Slice and array relationship in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of modifying a slice element

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    arr := [3]int{1, 2, 3}
    s := arr[:]
    s[1] = 10
    fmt.Println(arr)
}

A [1 2 3]

B [1 10 3]

C [10 2 3]

D [1 10 10]

💡 Hint

Slices share the underlying array with the original array.

Changing an element of the slice changes the underlying array because the slice references the same memory.

❓ Predict Output

intermediate

Length and capacity of a slice

What will be printed by this Go code?

Go

package main
import "fmt"
func main() {
    arr := [5]int{1, 2, 3, 4, 5}
    s := arr[1:4]
    fmt.Println(len(s), cap(s))
}

A 3 4

B 3 3

C 4 4

D 4 3

💡 Hint

Length is the number of elements in the slice; capacity is from the start of the slice to the end of the array.

The slice s covers elements at indices 1,2,3 (length 3). Capacity is from index 1 to end of arr (5 elements total), so capacity is 4.

🔧 Debug

advanced

Why does this slice append not change the original array?

Consider this Go code snippet. What is the output and why?

Go

package main
import "fmt"
func main() {
    arr := [3]int{1, 2, 3}
    s := arr[:2]
    s = append(s, 4)
    fmt.Println(arr)
    fmt.Println(s)
}

A [1 2 3]\n[1 2 4]

B [1 2 4]\n[1 2 4]

C [1 2 3]\n[1 2 3 4]

D [1 2 3]\n[1 2 3]

💡 Hint

Appending to a slice may create a new underlying array if capacity is exceeded.

The slice s has length 2 and capacity 3. Appending one element increases length to 3, which fits in capacity. Append modifies the underlying array by setting arr[2]=4. So arr becomes [1 2 4]. Both arr and s reflect [1 2 4].

📝 Syntax

advanced

Identify the syntax error in slice declaration

Which option contains a syntax error in slice or array usage?

A s := arr[1:4]

B arr := [3]int{1, 2, 3}

C s := arr[1..3]

D s := []int{1, 2, 3}

💡 Hint

Go slice syntax uses a single colon, not two dots.

The slice syntax uses a single colon between indices, e.g., arr[1:3]. Using two dots causes a syntax error.

🧠 Conceptual

expert

Effect of modifying a slice after re-slicing

Given the code below, what is the output?

Go

package main
import "fmt"
func main() {
    arr := [5]int{10, 20, 30, 40, 50}
    s1 := arr[1:4]
    s2 := s1[1:3]
    s2[0] = 100
    fmt.Println(arr)
}

A [10 20 30 100 50]

B [10 100 30 40 50]

C [10 20 30 40 50]

D [10 20 100 40 50]

💡 Hint

Slices share the same underlying array. Indexes are relative to the original array.

s1 is arr[1:4] → elements at indices 1,2,3. s2 is s1[1:3] → elements at indices 2,3 of arr. Modifying s2[0] changes arr[2].