feat: Added diff() function to DataFrame

[NeonSpork]

Added a diff() function to mimic the functionality of the equivalent function in pandas as best as possible.

Can calculate difference between rows and columns both forwards and backwards, in addition to difference in relation to another dataframe or series.

Added the function to the DataFrame interface.

Added unit testing with as much coverage as I could. The error that is thrown if the axis is set as something other than 0 or 1 follows the convension from a similar function, but as far as I can tell TypeScript won't let you set axis to anything besides 0 or 1, so it is untestable.

The implementation is made using the built in tensorflow functionality for pretty much everything except nudging the data to compare, and I believe this is one of the faster ways to achieve this!

[NeonSpork]

Resolves #383 partially, there's no pct_change as of yet, but figuring out how to perform the calculations effectively and manipulate the dataframes took the most amount of time. I will try to implement a similar function for pct_change() as well 😃

[risenW]

Great work so far @NeonSpork ! and thanks for taking out time to contribute. I dropped some general suggestions, if you need any input from me, don't hesitate to ping me.

[NeonSpork]

Another comment. I have implemented the pct_change() as well, but am struggling with testing.

My expected return is for example 0.8 but JavaScript thinks it should be 0.80000000000012345 to provide a random example. Is there an easy way around this? @risenW

Edit: Since it breaks testing I'm doing pct_change() in a separate branch. I can make a separate pull request for that in a bit, so you can see more hands on!

[risenW]

Another comment. I have implemented the pct_change() as well, but am struggling with testing.

My expected return is for example 0.8 but JavaScript thinks it should be 0.80000000000012345 to provide a random example. Is there an easy way around this? @risenW

Edit: Since it breaks testing I'm doing pct_change() in a separate branch. I can make a separate pull request for that in a bit, so you can see more hands on!

Yeah, I have had that issue with float values as well. You can try to truncate/floor the value.

And yes, doing pct_change in a separate PR is better.

[NeonSpork]

We can cross that bridge in another PR! 👍 I'm trying to refactor and remove some duplication, should hopefully not take too long!

[NeonSpork]

Alright! I think I've resolved the issues you pointed out @risenW ?

The code has become more succinct with less duplication, and tests are passing!

[risenW]

@NeonSpork Everything looks good here. One last thing before I merge, can you also add test for the browser version. Just copy over what you currently have to the browser test folder, and then use the namespace dfd.DataFrame or dfd.Series to create Dataframe and Series. You can see examples in the danfojs-browser/tests/ folder

[NeonSpork]

Sounds great! I forgot about the browser tests, oops! Was just happy to see green checks 👍

I'll sort that out in both my PRs in a short bit!

[risenW]

LGTM ✅✅ Thanks for the awesome work @NeonSpork

[NeonSpork]

LGTM ✅✅ Thanks for the awesome work @NeonSpork

My pleasure man! I'm looking forward to contributing more in the future 🚀

[Coelmount]

@NeonSpork @risenW - this feature is exactly what I am searching for, however, I cannot find it in the docs. Could you maybe point me to the documentation?

[NeonSpork]

@NeonSpork @risenW - this feature is exactly what I am searching for, however, I cannot find it in the docs. Could you maybe point me to the documentation?

Hi! Glad you've found my contribution useful! When I wrote that contribution I also added pages to the docs, but for some reason they're not on the official documentation web page. Not sure why that is the case.

Either way the docs pages I added are still in the docs repo, and the examples and explanations for the diff() function can be found here:
https://github.com/javascriptdata/danfojs-doc/blob/master/api-reference%2Fdataframe%2Fdanfo.dataframe.diff.md

Maybe @risenW has some insight into why it's not on the main docs? Maybe the main index page doesn't include links to them?