What does << mean in Ruby?
Solution 1
It can have 3 distinct meanings:
'<<' as an ordinary method
In most cases '<<' is a method defined like the rest of them, in your case it means "add to the end of this array" (see also here).
That's in your particular case, but there are also a lot of other occasions where you'll encounter the "<<" method. I won't call it 'operator' since it's really a method that is defined on some object that can be overridden by you or implemented for your own objects. Other cases of '<<'
- String concatenation: "a" << "b"
- Writing output to an IO: io << "A line of text\n"
- Writing data to a message digest, HMAC or cipher: sha << "Text to be hashed"
- left-shifting of an OpenSSL::BN: bn << 2
- ...
Singleton class definition
Then there is the mysterious shift of the current scope (=change of self) within the program flow:
class A
class << self
puts self # self is the singleton class of A
end
end
a = A.new
class << a
puts self # now it's the singleton class of object a
end
The mystery class << self
made me wonder and investigate about the internals there. Whereas in all the examples I mentioned <<
is really a method defined in a class, i.e.
obj << stuff
is equivalent to
obj.<<(stuff)
the class << self
(or any object in place of self) construct is truly different. It is really a builtin feature of the language itself, in CRuby it's defined in parse.y as
k_class tLSHFT expr
k_class
is the 'class' keyword, where tLSHFT
is a '<<' token and expr
is an arbitrary expression. That is, you can actually write
class << <any expression>
and will get shifted into the singleton class of the result of the expression. The tLSHFT
sequence will be parsed as a 'NODE_SCLASS' expression, which is called a Singleton Class definition (cf. node.c)
case NODE_SCLASS:
ANN("singleton class definition");
ANN("format: class << [nd_recv]; [nd_body]; end");
ANN("example: class << obj; ..; end");
F_NODE(nd_recv, "receiver");
LAST_NODE;
F_NODE(nd_body, "singleton class definition");
break;
Here Documents
Here Documents use '<<' in a way that is again totally different. You can define a string that spans over multiple lines conveniently by declaring
here_doc = <<_EOS_
The quick brown fox jumps over the lazy dog.
...
_EOS_
To distinguish the 'here doc operator' an arbitrary String delimiter has to immediately follow the '<<'. Everything inbetween that initial delimiter and the second occurrence of that same delimiter will be part of the final string. It is also possible to use '<<-', the difference is that using the latter will ignore any leading or trailing whitespace.
Solution 2
Mostly used in arrays to append the value to the end of the array.
a = ["orange"]
a << "apple"
puts a
gives this
["orange", "apple"]result.
Solution 3
'a << b' means append b to the end of a
Solution 4
In ruby '<<' operator is basically used for:
Appending a value in the array (at last position)
[2, 4, 6] << 8 It will give [2, 4, 6, 8]
It also used for some active record operations in ruby. For example we have a Cart and LineItem model associated as cart has_many line_items. Cart.find(A).line_items will return ActiveRecord::Associations object with line items that belongs to cart 'A'.
Now, to add (or say to associate) another line_item (X) to cart (A),
Cart.find(A).line_items << LineItem.find(X)
Now to add another LineItem to the same cart 'A', but this time we will not going to create any line_item object (I mean will not create activerecord object manually)
Cart.find(A).line_items << LineItem.new
In above code << will save object and append it to left side active record association array.
And many others which are already covered in above answers.
Solution 5
It's the operator which allows you to feed existing arrays, by appending new items.
In the example above you are just populating the empty array threads
with 5 new threads.
Comments
-
Vyacheslav Loginov over 2 years
I have code:
def make_all_thumbs(source) sizes = ['1000','1100','1200','800','600'] threads = [] sizes.each do |s| threads << Thread.new(s) { create_thumbnail(source+'.png', source+'-'+s+'.png', s) } end end
what does
<<
mean? -
Zabba over 12 yearsOne more use of
<<
is in class inheritance -
Kelvin over 11 yearsVery thorough answer. Might confuse beginners, but as a wise man (might've) said, "ask a broad question, get a thorough answer".
-
BenKoshy almost 8 yearsHi thank you @emboss: so << is the equivalent of calling the push method on an array?
-
Ammar Shah over 5 yearsNot everybody knows this behavior of <<. Thanks for mentioning it.
-
psychoslave over 2 yearsThis is one of the most thorough but still focus on topic answer I red on stackoverflow I think. I especially appreciated how you gave the information of what the esoteric identifiers meant. This culture of cryptic names widely spread in our industry is just throwing more hindrances at topics already hard enough to grasp by themselves. 🤔