How `cap` built-in differs from `len` one in Golang.

cap_vs_len.go

package main

import "fmt"

func main() {
	/*
		array address is 0xc000016420.
		array values are [5]int{1, 2, 3, 4, 5}.
		array type is [5]int.
		array length is 5.
		array capacity is 5.

		s address is 0xc00000c060.
		s values are []int{1, 2}.
		s type is []int.
		s length is 2.
		s capacity is 5.
	*/
	array := [5]int{1, 2, 3, 4, 5}
	s := array[0:2]

	fmt.Printf("array address is %p.\n", &array)
	fmt.Printf("array values are %#v.\n", array)
	fmt.Printf("array type is %T.\n", array)
	fmt.Printf("array length is %d.\n", len(array))
	fmt.Printf("array capacity is %d.\n", cap(array))
	fmt.Println()
	fmt.Printf("s address is %p.\n", &s)
	fmt.Printf("s values are %#v.\n", s)
	fmt.Printf("s type is %T.\n", s)
	fmt.Printf("s length is %d.\n", len(s))
	fmt.Printf("s capacity is %d.\n", cap(s))

}

package main

import "fmt"

func main() {
	/*
		[]
		[0 0 0 0 0]
		[0 0]
		Length is 2
		Capacity is 3
	*/
	a := make([]int, 0, 5)
	fmt.Println(a)

	a = a[:cap(a)]
	fmt.Println(a)

	a = a[2:4]
	fmt.Println(a)
	fmt.Printf("Length is %d\n", len(a))
	fmt.Printf("Capacity is %d\n", cap(a))
}