Why does one of my two slices panic with "runtime error: index out of range"?

Question

func main() {
        str1 := make([]string, 10)
        str2 := []string{}
        fmt.Println(str1[0]) *No error*
        fmt.Println(str2[0]) *error*
    }

Why does fmt.Println(str2[0]) show an error in Go?

Answer 1

The Go Programming Language Specification

Index expressions

A primary expression of the form

a[x]

denotes the element of the array, pointer to array, slice, or string a indexed by x. The value x is called the index,

the index x is in range if 0 <= x < len(a), otherwise it is out of range

---

[]string{} is the same as make([]string, 0) Therefore, 0 >= len(str2) and str2[0] is out of range..

package main

import (
    "fmt"
)

func main() {
    str1 := make([]string, 10)
    fmt.Println(len(str1), cap(str1), str1)
    str2 := []string{}
    fmt.Println(len(str2), cap(str2), str2)
    fmt.Println(str1[0]) // *No error*
    fmt.Println(str2[0]) // *error*
}

Playground: https://play.golang.org/p/p31fUyb4pqW

Output:

10 10 [         ]
0 0 []

panic: runtime error: index out of range [0] with length 0