What are hashes in Ruby?

Question

This code is in my book, there are 2 lines that I don't understand.

# library.rb

class Library
   def initialize
       @books ={}  #creating a new hash
   end
   def addBook(book) 
       @books[book.category]||=[]          #category is a symbol. I DONT UNDERSTAND THIS LINE
       @books[book.category] << book       #AND THIS ONE
    end
end

What do you mean by _category is a symbol_ ? It is a public property of the `book` object passed as a parameter to `addBook`. — Michael Berkowski, Mar 20 '15 at 15:13
Note there's a syntax error in the line where you append `book` to the array in `book.category`'s hash entry. — Dave Newton, Mar 20 '15 at 15:21

Sonalkumar sute · Accepted Answer · 2015-03-20T15:32:34.147

2

@books ={}

Above line will create a new Hash

@books[book.category]||=[]

This means @books is a Hash and book.category is it's key and if that key not exist assign an empty array

||= -> So this means or-equals

|| means if @books has value it will not assign empty array, else it will put an empty array

So if @books[book.category] is a Array, in which you can insert as many category values

In this line we will insert book value into the hash, where book.category is the key

   @books[book.category] << book

If you try this

@books ={}
@books[book.category].push(book) # This will give you the error `undefined method 'push' for nilclass`

because

@books[book.category].class will return NilClass

edited Mar 20 '15 at 15:32

answered Mar 20 '15 at 15:13

Sonalkumar sute

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Great answer, but what exactly this: ||= means? – TheKilz Mar 20 '15 at 15:15
@TheKilz "or-equals", e.g., if there's nothing in that hash entry yet, put the empty array into it. – Dave Newton Mar 20 '15 at 15:22
1

@TheKilz, x ||= [] is a shorthand of x = x || [] – Tien Nguyen Mar 20 '15 at 15:23
`a ||= b` It means "if a is false, nil or undefined, then evaluate b and set a to the result" – DinhNgocHien Mar 20 '15 at 15:24
It's also called the "lazy initializer", that is it's often used to initialize something only if it hasn't been done before. In other words "`x` or assign *this* to `x` if that hasn't happened yet". – tadman Mar 20 '15 at 15:26
1

@TienNguyen I think x ||= [] is equal to x || x = [] – DinhNgocHien Mar 20 '15 at 15:27
@DinhNgocHien, it's syntax sugar like +=, -=, ... that is common in many languages – Tien Nguyen Mar 20 '15 at 15:29
2

@DinhNgocHien Strictly speaking, it's actually neither. http://stackoverflow.com/a/27628662/1157054 – Ajedi32 Mar 20 '15 at 15:31
@TienNguyen `x ||= []` being a shorthand of `x = x || []` is a misconception: http://www.rubyinside.com/what-rubys-double-pipe-or-equals-really-does-5488.html – sebkkom Mar 20 '15 at 15:37
@Ajedi32 yes, it is indeed my mistake. *confused* – Tien Nguyen Mar 20 '15 at 15:39

score 0 · Answer 2 · answered Mar 20 '15 at 15:31

a ||= b means "if a is logically false (nil, false, undefined), assign b to it". You can read more about ||=, called "double pipe or equals" here.

So, in your code, @books[book.category] ||= [] means that an empty array ([]) will be assigned to books[book.category] if it's nil - otherwise it will remain intact.

Then, in the line below (@books[book.category] << book), book is appended (<<) to the array. Notice, however, that you have a syntax error (you open a { and you close with a [).

What are hashes in Ruby?

2 Answers2