map with multiple input matrices / arrays

[dvd101x]

Hi,

Currently map uses a single input matrix or array and a callback function with a single input

const square = math.square
const x = [1, 2, 3]
math.map(x, square) // [1, 4, 9]

What if it could work with multiple inputs?

const sumOfSquares = (a, b) => a**2 + b**2
const x = [1, 2, 3]
const y = [4, 5, 6]
math.map(x, y, sumOfSquares) // [17, 29, 45]

And include broadcasting

const sumOfSquares = (a, b) => a**2 + b**2
const x = [1, 2]
const y = [[3], [4]]
math.map(x, y, sumOfSquares) // [[10, 13], [17, 20]]

I would like to discuss the idea of map using multiple broadcasted inputs.

[josdejong]

Nice idea, I like it 👍

[dvd101x]

I'm glad you like it.

Is it ok if I attempt a PR for this new functionality?

[josdejong]

Yes, that sounds good!

[dvd101x]

Hi Jos,

I started to look into the arguments of the callback function.

the callback is invoked with three arguments: the value of the entry, the index at which that entry occurs, and the full matrix/array being traversed

But with N matrices or arrays being mapped, it would be like so.

N	callback
1	callback(value, index, array)
2	callback(valueA, valueB, index, broadcastedArrayA, broadcastedArrayB)
3	callback(valueA, valueB, valueC, index, broadcastedArrayA, broadcastedArrayB, broadcastedArrayC)

With this change it would be something like

the callback is invoked with 2N+1 arguments: the N values of the entry, the index at which that entry occurs, and the N full broadcasted matrix/array being traversed where N is the number of matrices being mapped.

I would like to validate in the meantime if this form makes the most sense.

[josdejong]

Your proposed API makes sense indeed. It looks a bit unwieldy, but in practice, the broadcastedArrayA, broadcastedArrayB, broadcastedArrayC part is not often needed, and this very order makes most sense since it is consistent with the case of 1 matrix. In typical usage, it will be callback(valueA, valueB, valueC) and sometimes callback(valueA, valueB, valueC, index), which is easy to follow.

[lulunac27a]

It's based on Array.prototype.map function in JavaScript

[dvd101x]

Thank you for the reference.

[josdejong]

This is implemented now in v13.1.0, see #3228.