var wg sync.WaitGroup
wg.Add(1)
go func(wg sync.WaitGroup) {
wg.Wait()
print("A")
}(wg)
wg.Done()
print("B")

What does this print?
a. AB
b. B
c. BA
d. any one of the above
i, f := 200, 2e2
x, y, z := 200+2e2, 200+f, i+2e2
fmt.Printf("%T %T %T", x, y, z)

What does this print?
a. int float64 int
b. float64 float64 int
c. float64 float64 float64
d. None of the above
s := []int{1, 2, 3}[1:2]
t := append(s, 4, 5, 6)
fmt.Println(len(s), cap(t))

What does this print?
a. 1 4
b. 2 5
c. 4 4
d. 4 6
const idx = 2
a := [...]int{idx: 1, idx+1: 2}
fmt.Println(a[3])

What does this print?
a. 0
b. 1
c. 2
d. compilation error: invalid array index 3
a := 1
var b interface{} = a
a = 2
fmt.Println(b)

What does this print?
a. 1
b. 2
c. (interface {})(1)
d. (interface {})(2)
const (
a = 0
b
c = iota
d
)
fmt.Println(a+b+c+d)

What does this print?
a. 1
b. 2
c. 5
d. 6
a := [...]int{1, 2}
_ = append(a[:1], 3)
_ = append(a[:1], 4, 5)
fmt.Println(a[1])

What does this print?
a. 0
b. 2
c. 3
d. 4
s := bytes.NewBufferString("string")
a := s.String()
b := s.Bytes()
m := make(map[interface{}]int)
m[a] = 1
m[b] = 2
log.Println(m[a])

What does this print?
a. 1
b. 2
c. compile error: invalid map key
d. panics at run-time
m := make(map[int]*struct{ i int })
m[3] = &struct{ i int }{1}
v := m[3]
v.i++
fmt.Println(m[3].i)

What does this print?
a. 0
b. 1
c. 2
d. 3
a := time.Now().Year
b, _ := strconv.Atoi(time.Now().Format("06"))
fmt.Println(b - a())

What does this print?
a. 6
b. 0
c. -2000
d. 18 (current year this century)
i := 2
switch i {
case 2:
if i % 2 == 0 {
fallthrough
}
i = 6
case 3:
i++
fallthrough
default:
i++
}
print(i)

What does this print?
a. 4
b. 5
c. 6
d. compile error: fallthrough statement out of place
s := []int{10, 12, 14}
for _, v := range s {
v++
}
fmt.Println(s[1])

What does this print?
a. 11
b. 12
c. 13
d. 14
s := []int{1, 2, 3, 4}
t := make([]int, 2, 3)
copy(t, s)
fmt.Println(t)

What does this print?
a. [1 2]
b. [1 2 3]
c. [1 2 3 4]
d. [0 0 3 4]
a := struct{}{}
b := struct{}{}

fmt.Printf("%t", &a == &b)

What does this print?
a. false
b. true
c. bool
d. the exact behaviour of this code is undefined
var a, b interface{}

a = "str" + "ing"
b = "string"

m := make(map[interface{}]int)
m[a] = 1
m[b] = 2

print(m[a])

What does this print?
a. 0
b. 1
c. 2
d. compile error: invalid map key
var t interface{} = 1

switch t := t.(type) {
case int, uint:
fmt.Println(reflect.TypeOf(&t))

default:
print("default")
}

What does this print?
a. *int
b. *uint
c. default
d. *interface{}
fmt.Printf("%q\n", string(65))

What does this print?
a. "65"
b. "A"
c. "\x41"
d. "�"
const i1, i2, i3 = 2, 3, 2
a := [...]int{i1: i1, i2: i2, i3: i3}

print(len(a))

What does this print?
a. 2
b. 3
c. 4
d. compile error: duplicate index in array literal: 2
type intA int
type intB = int
var a intA = 1
var b intB = 2
print(int(a) | b)

What does this print?
a. 0
b. 1
c. 3
d. compile error: invalid operation: int(a) | b (mismatched types int and intB)
ctx, cancel := context.WithCancel(context.Background())

cancel()

fmt.Println(ctx.Err())

What does this print?
a. <nil>
b. deadline exceeded
c. context canceled
d. does not print anything but causes a panic at run-time
const (
a, b = iota, iota
c, d = iota+1, iota+2
)

print(a+b+c+d)

What does this print?
a. 5
b. 6
c. 7
d. 9
var a, b interface{}
a = "main"
b = main

fmt.Printf("%t\n", a == b)

What does this print?
a. false
b. true
c. compile error: invalid interface type
d. panics at run-time
fmt.Printf("%q", "A\xbc")

What does this print?
a. "A\xbc"
b. A
c. A�
d. "Abc"

Software Development

Wednesday 31 October 2018

Go Puzzles

2 comments: