How many times does Ruby evaluate a loop's enumerated collection?

In Ruby, if I were to loop over a collection, how many times would Ruby evaluate the enumerated collection?

Specifically, I'd like to sort a collection, and loop over the sorted collection. Since I have no need for keeping a copy of the sorted collection around, I figured I'd just write the loop as:

for item in @items.sort{ |a,b| b.created_at <=> a.created_开发者_如何学运维at } do
    #do some stuff
end

However, after crafting that lovely bit of code I began to wonder how many times I might be actually calling sort.

Would the above line indeed only sort the collection once? Or will Ruby end up sorting it N times for each item in the collection?

You're calling sort once.

Except for scoping differences,

for x in xs do
  some_stuff
end

is the same as

xs.each do |x|
  some_stuff
end

And of course when you do foo.bar(baz), foo is evaluated exactly once, no matter what bar does.

That's equivalent to sorting the whole collection once, and then iterating it once.

Equivalent to:

@items.sort{ |a,b| b.created_at <=> a.created_at }.each do |item|
  # do some stuff
end

Even cleaner and much faster:

@items.sort_by {|a| a.created_at}.reverse

You should pretty much always use sort_by instead of sort if you can (and you almost always can), because it evaluates the sort-key function only once per item. And it lets you write half as much comparison code!