Why am I getting objects printed twice?

问题

When I am in irb or in rails and I create some iteration with each, I am getting the whole struct printed again in my terminal or inside the browser. Example:

a = [1,2,3,4]

a.each do |number|
  puts n
end

The result in irb terminal or inside the browser:

1
2
3
4

=> [1,2,3,4]

Why does this => [1,2,3,4] appear inside the browser? I can't create a single list in my page because the whole structure appears.

回答1:

Every expression in Ruby returns a value; in irb, the value returned by the expression you've just executed is displayed after =>.

The return value of Enumerable::each is the object that called each - in this case, the array [1,2,3,4]

回答2:

You see them printed twice: once as side effect, second time as the return value. irb always puts the last return value. The return value for each is its receiver. You cannot avoid that when using irb, but they will not show up when you run the script as standalone software. The First 1, ..., 4 are the outputs of your puts. They are called side effects.

回答3:

Because ruby returns in everything even blocks. So your each statement is returning. You're seeing it output because irb shows you the return of everything. In a script it would only output once.

回答4:

Run it outside irb, and you shall be enlightened.

ruby -e `[1,2,3,4].each {|ele| puts ele}`

来源：https://stackoverflow.com/questions/14363330/why-am-i-getting-objects-printed-twice