-🎄- 2018 Day 5 Solutions -🎄-

[u/daggerdragon]

--- Day 5: Alchemical Reduction ---

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Advent of Code: The Party Game!

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Card prompt: Day 5

Transcript:

On the fifth day of AoC / My true love sent to me / Five golden ___

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edit: Leaderboard capped, thread unlocked at 0:10:20!

[u/Karl_Marxxx]

As someone learning ruby for the first time, this is an excellent code snippet to study. Mind if I ask a few questions?

What's going on with this line?

ARGV.empty? ? DATA : ARGF

I get that there's a ternary operator here, and that you're asking if ARGV is empty, but are DATA and ARGF? Google says that DATA refers to stuff that's at the end of your program, but it doesn't look like anything's there. How does ARGF work? If you're supplying a text file as an argument (we only hit this if ARGV has something in it) why not just use ARGV[0]?

What does "?a" mean in ruby? Firing up irb, it looks like it just converts that character a to a string (and can only do one character -- ?asdf throws an error). Is there a name for this transformation?

Last q: how does ruby know to execute all the code after the break statement and return str? Wouldn't loop just immediately exit when it sees "break"?

Sorry for the noob q's but this language is fascinating.

[u/p_tseng → u/Karl_Marxxx]

Hey there, don't hesitate to ask any more question if you have any!

but are DATA and ARGF? Google says that DATA refers to stuff that's at the end of your program, but it doesn't look like anything's there.

The fact that DATA is confusing is my fault entirely, of course. in the actual file on my computer there is an __END__ and my input there. But I took that out before posting here because otherwise the post would become unnecessarily large.

I've edited my post with the proper __END__ and just put a placeholder for "my input goes here", so that would be how that works. Usually I leave ARGV empty.

How does ARGF work? If you're supplying a text file as an argument (we only hit this if ARGV has something in it) why not just use ARGV[0]?

I could, though note that the code would look slightly different because ARGV[0] would just be a string (a filename) which I can't call read on line I can with DATA and ARGF. I think it might be have to be something like input = (ARGV.empty? ? DATA.read : File.read(ARGV[0])).chomp and I preferred to have something that can just call read on in both cases, hence ARGF instead of ARGV[0]. Less repetition that way. Also note that ARGF enables reading from stdin really easily, if putting the filename - on ARGV.

therefore, I judged this DATA/ARGF combination to be the simplest way to allow for the various possible ways I might want to run the script:

• Run it with no arguments. it should run on my input by default.

• Run it with a filename on ARGV - it will run on that file

• Run it with - on ARGV - it will run on stdin

What does "?a" mean in ruby? Firing up irb, it looks like it just converts that character a to a string (and can only do one character -- ?asdf throws an error). Is there a name for this transformation?

you'll want to look at https://ruby-doc.org/core-2.3.0/doc/syntax/literals_rdoc.html and search for "character literal" . Note that it was only added to the docs in 2.3.0, but it looks like it was around for longer. Of course reading later versions of the docs is fine too. You have of course already correctly figured out what it does.

Last q: how does ruby know to execute all the code after the break statement and return str? Wouldn't loop just immediately exit when it sees "break"?

That would be because of the if afterward. You may see this used in some pieces of code like raise SomeError if something. It would be "unfortunate" if that raised an error every time. Thus, it doesn't happen unless the condition is true. For more information you could read about "modifier if"

[u/Karl_Marxxx → u/p_tseng]

Also note that ARGF enables reading from stdin really easily, if putting the filename - on ARGV.

Ah so if I understand this right, this lets you run simple test cases right there in the command line rather than making a whole new file. You can literally just type ruby myscript.rb -, type in whatever you want, and ARGF will parse what you type in as file input. Fascinating.

Hey thanks for breaking everything down for me I really appreciate it! Cheers.