Defining methods in Go - Mini Project: Build & Apply

Defining methods in Go

📖 Scenario: You are creating a simple program to represent a Car with its brand and speed. You want to add a method to the Car that can display its current speed in a friendly message.

🎯 Goal: Build a Go program that defines a Car struct and adds a method DisplaySpeed to it. The method should print the car's brand and speed in a sentence.

📋 What You'll Learn

Create a struct called Car with fields Brand (string) and Speed (int).

Create a variable myCar of type Car with brand "Toyota" and speed 120.

Define a method DisplaySpeed with a receiver of type Car that prints the brand and speed.

Call the DisplaySpeed method on myCar to show the output.

💡 Why This Matters

🌍 Real World

Defining methods on structs is how Go programs model real-world objects with both data and actions, like cars, users, or bank accounts.

💼 Career

Understanding methods and structs is essential for Go developers building backend services, tools, or any software that needs organized data and behavior.

1

Create the Car struct and a variable

Create a struct called Car with fields Brand of type string and Speed of type int. Then create a variable called myCar of type Car with Brand set to "Toyota" and Speed set to 120.

Go

// Define the Car struct and create myCar variable
// Your code here

Hint

Use type Car struct { ... } to define the struct. Create myCar with Car{Brand: "Toyota", Speed: 120}.

2

Add a method DisplaySpeed to Car

Define a method called DisplaySpeed with a receiver of type Car. The method should print the message: "The is moving at km/h." using fmt.Printf. Use receiver name c.

Go

package main

import "fmt"

type Car struct {
    Brand string
    Speed int
}

// Define DisplaySpeed method for Car here
// Your code here

func main() {
    myCar := Car{Brand: "Toyota", Speed: 120}
}

Hint

Use func (c Car) DisplaySpeed() to define the method. Use fmt.Printf to print the message with c.Brand and c.Speed.

3

Call the DisplaySpeed method on myCar

Inside the main function, call the DisplaySpeed method on the variable myCar.

Go

package main

import "fmt"

type Car struct {
    Brand string
    Speed int
}

func (c Car) DisplaySpeed() {
    fmt.Printf("The %s is moving at %d km/h.\n", c.Brand, c.Speed)
}

func main() {
    myCar := Car{Brand: "Toyota", Speed: 120}
    // Call DisplaySpeed method on myCar
    // Your code here
}

Hint

Call the method with myCar.DisplaySpeed() inside main.

4

Print the output

Run the program so it prints the message from the DisplaySpeed method.

Go

package main

import "fmt"

type Car struct {
    Brand string
    Speed int
}

func (c Car) DisplaySpeed() {
    fmt.Printf("The %s is moving at %d km/h.\n", c.Brand, c.Speed)
}

func main() {
    myCar := Car{Brand: "Toyota", Speed: 120}
    myCar.DisplaySpeed()
    // Program output should appear above
}

Hint

Run the program to see the message printed exactly as: The Toyota is moving at 120 km/h.

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