len and cap
- Go course
- Beginner
- Lesson 45/90
- 4 min
Learn the length and capacity of a slice.
len and cap
len(s) is the number of elements in the slice; cap(s) is the total room available in the underlying array from the slice's start. Length can't exceed capacity.
s := make([]int, 2, 5)
fmt.Println(len(s), cap(s)) // 2 5
s = append(s, 1, 2, 3)
fmt.Println(len(s), cap(s)) // 5 5
s = append(s, 9)
fmt.Println(len(s), cap(s)) // 6 10 (new array)When capacity fills up, Go usually doubles it (large slices grow by less).
The cost of growth
On every growth all elements are copied to a new array. Appending 1 million elements one by one means about 20 growths; starting with make([]T, 0, 1_000_000) avoids all of these copies. If you keep a small sub-slice of a large slice, the large underlying array stays in memory too; copy the small piece with slices.Clone if needed.
Practice Watch the growth
Append the numbers 1 to 5 one by one to an empty []int; after each append print len and cap with fmt.Println(len(s), cap(s)).
Starter code
package main
import "fmt"
func main() {
// buraya yaz
}Sample solution
package main
import "fmt"
func main() {
var s []int
for i := 1; i <= 5; i++ {
s = append(s, i)
fmt.Println(len(s), cap(s))
}
}Expected output
1 1 2 2 3 4 4 4 5 8
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 For <code>s := []int{1, 2, 3, 4, 5}</code>, what is <code>cap(s[2:])</code>?
- 2
- 0
- 3 Correct answer
- 5
Explanation: A sub-slice's capacity is the room from its start to the end of the underlying array: 5 - 2 = 3.
2 Capacity
s := make([]int, 0, 8)
fmt.Println(____(s)) // 8s := make([]int, 0, 8)
fmt.Println(cap(s)) // 8Explanation: cap gives the slice's total capacity in the underlying array.
3 Slice and len/cap
| Term | Meaning |
|---|---|
make([]int, 3) |
3 / 3 |
make([]int, 0, 4) |
0 / 4 |
[]int{1, 2}[1:] |
1 / 1 |
var s []int |
0 / 0 |