Cannot use map function inside evaluate

[WilcovS]

I'm currently evaluating various functions using evaluate, but I can't figure out how to use functions that accept functions as their arguments.

Example:
math.evaluate('map([1, 2, 3], x => x+x)') // expect [2,4,6]
But instead, I get:

Value expected (char 19)

Any mathjs function that accepts a callback or function as an argument seems to not work, but all the functions that don't, do.
this works for example:
math.evaluate('add(1,2,3)') // returns 6

Am I missing something, or is this a limitation of the library?

[dvd101x]

Hi,

The syntax for arrow functions in javascript is not the syntax for functions in the parser.

This is not a function x => x+x, instead try f(x) = x+x personally if I wont be using f later I tend to use _(x) = x+x that will make _ into a function but that reminds me I wont be using that later.

You could try the following and get the expected result.

math.evaluate('map([1, 2, 3], _(x) = x+x)')

Here is a reference for defining functions.
https://mathjs.org/docs/expressions/syntax.html#differences-from-javascript

[WilcovS]

Thank you! I had missed that page. Perhaps it's a good idea to link to it from the evaluate doc?
https://mathjs.org/docs/reference/functions/evaluate.html

[josdejong]

Good idea. Addressed via 61c5d07

[dvd101x]

Somehow this also works.

math.evaluate('map([1, 2, 3], x+x)')

Jos, I recall at some point you mentioned this was undocumented. If it's still undocumented, what is a good way to document it?

In some curious cases this notation causes errors.

math.evaluate('map([1, 2, 3], val + idx)') // error
math.evaluate('map([1, 2, 3], x)')         // error

[josdejong]

mathjs tries to detect a single undefined variable and assumes that will be the variable to be used to iterate over the matrix values. In your example math.evaluate('map([1, 2, 3], val + idx)') there are two undefined variables. The first can be used to iterate over, but the second remains undefined (when evaluating this, you'll get an error that gives a clue: "Undefined symbol idx"). Does that make sense?

This is handled by the compileInlineExpression function here: https://github.com/josdejong/mathjs/blob/develop/src/expression/transform/map.transform.js#L29-L30

I did look around in the documentation but indeed I think the documentation is missing. It would be neat to add a section about map and forEach on the Syntax page. Is that something you would like to pick up @dvd101x ? (if not I'll do it).

[dvd101x]

Hi @josdejong, I will do it.