Diff function

[Veeloxfire]

Add function that returns difference between adjacent members of an array or matrix

Currently no way to decide which axis for multi-dimensional arrays but I could work on that

[Veeloxfire]

That included a some old commits sorry

[Veeloxfire]

#1634 and I forgot to link it

[josdejong]

Nice, thanks for your work @Veeloxfire .

I see the function diff currently supports 1-d matrices only. That is a nice and useful first version. I think we could go two ways:

1. Built a check in to throw an error when the input is not a 1-d matrix.
2. Go for the next step already: allow 2-d matrices too, and then allow selecting an axis along which to calculate the diff. This version would also require a check for the number of dimensions I think.

What would you like to do? Round it up or go for the next step right away?

[Veeloxfire]

Ill start trying to get it to work for multi-dimensional so it more mirrors the numpy version as that was the original suggestion

[josdejong]

Sounds good 👍 . It will also be consistent with mathjs functions like sum, max, min, etc.

[Veeloxfire]

Finally got around to doing this. The only thing its doesnt have that the numpy version does is the ability to say how many times to find the difference. But python has the ability to select parameters with the function(param=num) feature so it makes more sense to have optional parameters. I think to include that here would just be confusing and lead to hard to diagnose errors

[josdejong]

Cool! I'll do some testing this weekend.

[josdejong]

Maybe this example should be a 2d array so you can actually see what the dimension does, like

'diff([[1, 2], [3, 6]], 0)',
'diff([[1, 2], [3, 6]], 1)',

[Veeloxfire]

Agreed, I think I did this because it was just a copy and paste but yes should definitely have more comprehensive examples

[josdejong]

I think here the dimension should be specified, like math.diff(arr, 1)?

[josdejong]

Looks very good @Veeloxfire 👍

I made two small remarks in the code, and here three more remarks:

1. Can you add some unit tests testing with BigNumber, Fraction, Unit? I tested the following it and it all works:

   assert.deepStrictEqual(math.diff([[1, 2], [3, 6]]), [[2, 4]])
   assert.deepStrictEqual(math.diff([[1, 2], [3, 6]], 0), [[2, 4]])
   assert.deepStrictEqual(math.diff([[1, 2], [3, 6]], 1), [[1], [3]])
   
   assert.deepStrictEqual(math.diff(math.bignumber([[1, 2], [3, 6]])), math.bignumber([[2, 4]]))
   assert.deepStrictEqual(math.diff(math.bignumber([[1, 2], [3, 6]]), 0), math.bignumber([[2, 4]]))
   assert.deepStrictEqual(math.diff(math.bignumber([[1, 2], [3, 6]]), 1), math.bignumber([[1], [3]]))
   
   assert.deepStrictEqual(math.diff(math.fraction([[1, 2], [3, 6]])), math.fraction([[2, 4]]))
   assert.deepStrictEqual(math.diff(math.fraction([[1, 2], [3, 6]]), 0), math.fraction([[2, 4]]))
   assert.deepStrictEqual(math.diff(math.fraction([[1, 2], [3, 6]]), 1), math.fraction([[1], [3]]))
   
   assert.deepStrictEqual(math.diff([math.unit('2 cm'), math.unit('8 cm')]), [math.unit('6 cm')])

2. Can you add a unit test testing that an error is thrown when you pass and invalid dimension (like 999) or invalid input argument (like not passing an array but a value or something)

3. The expression parser of mathjs is 1-based instead of 0-based, see docs. This means that we have to write a transform function for diff so that when using it inside the expression parser, you can pass 1-based dimensions instead of 0-based. Functions like sum also have such a transform. See:

   https://github.com/josdejong/mathjs/tree/develop/src/expression/transform
   https://github.com/josdejong/mathjs/blob/develop/src/expression/transform/sum.transform.js

   If you're not sure what/how to do this I can implement the transform function for diff and a unit test for it, no problem at all.

[josdejong]

@Veeloxfire would be great if you can have a look at my feedback

[Veeloxfire]

Sorry I didnt see this! Yep Ill look in to it

[Veeloxfire]

I will have to make another pull request to update this because I decided to reclone the repository to refresh my offline one but I stupidly deleted the old one

Ill make sure to reference this one when I do

[josdejong]

👍 thanks

[Veeloxfire]

I'll close this pull request now as its moved to #1920

[josdejong]

👍 I'll look into your new PR soon