How do you append data to array struct with * pointer

i'm learning the basics of golang, and i stumbled upon something that i may lack understanding about it, this code works fine:

package main

import (
	"fmt"
)

type Category struct {
	Id   int
	Name string
}

type Book struct {
	Id         int
	Name       string
	Categories []Category
}

func main() {

	var book Book

	book.Id = 1
	book.Name = "Vanaraj"

	for i := 0; i < 10; i++ {
		book.Categories = append(book.Categories, Category{
			Id:   10,
			Name: "Vanaraj",
		})
	}

	fmt.Println(book)

}

Output:
{1 Vanaraj [{10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj}]}

But adding * pointer on []Category break the program

package main

import (
	"fmt"
)

type Category struct {
	Id   int
	Name string
}

type Book struct {
	Id         int
	Name       string
	Categories *[]Category
}

func main() {

	var book Book

	book.Id = 1
	book.Name = "Vanaraj"

	for i := 0; i < 10; i++ {
		book.Categories = append(book.Categories, Category{
			Id:   10,
			Name: "Vanaraj",
		})
	}

	fmt.Println(book)

}

Output:
./prog.go:26:27: first argument to append must be slice; have *[]Category

Now let's say i can't remove the * from []Category on the second block of code (Case example is importing a package), how can i still append data like the first block of the code with only adding codes to my main() function? <br>
Thanks in advance

It's the same as using an int value or an *int pointer.

var p *int

You have to initialize it first, e.g. with using the builtin new():

p = new(int)

You assign to the pointed value:

*p = 3
fmt.Println(*p)

The same goes with pointer slices: you first have to initialize it, e.g. with using the builtin new():

book.Categories = new([]Category)

And you dereference the pointer using *:

*book.Categories = append(*book.Categories, Category{
	Id:   10,
	Name: "Vanaraj",
})

Try it on the Go Playground.

Please note that slices are slice headers which already contain a pointer to a backing array. So using pointers to slices is very rare, usually unneeded and just complicates things.

func main() {

var book Book

book.Id = 1
book.Name = "Vanaraj"
book.Categories = new([]Category)

for i := 0; i < 10; i++ {
    *book.Categories = append(*book.Categories, Category{
        Id:   10,
        Name: "Vanaraj",
    })
}

fmt.Println(book)}

Ouput : {1 Vanaraj 0xc00000c030}

And to print categories you can use fmt.Println(book.Categories)