Zero values in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of uninitialized variables

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    var a int
    var b string
    var c bool
    fmt.Printf("%d %q %t", a, b, c)
}

A 0 "" false

B 0 "0" false

C 0 "nil" false

D 0 "" true

💡 Hint

Think about what Go assigns to variables when you don't set them.

In Go, uninitialized variables get their zero values. For int it's 0, for string it's an empty string "", and for bool it's false.

❓ Predict Output

intermediate

Zero value of a slice

What will this Go program print?

Go

package main
import "fmt"
func main() {
    var s []int
    fmt.Println(s == nil)
    fmt.Println(len(s))
}

A

false
0

B

true
-1

C

true
0

D

false
-1

💡 Hint

Slices have a zero value of nil and length zero.

The zero value of a slice is nil. So s == nil is true. The length of a nil slice is 0.

❓ Predict Output

advanced

Zero value of a map and its behavior

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    var m map[string]int
    fmt.Println(m == nil)
    fmt.Println(m["key"])
    m["key"] = 10
}

A

false
0
panic: assignment to entry in nil map

B

false
0
no panic

C

true
0
no panic

D

true
0
panic: assignment to entry in nil map

💡 Hint

Maps have zero value nil, reading from nil map is allowed, writing causes panic.

The zero value of a map is nil. Reading from a nil map returns the zero value of the element type (0 here). But writing to a nil map causes a runtime panic.

❓ Predict Output

advanced

Zero value of a struct with embedded fields

What will this Go program print?

Go

package main
import "fmt"
type Inner struct {
    X int
}
type Outer struct {
    Inner
    Y string
}
func main() {
    var o Outer
    fmt.Printf("%d %q", o.X, o.Y)
}

A 0 ""

B 0 "nil"

C 0 "0"

D panic at runtime

💡 Hint

Embedded structs get zero values too.

The embedded struct Inner is zero-initialized, so o.X is 0. The string Y is empty string "".

❓ Predict Output

expert

Zero value of interface and type assertion

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    var i interface{}
    fmt.Println(i == nil)
    v, ok := i.(int)
    fmt.Println(v, ok)
}

A

false
0 false

B

true
0 false

C

true
0 true

D

false
0 true

💡 Hint

An uninitialized interface is nil; type assertion fails safely.

The zero value of an interface is nil. So i == nil is true. The type assertion i.(int) fails, so ok is false and v is zero value of int (0).