Fill a List with a for-loop

问题

Why can't I fill a list with this simple for-loop?

new_data = []
for data <- old_data do
  new_data = List.insert_at(new_data, -1, data)
end

After this operation my new_data list is still empty, even though the loop is N times executed.

回答1:

In Elixir, you can't mutate the value your variable is referencing as explained in Are Elixir variables really immutable?. For in this instance is not a "loop" it is a list comprehension.

You can assign to the result of a comprehension with:

new_data = for data <- old_data do
  data
end

In your line:

new_data = List.insert_at(new_data, -1, data)

The new_data variable is local to the scope of the comprehension. You can use your previous new_data value, but you won't be able to rebind for the outside scope. Which is why new_data is still [] after your comprehension. The scoping rules are explained in http://elixir-lang.readthedocs.org/en/latest/technical/scoping.html

来源：https://stackoverflow.com/questions/33830686/fill-a-list-with-a-for-loop