in Ruby, why I need to define a hash, but don't need to define variables?

I mean, I don't declare

my_var = new variable

o开发者_如何学运维r something like that. I just go with

my_var = 1;

Similarly, why can't I just

books["War and peace"] = :masterpiece

Why I need to define in advance?

books = {}

books["War and peace"] = :masterpiece calls the []= method on books with "War and peace" and :masterpiece as its arguments. If books doesn't exist, you can't call a method on it.

Or to approach the question a different way: If ruby did do some magic to automatically initialize variables, when you use []= on them, how should ruby know that you want books to be a hash in the above example? Any class can have [] and []= operators which accept strings as an index.

Doing:

my_var = 1

Is defining and assigning the local variable. With a hash it could be done like this:

books = { 'War and Peace' => :masterpiece }