Matrix slicing in the expression language

[herdrick]

Using the mathjs command line tool:

$ mathjs
> M = [[1, 2], [3, 4]]
M = [[1, 2], [3, 4]]
> M[:, 1]
[[1], [3]]
> 

That is surprising. In other tools, like MATLAB and Python Numpy, that slicing would give you a column,
[1, 3]. Can someone point me to docs about matrix slicing in the expression language?

(Corrections about my use of terminology, or anything else, are very welcome.)

[dvd101x]

Hi, this is a good place to read about slicing.

https://mathjs.org/docs/expressions/syntax.html#matrices
https://mathjs.org/docs/datatypes/matrices.html#getting-or-replacing-subsets
https://mathjs.org/docs/reference/functions/subset.html

Regarding what you are describing, it's something that currently happens only for scalar results.

a = [[1,2],[3,4]]
a[1, 1]    # 1
a[1, :]    # [[1, 2]]

Some relevant discussions:

July 26 2022 #2344

In July 20 2023 at #2994 it was stated that

Maybe a proposal could be that if the index comes from an array, then the result should match the source regardless if it has singleton dimensions.

A = [1, 2; 3, 4]
A[1, 2]          # shall return number 2
A[[1], 2]        # shall return [2]
A[1, [2]]        # shall return [2]
A[[1], [2]]      # shall return [2]

In July 24 2023 at #2994 #2344 it was agreed that

Indexing any dimension with a scalar eliminates that dimension

A[2,3]  --> 6
A[[1,2],[2,3]] --> [2,3; 5,6]
A[[2],[2,3]] --> [[5, 6]]
A[2,[2,3]] --> [5,6]
A[[1,2],[2]] --> [2; 5]  // which is the same as [[2], [5]]
A[[1,2],2] --> [2,5]
A[[2], [2]] --> [[5]]
A[2, [2]] --> [5]
A[[2], 2] --> [5]

I think there is an agreement with what you describe, just not implemented yet.

[gwhitney]

Yes, agree with all of dvd101x's comments, except the agreed-upon conventions to change to are in #2344 (not #2994). Would love to have someone implement those finalized conventions!

[dvd101x]

My bad, I just did an edit to reflect the correct discussion.

[herdrick]

Thanks @dvd101x