Creating struct values in Go - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of struct literal with field names

What is the output of this Go program?

Go

package main
import "fmt"
type Person struct {
    Name string
    Age  int
}
func main() {
    p := Person{Name: "Alice", Age: 30}
    fmt.Println(p)
}

A {Name: Alice, Age: 30}

B {Alice 30}

C Person{Name: "Alice", Age: 30}

D {Name:Alice Age:30}

💡 Hint

Look at how fmt.Println prints structs by default.

In Go, fmt.Println prints struct values without field names, just the values in order inside braces.

❓ Predict Output

intermediate

Output of struct literal without field names

What is the output of this Go program?

Go

package main
import "fmt"
type Point struct {
    X int
    Y int
}
func main() {
    p := Point{10, 20}
    fmt.Println(p)
}

A Point{10, 20}

B {X=10, Y=20}

C {10 20}

D {X:10 Y:20}

💡 Hint

Struct literals can omit field names if values are in order.

When printing, fmt.Println shows struct fields as values in braces without field names.

❓ Predict Output

advanced

Output of nested struct values

What is the output of this Go program?

Go

package main
import "fmt"
type Address struct {
    City  string
    State string
}
type User struct {
    Name    string
    Address Address
}
func main() {
    u := User{Name: "Bob", Address: Address{City: "NY", State: "NY"}}
    fmt.Println(u)
}

A User{Name: "Bob", Address: Address{City: "NY", State: "NY"}}

B {Name:Bob Address:{City:NY State:NY}}

C {Bob {City:NY State:NY}}

D {Bob {NY NY}}

💡 Hint

fmt.Println prints nested structs recursively without field names.

fmt.Println prints nested structs showing values inside braces without field names.

❓ Predict Output

advanced

Output of pointer to struct literal

What is the output of this Go program?

Go

package main
import "fmt"
type Car struct {
    Brand string
    Year  int
}
func main() {
    c := &Car{Brand: "Tesla", Year: 2022}
    fmt.Println(*c)
}

A {Tesla 2022}

B &{Tesla 2022}

C {Brand:Tesla Year:2022}

D Car{Brand: "Tesla", Year: 2022}

💡 Hint

Dereferencing pointer prints the struct value.

Dereferencing the pointer with *c gives the struct value, printed without field names.

🧠 Conceptual

expert

Number of fields in struct literal with embedded struct

Given the structs below, how many fields does the struct literal have when creating a value of type Employee?

Go

type Person struct {
    Name string
    Age  int
}
type Employee struct {
    Person
    ID int
}

// Creating value:
// e := Employee{Person{Name: "Eve", Age: 28}, 101}

A 2 fields: Person and ID

B 3 fields: Name, Age, and ID

C 1 field: ID only

D 4 fields: Person, Name, Age, and ID

💡 Hint

Embedded structs count as one field in the outer struct literal.

Employee struct has two fields: an embedded Person struct and an int ID. The literal uses two values.

Practice

1. What is the correct way to create a struct value with fields Name as "Alice" and Age as 30 in Go?

easy

A. person := Person("Alice", 30)

B. person := Person{30, "Alice"}

C. person := Person{Name: "Alice", Age: 30}

D. person := Person{Name = "Alice", Age = 30}

Solution

1. Step 1: Understand struct literal syntax

   In Go, struct values are created using the syntax StructType{FieldName: value, ...} for clarity and correctness.

2. Step 2: Match the syntax with the given options

   person := Person{Name: "Alice", Age: 30} uses the correct syntax with field names and values. Options A and C use positional values without field names, which is allowed but less clear and can cause errors if order changes. person := Person{Name = "Alice", Age = 30} uses '=' which is invalid in struct literals.

3. Final Answer:

   person := Person{Name: "Alice", Age: 30} -> Option C

4. Quick Check:

   Use field names with colons in struct literals = B [OK]

Hint: Use field names with colons to create structs clearly [OK]

Common Mistakes:

• Using parentheses instead of braces
• Using '=' instead of ':' in struct literals
• Mixing field order without names causing confusion

2. Which of the following is the correct syntax to create a struct value with zero values for all fields in Go?

easy

A. p := Person{}

B. p := Person()

C. p := new(Person)

D. p := Person(nil)

Solution

1. Step 1: Recall zero value struct creation

   In Go, creating a struct with zero values for all fields is done by using empty braces after the struct type: StructType{}.

