Slice length and capacity in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Understanding slice length and capacity after slicing

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    s := []int{10, 20, 30, 40, 50}
    t := s[1:3]
    fmt.Println(len(t), cap(t))
}

A 2 3

B 3 4

C 2 4

D 3 5

💡 Hint

Remember: length is the number of elements in the slice, capacity is from the start of the slice to the end of the underlying array.

The slice t = s[1:3] has length 3-1=2. Its capacity is from index 1 to the end of s, which is 5-1=4.

❓ Predict Output

intermediate

Effect of append on slice capacity

What will be printed by this Go program?

Go

package main
import "fmt"
func main() {
    s := make([]int, 2, 4)
    s[0], s[1] = 1, 2
    s = append(s, 3)
    fmt.Println(len(s), cap(s))
}

A 3 3

B 3 4

C 4 4

D 2 4

💡 Hint

Appending increases length by 1; capacity stays the same if there is room.

Initial slice has length 2 and capacity 4. Append adds one element, length becomes 3, capacity remains 4.

❓ Predict Output

advanced

Slice capacity after reslicing

What is the output of this Go code?

Go

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

A 3 4

B 2 4

C 3 3

D 2 3

💡 Hint

Capacity of a slice is from its start index to the end of the underlying array.

t = s[1:4] has length 3 and capacity 4 (from index 1 to 5). u = t[1:3] starts at s index 2, length 2, capacity 3 (from index 2 to 5).

❓ Predict Output

advanced

Appending beyond capacity creates new array

What will this Go program print?

Go

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

A 4 6

B 4 4

C 5 6

D 5 4

💡 Hint

Appending more elements than capacity causes a new underlying array with larger capacity.

t starts with length 2 and capacity 3. Appending two elements increases length to 4, exceeding capacity, so Go allocates a new array with larger capacity, usually doubling to 6.

❓ Predict Output

expert

Slice capacity and length with nil slice and append

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    var s []int
    fmt.Println(len(s), cap(s))
    s = append(s, 1)
    fmt.Println(len(s), cap(s))
}

A

0 0
1 1

B

0 0
1 0

C

0 1
1 1

D

0 0
1 2

💡 Hint

A nil slice has zero length and capacity. Append allocates new underlying array.

Initially s is nil, so length and capacity are 0. After append, length is 1 and capacity is at least 1.