Defining methods in Go - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a method named Greet for the Person struct.

Go

type Person struct {
    Name string
}

func (p [1]) Greet() string {
    return "Hello, " + p.Name
}

Drag options to blanks, or click blank then click option'

Aperson

B*Person

CPerson

Dp

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name instead of a type name as receiver.

Forgetting parentheses around the receiver.

The method receiver must be a type name or pointer type. Here, Person is correct to define the method on the struct value.

2fill in blank

medium

Complete the code to define a method Area for the Rectangle struct that returns the area as an int.

Go

type Rectangle struct {
    Width, Height int
}

func (r [1]) Area() int {
    return r.Width * r.Height
}

Drag options to blanks, or click blank then click option'

Arectangle

B*Rectangle

Crect

DRectangle

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name instead of a type name.

Omitting the pointer when the method needs to modify the struct.

Using a pointer receiver *Rectangle allows the method to work efficiently and modify the struct if needed.

3fill in blank

hard

Fix the error in the method receiver declaration to correctly define method Double for Counter.

Go

type Counter struct {
    Value int
}

func (c [1]) Double() {
    c.Value = c.Value * 2
}

Drag options to blanks, or click blank then click option'

A*Counter

Bcounter

CCounter

Dc

Attempts:

3 left

💡 Hint

Common Mistakes

Using a value receiver which copies the struct and does not modify the original.

Using a variable name instead of a type name.

To modify the struct field inside the method, the receiver must be a pointer *Counter.

4fill in blank

hard

Fill both blanks to define a method IsSquare for Rectangle that returns true if width equals height.

Go

func (r [1]) IsSquare() bool {
    return r.Width [2] r.Height
}

Drag options to blanks, or click blank then click option'

A*Rectangle

BRectangle

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using a value receiver when pointer is preferred.

Using the wrong comparison operator.

The method uses a pointer receiver *Rectangle and compares width and height with ==.

5fill in blank

hard

Fill all three blanks to define a method UpdateName for Person that sets the Name field to a new value.

Go

func (p [1]) UpdateName(newName [2]) {
    p.Name = [3]
}

Drag options to blanks, or click blank then click option'

A*Person

Bstring

CnewName

DPerson

Attempts:

3 left

💡 Hint

Common Mistakes

Using a value receiver which does not modify the original struct.

Forgetting to use the parameter name when assigning.

The method uses a pointer receiver *Person to modify the struct, takes a string parameter, and assigns it to p.Name.

Practice

1. What is the purpose of defining a method with a receiver in Go?

easy

A. To import external packages into the program

B. To create a standalone function unrelated to any type

C. To associate a function with a specific type so it can act on its data

D. To declare a global variable accessible everywhere

Solution

1. Step 1: Understand what a receiver does

   A receiver connects a method to a type, allowing the method to access or modify that type's data.

2. Step 2: Compare with other options

   Options A, B, and C describe unrelated concepts: imports, standalone functions, and global variables.

3. Final Answer:

   To associate a function with a specific type so it can act on its data -> Option C

4. Quick Check:

   Receiver links method to type = D [OK]

Hint: Receiver connects method to type's data [OK]

Common Mistakes:

• Thinking methods are standalone functions
• Confusing methods with variables
• Believing receivers import packages

2. Which of the following is the correct syntax to define a method named Speak for a struct type Dog?

easy

A. func (d Dog) Speak() { }

B. func Speak(d Dog) { }

C. func Dog.Speak() { }

D. func Speak() Dog { }

Solution

1. Step 1: Recall Go method syntax

   Methods use the form: func (receiver Type) MethodName() { }.

2. Step 2: Match syntax to options

   func (d Dog) Speak() { } matches this pattern exactly. func Speak(d Dog) { } is a function, not a method. func Dog.Speak() { } is invalid syntax. func Speak() Dog { } is a function returning Dog, not a method.

3. Final Answer:

   func (d Dog) Speak() { } -> Option A

4. Quick Check:

   Method syntax = func (receiver Type) Name() [OK]

Hint: Method syntax always has receiver in parentheses before name [OK]

Common Mistakes:

• Omitting receiver parentheses
• Using dot notation in function name
• Confusing methods with functions

3. What will be the output of this Go program?

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

medium

A. 6

B. 5

C. 0

D. Compilation error

Solution

1. Step 1: Understand receiver type in Increment method

   The method uses a value receiver (c Counter), so it works on a copy of the struct, not the original.

2. Step 2: Trace the method call and print

   Increment increases the copy's value, but original c.value remains 5. So printing c.value outputs 5.

3. Final Answer:

   5 -> Option B

4. Quick Check:

   Value receiver does not modify original = 5 [OK]

Hint: Value receiver copies data; original stays unchanged [OK]

Common Mistakes:

• Assuming value receiver changes original data
• Confusing pointer and value receivers
• Expecting incremented output

4. Identify the error in this method definition:

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)
}

medium

A. No error; code runs and prints 'Bob'

B. Cannot call method with pointer receiver on value variable

C. Method must return a value

D. Missing import of fmt package

Solution

1. Step 1: Check pointer receiver usage

   In Go, methods with pointer receivers can be called on value variables; Go automatically takes address.

2. Step 2: Verify imports and method behavior

   Code imports fmt, method sets name correctly, so p.name becomes "Bob" and prints it.

3. Final Answer:

   No error; code runs and prints 'Bob' -> Option A

4. Quick Check:

   Pointer receiver works on value variable = B [OK]

Hint: Pointer receiver methods can be called on values directly [OK]

Common Mistakes:

• Thinking pointer receiver requires pointer variable
• Forgetting Go auto-addressing
• Expecting compile error

5. You want to define a method Reset for a struct Timer that sets its seconds field to zero. Which receiver type should you use and why? Options: A) Pointer receiver, because it modifies the original struct's field B) Value receiver, because it copies the struct and resets the copy C) Value receiver, because methods cannot have pointer receivers D) Pointer receiver, but it cannot modify fields inside the struct

hard

A. Pointer receiver, but it cannot modify fields inside the struct

B. Value receiver, because it copies the struct and resets the copy

C. Value receiver, because methods cannot have pointer receivers

D. Pointer receiver, because it modifies the original struct's field

Solution

1. Step 1: Understand effect of receiver types on struct modification

   Value receivers work on copies, so changes do not affect original struct fields.

2. Step 2: Choose pointer receiver to modify original

   Pointer receivers allow methods to change the original struct's fields, so Reset can set seconds to zero effectively.

3. Final Answer:

   Pointer receiver, because it modifies the original struct's field -> Option D

4. Quick Check:

   Pointer receiver modifies original struct = A [OK]

Hint: Use pointer receiver to change original struct fields [OK]

Common Mistakes:

• Using value receiver expecting original change
• Believing methods can't have pointer receivers
• Thinking pointer receiver can't modify fields