2. Step 2: Evaluate options

   p := Person{} uses Person{} which creates a struct with zero values. p := Person() uses parentheses which is invalid syntax. p := new(Person) uses new(Person) which returns a pointer, not a struct value. p := Person(nil) uses nil which is invalid for struct values.

3. Final Answer:

   p := Person{} -> Option A

4. Quick Check:

   Empty braces create zero-value struct = C [OK]

Hint: Use empty braces {} to create zero-value struct [OK]

Common Mistakes:

• Using parentheses instead of braces
• Confusing new() pointer with struct value
• Passing nil to struct literal

3. Given the struct and code below, what will be printed?

type Person struct {
    Name string
    Age  int
}

func main() {
    p := Person{Name: "Bob", Age: 25}
    p.Age = 26
    fmt.Println(p)
}

medium

A. {Bob 25}

B. {Bob 26}

C. Person{Name: "Bob", Age: 26}

D. Syntax error

Solution

1. Step 1: Create struct and modify field

   The struct Person is created with Name "Bob" and Age 25. Then the Age field is updated to 26.

2. Step 2: Print the struct value

   Printing the struct with fmt.Println shows the field values in braces. Since Age was updated to 26, the output is {Bob 26}.

3. Final Answer:

   {Bob 26} -> Option B

4. Quick Check:

   Updated field value shows in output = D [OK]

Hint: Changing a field after creation updates printed struct [OK]

Common Mistakes:

• Assuming original Age remains unchanged
• Expecting struct type name in print output
• Confusing syntax error with valid code

4. What is wrong with the following code snippet?

type Person struct {
    Name string
    Age  int
}

func main() {
    p := Person{Name: "Eve", Age: "30"}
    fmt.Println(p)
}

medium

A. Age field has wrong type value

B. Missing parentheses in struct creation

C. Name field is not initialized

D. fmt.Println cannot print structs

Solution

1. Step 1: Check field types in struct literal

   The struct Person has Age as int type, but the code assigns "30" which is a string, causing a type mismatch error.

2. Step 2: Identify the error cause

   Assigning a string to an int field is invalid in Go and causes a compile-time error.

3. Final Answer:

   Age field has wrong type value -> Option A

4. Quick Check:

   Field types must match exactly = A [OK]

Hint: Match field types exactly when creating struct values [OK]

Common Mistakes:

• Assigning string to int field
• Confusing struct literal syntax errors
• Thinking fmt.Println can't print structs

5. How can you create a slice of struct values representing three people with names "Anna", "Ben", "Cara" and ages 22, 28, 31 respectively in Go?

hard

A. people := []Person{Person("Anna", 22), Person("Ben", 28), Person("Cara", 31)}

B. people := [Person]{Name: "Anna", Age: 22, Name: "Ben", Age: 28, Name: "Cara", Age: 31}

C. people := []Person{Name: "Anna", Age: 22, Name: "Ben", Age: 28, Name: "Cara", Age: 31}

D. people := []Person{{Name: "Anna", Age: 22}, {Name: "Ben", Age: 28}, {Name: "Cara", Age: 31}}

Solution

1. Step 1: Understand slice of structs syntax

   A slice of structs is created using []StructType{...} with each element as a struct literal.

2. Step 2: Check each option for correct syntax

   people := []Person{{Name: "Anna", Age: 22}, {Name: "Ben", Age: 28}, {Name: "Cara", Age: 31}} correctly creates a slice with three struct literals using field names. people := [Person]{Name: "Anna", Age: 22, Name: "Ben", Age: 28, Name: "Cara", Age: 31} uses array syntax but incorrect literal format. people := []Person{Name: "Anna", Age: 22, Name: "Ben", Age: 28, Name: "Cara", Age: 31} tries to create slice but misses braces for each struct. people := []Person{Person("Anna", 22), Person("Ben", 28), Person("Cara", 31)} uses parentheses which is invalid for struct literals.

3. Final Answer:

   people := []Person{{Name: "Anna", Age: 22}, {Name: "Ben", Age: 28}, {Name: "Cara", Age: 31}} -> Option D

4. Quick Check:

   Slice of structs uses []Type{{...}, {...}} = A [OK]

Hint: Use double braces for slice of struct literals [OK]

Common Mistakes:

• Using parentheses instead of braces for structs
• Missing braces around each struct in slice
• Confusing array and slice syntax