Tour of Go 2.0

Slice length and capacity

Every slice has two numbers you should know: length and capacity.

  • Length (len(s)): how many elements the slice currently holds.
  • Capacity (cap(s)): how many elements are available in the underlying array, starting from the slice’s first element.

You can grow a slice by re-slicing it — but only up to its capacity. Go beyond that and you’ll get a runtime panic.

Try it: Modify one of the slice operations in the example to exceed its capacity and observe the error.

main.go ▶ Run in Playground 
package main

import "fmt"

func main() {
	s := []int{2, 3, 5, 7, 11, 13}
	printSlice(s)

	// Slice the slice to give it zero length.
	s = s[:0]
	printSlice(s)

	// Extend its length.
	s = s[:4]
	printSlice(s)

	// Drop its first two values.
	s = s[2:]
	printSlice(s)
}

func printSlice(s []int) {
	fmt.Printf("len=%d cap=%d %v\n", len(s), cap(s), s)
}