Passing values vs pointers in Go - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of modifying a struct passed by value

What is the output of this Go program?

Go

package main
import "fmt"
type Point struct {
  X, Y int
}
func move(p Point) {
  p.X = 10
  p.Y = 20
}
func main() {
  pt := Point{1, 2}
  move(pt)
  fmt.Println(pt)
}

A {1 2}

B {10 20}

C Compilation error

D {0 0}

💡 Hint

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

The function move receives a copy of the struct Point. Changes inside move do not affect the original variable pt in main. So pt remains {1 2}.

❓ Predict Output

intermediate

Output of modifying a struct passed by pointer

What is the output of this Go program?

Go

package main
import "fmt"
type Point struct {
  X, Y int
}
func move(p *Point) {
  p.X = 10
  p.Y = 20
}
func main() {
  pt := Point{1, 2}
  move(&pt)
  fmt.Println(pt)
}

A {10 20}

B {0 0}

C Compilation error

D {1 2}

💡 Hint

Check if the function receives a pointer and modifies the original struct.

The function move receives a pointer to Point. It modifies the original struct fields through the pointer. So pt changes to {10 20}.

❓ Predict Output

advanced

Effect of pointer vs value on slice modification

What is the output of this Go program?

Go

package main
import "fmt"
func modifySlice(s []int) {
  s[0] = 100
  s = append(s, 200)
}
func main() {
  slice := []int{1, 2, 3}
  modifySlice(slice)
  fmt.Println(slice)
}

A [100 2 3 200]

B [100 2 3]

C [1 2 3]

D Runtime panic

💡 Hint

Remember slices are references but append may create a new underlying array.

The function modifies the first element of the original slice, so s[0] becomes 100. But append creates a new slice inside the function, which does not affect the original slice length. So original slice remains length 3.

❓ Predict Output

advanced

Output when passing map by value and modifying it

What is the output of this Go program?

Go

package main
import "fmt"
func modifyMap(m map[string]int) {
  m["a"] = 100
  m = make(map[string]int)
  m["b"] = 200
}
func main() {
  myMap := map[string]int{"a": 1}
  modifyMap(myMap)
  fmt.Println(myMap)
}

A map[a:1]

B map[b:200]

C map[a:100]

D Compilation error

💡 Hint

Maps are reference types in Go. Changes to the map inside the function affect the original map.

The function modifies the original map by setting key "a" to 100. Then it assigns a new map to the local variable m, but this does not affect the original map outside. So the original map has key "a" with value 100.

🧠 Conceptual

expert

Understanding pointer semantics with interface values

Consider this Go code snippet. What will be printed? package main import "fmt" type Data struct { Value int } func modify(d interface{}) { switch v := d.(type) { case *Data: v.Value = 42 case Data: v.Value = 42 } } func main() { d1 := Data{1} d2 := &Data{1} modify(d1) modify(d2) fmt.Println(d1.Value, d2.Value) }

A 42 42

B Compilation error

C 42 1

D 1 42

💡 Hint

Think about whether the modify function changes the original data or a copy depending on the type assertion.

When modify receives d1 (a value), the case Data matches and modifies a copy, so d1.Value stays 1. When modify receives d2 (a pointer), the case *Data matches and modifies the original, so d2.Value becomes 42.