How is each_with_object supposed to work?

Question

I'm trying to figure out how each_with_object is supposed to be used.

I have a sum example that doesn't work:

> (1..3).each_with_object(0) {|i,sum| sum+=i}
=> 0

I would assume that the result would be 6 ! Where is my mistake ?

Answer 1

each_with_object does not work on immutable objects like integer.

(1..3).each_with_object(0) {|i,sum| sum += i} #=> 0

This is because each_with_object iterates over a collection, passing each element and the given object to the block. It does not update the value of object after each iteration and returns the original given object.

It would work with a hash since changing value of a hash key changes it for original object by itself.

(1..3).each_with_object({:sum => 0}) {|i,hsh| hsh[:sum] += i}
#=> {:sum => 6}

String objects are interesting case. They are mutable so you might expect the following to return "abc"

("a".."c").each_with_object("") {|i,str| str += i} # => ""

but it does not. This is because str += "a" returns a new object and the original object stays the same. However if we do

("a".."c").each_with_object("") {|i,str| str << i} # => "abc"

it works because str << "a" modifies the original object.

For more info see ruby docs for each_with_object

For your purpose, use inject

(1..3).inject(0) {|sum,i| sum += i} #=> 6
# or
(1..3).inject(:+) #=> 6

Answer 2

A simple, but common example of using each_with_object is when you need to build a hash depending on elements in an array. Very often you see something like:

hash = {}
[1, 2, 3, 4].each { |number| hash[number] = number**2 }
hash

Using each_with_object avoids the explicit initialization and return of the hash variable.

[1,2,3,4].each_with_object({}) { |number, hash| hash[number] = number**2 }

I advise reading the docs for inject, tap, and each_with_index. These methods are helpful when you aim for short and readable code.

Answer 3

The documentation of Enumerable#each_with_object is very clear :

Iterates the given block for each element with an arbitrary object given, and returns the initially given object.

In your case, (1..3).each_with_object(0) {|i,sum| sum+=i},you are passing 0,which is immutable object. Thus here the intial object to each_with_object method is 0,so method returns 0.It works at it is advertised. See below how each_with_object works ,

(1..3).each_with_object(0) do |e,mem|
    p "#{mem} and #{e} before change"
    mem = mem + e
    p mem
end

# >> "0 and 1 before change"
# >> 1
# >> "0 and 2 before change"
# >> 2
# >> "0 and 3 before change"
# >> 3

That means in every pass,mem is set to initial passed object. You might be thinking of the in first pass mem is 0,then in next pass mem is the result of mem+=e,i.e. mem will be 1.But NO,in every pass mem is your initial object,which is 0.