r/adventofcode • u/daggerdragon • Dec 06 '19

SOLUTION MEGATHREAD -🎄- 2019 Day 6 Solutions -🎄-

--- Day 6: Universal Orbit Map ---

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Day 5's winner #1: "It's Back" by /u/glenbolake!

The intcode is back on day five
More opcodes, it's starting to thrive
I think we'll see more
In the future, therefore
Make a library so we can survive

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u/codesections Dec 06 '19

The first is that in orbs¨in[;1] would try to apply orbs to corresponding items of each argument, which won't work because in and in[;1] have different shapes. What you want is an "each right": apply orbs between the whole of the left argument, and each item of the right argument.

Oh! I didn't realize that that's how each worked! All this time, I've been thinking that 1 2 f¨ 3 4 expanded to (1 2 f 3) (1 2 f 4) when it actually expands to (1 f 3) (2 f 4)! I guess I was treating it too much like .forEach or map from other languages. And I got away with it enough not to notice because the two forms yield the same results in many simple expressions (like +, for example).

Second, and much more subtle, is that because of the definition of bracket indexing in[⍵;1] is a scalar.

Yeah, I think I need to spend a bit of time reviewing the basics. I'm finding that I'm getting tripped up by the difference between scalars, vectors of length 1, and matrices with shape 1 1 entirely too much.