Majestic Sea Creature - Latest Comments in http://blog.majesticseacreature.com/archives/2008.08/first_post.html

Re: http://blog.majesticseacreature.com/archives/2008.08/first_post.html

Gregory Brown — Tue, 21 Oct 2008 13:59:00 -0000

I don't think I've ever seen anyone use <<= before, the fact that it works is basically an accident.

It expands out to:

obj = obj << other

Which is very confusing: "Append to this object, then re-assign it"

In this case, if you used that, you'd just be working around the fact that your Hash.new { .. } definition was written incorrectly.

Hash.new { |h,k| h[k] = [] } is idiomatic for these purposes :)

jgn — Tue, 21 Oct 2008 13:48:56 -0000

Oh, right. You'd have to do archives[e.archive] <<= e as well.

Gregory Brown — Tue, 21 Oct 2008 12:42:29 -0000

Unfortunately, no, that won't work.

The code you have shown will return an array, that's true, but it never assigns it to the Hash. So a new array will be built every time:

>> a = Hash.new { [] }
=> {}
>> a[:b]
=> []
>> a[:b] << 1
=> [1]
>> a[:b]
=> []

This is why Hash.new passes the hash itself and the key in question into the block. You need to do something with that to persist any data.

jgn — Tue, 21 Oct 2008 12:00:29 -0000

Hey, Greg, in self.build_archive_indices(entries) wouldn't this be sufficient?

archives = Hash.new { [] }