Initializing an empty array of struct type in golang

Question

I have initialized a struct :

type DayAndTime struct {
    days string
    time string
  }

I have initialized an empty set of array of this DayAndTime type:

day := []DayAndTime{}

And put a value in it: day[0] = DayAndTime{"Monday", "8.00 PM"}

But it shows a runtime error:

panic: runtime error: invalid memory address or nil pointer dereference

Why is this happenning and what could be a possible solution?

edit: It's actually a slice not an array.

Answer 1

Here you have a zero-length slice, the len and cap functions will both return 0 for a zero-valued slice

A slice cannot be grown beyond its capacity. Attempting to do so will cause a runtime panic, just as when indexing outside the bounds of a slice or array. Similarly, slices cannot be re-sliced below zero to access earlier elements in the array.

You may us make to initialize the slice with capacity and assign with index or use append to add values to slice

Both are valid code

var day []DayAndTime
day = append(day, DayAndTime{"Monday", "8.00 PM"})

or

var day = make([]DayAndTime, 1)
day[0] = DayAndTime{"Monday", "8.00 PM"}

Using append is recomended

Here is a sample code justifying/explaining the answer https://play.golang.org/p/ajsli-6Vqw

package main

import (
    "fmt"
)

type DayAndTime struct {
    days string
    time string
}

func ZeroLength() {
    var day = []DayAndTime{}
    fmt.Println("Hello, playground", cap(day), len(day), day)
}

func AppendArray() {
    var day = []DayAndTime{}
    day = append(day, DayAndTime{"Monday", "8.00 PM"})
    fmt.Println("Hello, playground", cap(day), len(day), day)
}

func SetIndex() {
    var day = make([]DayAndTime, 1)
    day[0] = DayAndTime{"Monday", "8.00 PM"}
    fmt.Println("Hello, playground", cap(day), len(day), day)
}

func main() {
    ZeroLength()
    AppendArray()
    SetIndex()
}