Extend diff() to arbitrary dimensions

[juliohm]

I recently saw a post by Tim discussing the new CartesianIndex type. Does it help to write a more general diff() that works with N-dimensional arrays? Currently it only supports matrices (i.e. 2D arrays).

[juliohm]

At least have it working in 3 dimensions would be great.

[timholy]

Sorry I didn't see this. Yes, absolutely. You could almost copy the final example verbatim and then tweak 1 or 2 lines.

[juliohm]

Hi Tim! Could you solve this issue? I bookmarked your post and will read it again, it is really useful for the kinds of algorithms I been writing recently.

[tkelman]

Tim's supposed to be on vacation :)

[juliohm]

Oh, I didn't knew that! Enjoy your vacation Tim!

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-Júlio

[timholy]

Thanks, I am enjoying my vacation.

I may well get around to implementing this at some point, but the teacher in me can't resist pointing out that by asking someone else to do this for you, you're throwing away a fantastic learning opportunity. Contributing code to julia or its packages---while it might seem like extra work---is basically how you "buy" a ticket to getting free tutoring in how to write efficient code. If you really want to learn how to use the material in that blog post independently, you couldn't ask for a better way to learn it.

[juliohm]

Totally agree Tim! I'll take a look into it immediately after I finish some coding for my research. :) Thanks again for the blog post, amazing tips as usual.

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-Júlio

[juliohm]

Didn't have the time to look into it, anyone feel free to fix this issue!

[sg1101]

Hi @juliohm. I would like to try and solve this issue. I am new to open source and had never contributed till now. Can you please guide how to proceed. Thank You :)

[StefanKarpinski]

Get an implementation working and make a PR after reading through CONTRIBUTING.

[juliohm]

@sg1101 have you had the chance to work on this?

[sg1101]

No Julio, I didn't work on it.

[felixrehren]

@juliohm I gave it a first pass a little while ago, see
https://gist.github.com/felixrehren/21b300061ccf9bf3fa5396889c799b48
The file contains code (accum and decum) to calculate multidimensional summed area tables accum(+,A) and recover, from a summed area table, the original array: decum(+,accum(+,A)) == A. It works for any dimension and also works for multiplication * or probably any symmetric binary operation.

It has problems -- seems to spend a lot of time inferring, probably not best practice on writing array algorithms, and I wasn't sure about the basic design. (In particular, implementing summed area tables requires an inverse of +, and to keep it general I wrote a function inverse(::typeof(+),x) = -x. I like this, but am not sure if it is considered Julian) Also, I wasn't sure about changes due to 0.7 coming up