JuniorTheoryCommon

What does the program print?

package main

import "fmt"

func main() {
    a := []int{1, 2, 3, 4}
    b := a[1:3]
    b[0] = 9
    fmt.Println(a, len(b))
}

[1 9 3 4] 2

Expecting a to stay [1 2 3 4], as if a[1:3] made a copy: a sub-slice shares the array with a, so b[0] = 9 writes into a[1].
Expecting len(b) to be 3: in a[1:3] the end index is excluded, so b holds the elements at indexes 1 and 2.