len and cap

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.

Exercise 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)).

Show example 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))
    }
}

Test yourself

For <code>s := []int{1, 2, 3, 4, 5}</code>, what is <code>cap(s[2:])</code>?
- 2
- 0
- 3
- 5
Show answer

3 — A sub-slice's capacity is the room from its start to the end of the underlying array: 5 - 2 = 3.

Capacity

s := make([]int, 0, 8)
fmt.Println(____(s))  // 8

Show answer

s := make([]int, 0, 8)
fmt.Println(cap(s))  // 8

Slice and len/cap

make([]int, 3)

3 / 3
make([]int, 0, 4)

0 / 4
[]int{1, 2}[1:]

1 / 1
var s []int

0 / 0

Next lesson: Summary - Array and slice

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