Struct pointers in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of struct pointer field modification

What is the output of this Go program?

Go

package main
import "fmt"
type Person struct {
    Name string
    Age  int
}
func main() {
    p := Person{"Alice", 30}
    ptr := &p
    ptr.Age = 31
    fmt.Println(p.Age)
}

A Compilation error

B 0

C 30

D 31

💡 Hint

Remember that modifying a struct through its pointer changes the original struct.

The pointer ptr points to p. Changing ptr.Age changes p.Age because ptr references p directly.

❓ Predict Output

intermediate

Output when assigning struct pointer to another variable

What will be printed by this Go program?

Go

package main
import "fmt"
type Point struct {
    X, Y int
}
func main() {
    p1 := &Point{X: 1, Y: 2}
    p2 := p1
    p2.X = 10
    fmt.Println(p1.X)
}

A 0

B 1

C 10

D Compilation error

💡 Hint

Both p1 and p2 point to the same struct in memory.

p2 is assigned the pointer p1, so both point to the same Point struct. Changing p2.X changes p1.X.

🔧 Debug

advanced

Identify the runtime error in struct pointer usage

What error does this Go program produce when run?

Go

package main
import "fmt"
type Data struct {
    Value int
}
func main() {
    var d *Data
    d.Value = 5
    fmt.Println(d.Value)
}

A panic: runtime error: invalid memory address or nil pointer dereference

B Output: 5

C Compilation error: cannot assign to d.Value

D No output, program hangs

💡 Hint

Check if the pointer d is initialized before accessing its fields.

d is a nil pointer. Accessing d.Value causes a runtime panic due to nil pointer dereference.

🧠 Conceptual

advanced

Effect of passing struct pointer to a function

Consider this Go code snippet. What will be the output after calling updateAge(&p)?

Go

package main
import "fmt"
type Person struct {
    Age int
}
func updateAge(p *Person) {
    p.Age = 40
}
func main() {
    p := Person{Age: 25}
    updateAge(&p)
    fmt.Println(p.Age)
}

A 25

B 40

C 0

D Compilation error

💡 Hint

Passing a pointer allows the function to modify the original struct.

The function receives a pointer to p and sets p.Age to 40, modifying the original struct.

🔧 Debug

expert

Which option causes a runtime panic with struct pointers?

Given the struct type Node, which code snippet will cause a runtime panic?

Go

type Node struct {
    Value int
    Next  *Node
}

A

var n *Node
n.Value = 10

B

n := &Node{Value: 5}
n.Next = &Node{Value: 10}

C

var n Node
n.Value = 3

D

n := Node{Value: 1}
var p *Node = &n
p.Value = 2

💡 Hint

Check if the pointer variable is initialized before accessing fields.

Option A declares a nil pointer n and accesses n.Value, causing a runtime panic due to nil pointer dereference.

Practice

1. What does a struct pointer in Go hold?
type Person struct { Name string }

easy

A. Only the struct's first field value

B. A copy of the struct data

C. The memory address of a struct instance

D. A string representation of the struct

Solution

1. Step 1: Understand what a pointer stores

   A pointer stores the memory address of a variable, not the variable's value itself.

2. Step 2: Apply to struct pointers

   A struct pointer holds the address of the struct instance, allowing access or modification of the original struct.

3. Final Answer:

   The memory address of a struct instance -> Option C

4. Quick Check:

   Struct pointer = memory address [OK]

Hint: Pointers hold addresses, not copies. Remember & for address [OK]

Common Mistakes:

• Thinking pointer holds a copy of the struct
• Confusing pointer with struct value
• Assuming pointer holds only first field

2. Which of the following is the correct way to declare a pointer to a struct Person in Go?

easy

A. var *p Person

B. var p Person*

C. var p &Person

D. var p *Person

Solution

1. Step 1: Recall Go pointer syntax

   In Go, a pointer type is declared by placing * before the type name, e.g., *Person.

2. Step 2: Match correct declaration

   var p *Person uses var p *Person, which is the correct syntax for a pointer variable.

3. Final Answer:

   var p *Person -> Option D

4. Quick Check:

   Pointer declaration = *Type [OK]

Hint: Pointer type syntax: *TypeName, not after variable [OK]

Common Mistakes:

• Placing * after variable name
• Using & in declaration instead of *
• Incorrect order of * and type

3. What is the output of this Go program?

package main
import "fmt"
type Point struct { X, Y int }
func main() {
 p := Point{1, 2}
 ptr := &p
 ptr.X = 10
 fmt.Println(p.X)
}

medium

A. 1

B. 10

C. 0

D. Compilation error

Solution

1. Step 1: Understand pointer modification

   ptr points to p. Changing ptr.X modifies p.X because ptr references p's memory.

2. Step 2: Check printed value

   After setting ptr.X = 10, p.X becomes 10. So, printing p.X outputs 10.

3. Final Answer:

   10 -> Option B

4. Quick Check:

   Pointer changes original struct field [OK]

Hint: Changing pointer field changes original struct field [OK]

Common Mistakes:

• Thinking ptr.X is separate copy
• Expecting original p.X to remain 1
• Confusing pointer dereference syntax

4. Identify the error in this Go code snippet:

package main
import "fmt"
type User struct { Name string }
func updateName(u User) {
 u.Name = "Alice"
}
func main() {
 user := User{"Bob"}
 updateName(&user)
 fmt.Println(user.Name)
}

medium

A. Passing pointer to function expecting value

B. Missing import for fmt package

C. Cannot assign string to Name field

D. Function parameter should be a pointer to User

Solution

1. Step 1: Check function parameter type

   Function updateName expects a User value, but main passes &user (a *User pointer).

2. Step 2: Identify the type mismatch

   This causes a compilation error because the argument type (*User) does not match the parameter type (User).

3. Final Answer:

   Passing pointer to function expecting value -> Option A

4. Quick Check:

   Function param must match argument type [OK]

Hint: Match function param type with argument (& for pointer) [OK]

Common Mistakes:

• Passing pointer to value parameter without pointer receiver
• Ignoring need to modify original struct
• Assuming string assignment causes error

5. Given this struct and function:

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

What will happen when running this program?

hard

A. Compilation error at increment(c)

B. Output is 7

C. Output is 6

D. Runtime panic due to invalid pointer

Solution

1. Step 1: Analyze function calls

   increment expects a *Counter pointer. The first call increment(&c) is correct.

2. Step 2: Check second call increment(c)

   Passing c (a value) instead of *Counter causes a compilation error because types don't match.

3. Final Answer:

   Compilation error at increment(c) -> Option A

4. Quick Check:

   Pointer param requires pointer argument [OK]

Hint: Pointer params need pointer args; value won't auto-convert [OK]

Common Mistakes:

• Assuming Go auto-converts value to pointer
• Expecting output 7 without error
• Ignoring type mismatch in function call