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.

Practice

1. In Go, what does the len() function return when used on a slice?

easy

A. The maximum number of elements the slice can hold

B. The total memory size of the slice in bytes

C. The index of the last element in the slice

D. The number of elements currently in the slice

Solution

1. Step 1: Understand the purpose of len() on slices

   The len() function returns how many elements are currently stored in the slice.

2. Step 2: Differentiate from capacity

   Capacity is about how many elements the slice can hold before resizing, not the current count.

3. Final Answer:

   The number of elements currently in the slice -> Option D

4. Quick Check:

   len(slice) = current element count [OK]

Hint: Remember: len = current items, cap = max before grow [OK]

Common Mistakes:

• Confusing length with capacity
• Thinking len returns memory size
• Assuming len gives last index

2. Which of the following is the correct way to create a slice with length 3 and capacity 5 in Go?

easy

A. make([]int, 5, 3)

B. make([]int, 3, 5)

C. []int{3, 5}

D. make([]int, 3)

Solution

1. Step 1: Recall the make function signature for slices

   make([]T, length, capacity) creates a slice with given length and capacity.

2. Step 2: Match length and capacity values

   To get length 3 and capacity 5, use make([]int, 3, 5).

3. Final Answer:

   make([]int, 3, 5) -> Option B

4. Quick Check:

   make([]int, length, capacity) [OK]

Hint: make([]T, len, cap) sets length then capacity [OK]

Common Mistakes:

• Swapping length and capacity arguments
• Using slice literal instead of make
• Omitting capacity when needed

3. What will be the output of this Go code?

nums := make([]int, 2, 4)
nums[0] = 10
nums[1] = 20
fmt.Println(len(nums), cap(nums))

medium

A. 2 2

B. 0 4

C. 2 4

D. 4 4

Solution

1. Step 1: Analyze slice creation

   make([]int, 2, 4) creates a slice with length 2 and capacity 4.

2. Step 2: Check length and capacity after assignments

   Assigning values to existing indices does not change length or capacity.

3. Final Answer:

   2 4 -> Option C

4. Quick Check:

   len = 2, cap = 4 [OK]

Hint: Length stays same unless you append beyond capacity [OK]

Common Mistakes:

• Confusing length with capacity
• Assuming capacity equals length after assignment
• Thinking length changes when assigning values

4. Consider this code snippet:

arr := []int{1, 2, 3, 4, 5}
slice := arr[1:3]
fmt.Println(len(slice), cap(slice))

What is the correct output?

medium

A. 2 4

B. 2 2

C. 3 4

D. 3 5

Solution

1. Step 1: Determine length of the slice

   The slice is from index 1 to 3 (exclusive), so length = 3 - 1 = 2.

2. Step 2: Calculate capacity of the slice

   Capacity is from start index to end of original array: 5 - 1 = 4.

3. Final Answer:

   2 4 -> Option A

4. Quick Check:

   len = 2, cap = 4 from slice indices [OK]

Hint: Capacity = original array length - slice start index [OK]

Common Mistakes:

• Using slice length as capacity
• Confusing slice end index with capacity
• Assuming capacity equals original array length

5. You have a slice s := make([]int, 3, 5). You append two elements to it. What will be the length and capacity of s after these appends?

hard

A. Length 5, Capacity 5

B. Length 5, Capacity 10

C. Length 3, Capacity 5

D. Length 5, Capacity 6

Solution

1. Step 1: Understand initial slice size

   Slice s starts with length 3 and capacity 5.

2. Step 2: Append two elements within capacity

   Appending two elements increases length to 5, capacity remains 5 because it is not exceeded.

3. Final Answer:

   Length 5, Capacity 5 -> Option A

4. Quick Check:

   Appending within capacity keeps capacity same [OK]

Hint: Capacity grows only when length exceeds capacity [OK]

Common Mistakes:

• Assuming capacity doubles immediately after append
• Confusing length with capacity after append
• Thinking capacity increases by 1 per append