Defining methods in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method with pointer receiver

What is the output of this Go program?

Go

package main
import "fmt"
type Counter struct {
    count int
}
func (c *Counter) Increment() {
    c.count++
}
func main() {
    c := Counter{count: 5}
    c.Increment()
    fmt.Println(c.count)
}

A 0

B 5

C 6

D Compilation error

💡 Hint

Think about whether the method changes the original struct or a copy.

The method Increment has a pointer receiver, so it modifies the original Counter's count field. Starting from 5, it increments to 6.

❓ Predict Output

intermediate

Output of method with value receiver

What will this Go program print?

Go

package main
import "fmt"
type Number struct {
    value int
}
func (n Number) Double() {
    n.value = n.value * 2
}
func main() {
    num := Number{value: 10}
    num.Double()
    fmt.Println(num.value)
}

A 10

B 20

C 0

D Compilation error

💡 Hint

Does the method change the original struct or a copy?

The method Double has a value receiver, so it works on a copy of Number. The original value remains unchanged at 10.

🔧 Debug

advanced

Why does this setter method not update the struct?

This Go code does not update the name. What is the cause of the error?

Go

package main
import "fmt"
type Person struct {
    name string
}
func (p Person) SetName(newName string) {
    p.name = newName
}
func main() {
    p := Person{name: "Alice"}
    p.SetName("Bob")
    fmt.Println(p.name)
}

A Cannot assign to p.name because p is a value receiver and method cannot modify it

B The method SetName must return the new name to update p.name

C fmt.Println cannot access p.name because it is unexported

D Missing pointer receiver in SetName method to modify original struct

💡 Hint

Think about how methods with value receivers behave when modifying fields.

The method SetName uses a value receiver, so it modifies a copy of Person. To change the original, the receiver must be a pointer.

🧠 Conceptual

advanced

Method sets and interface implementation

Given the interface and struct below, which method set allows the struct to implement the interface?

Go

package main
import "fmt"
type Speaker interface {
    Speak() string
}
type Dog struct {}
func (d Dog) Speak() string {
    return "Woof!"
}
func (d *Dog) Speak() string {
    return "Bark!"
}
func main() {
    var s Speaker
    d := Dog{}
    s = d
    fmt.Println(s.Speak())
}

A Neither Dog nor *Dog implement Speaker because of method name conflict

B Dog implements Speaker because it has Speak() with value receiver

C Both Dog and *Dog implement Speaker interface

D *Dog implements Speaker because it has Speak() with pointer receiver

💡 Hint

Value receiver methods belong to both value and pointer types, pointer receiver methods belong only to pointer types.

The method with value receiver means Dog (value) implements Speaker. Pointer receiver methods only belong to *Dog. So Dog implements Speaker here.

❓ Predict Output

expert

Output of method with embedded struct and pointer receiver

What is the output of this Go program?

Go

package main
import "fmt"
type Animal struct {
    name string
}
func (a *Animal) Speak() string {
    return "I am " + a.name
}
type Cat struct {
    Animal
}
func (c Cat) Speak() string {
    return "Meow, " + c.name
}
func main() {
    c := Cat{Animal{name: "Whiskers"}}
    var a *Animal = &c.Animal
    fmt.Println(c.Speak())
    fmt.Println(a.Speak())
}

A

Meow, Whiskers
I am Whiskers

B

Meow, Whiskers
Meow, Whiskers

C

I am Whiskers
Meow, Whiskers

D Compilation error due to method conflict

💡 Hint

Remember that method calls depend on the type of the variable, not just the underlying data.

c.Speak() calls Cat's Speak method (value receiver), printing "Meow, Whiskers". a.Speak() calls Animal's Speak method (pointer receiver), printing "I am Whiskers".