Adding expm1 as expression

[trendelkampschroer]

Description

It would be nice to be able to have expm1, cf. https://en.wikipedia.org/wiki/Exponential_function#Computation, available as a polars expression. The inverse log1p is already available as an expression.

I am currently using

 t = (values / 2).tanh()
pl.when(values.abs() < 0.1).then(2.0 * t / (1.0 - t)).otherwise(values.exp() - 1.0)

with an arbitrarily defined threshold of 0.1 to get a 'stable' implementation of expm1 but it's probably sub-optimal, because of the conditional, and a user needs to define a meaningful threshold requiring knowledge of the "region of stability" of the naive exp(x) - 1 implementation.

Would be happy to try implementing this myself, but I have little/zero knowledge of Rust and experience with the complexity/scope of adding this feature to polars - but I am happy to receive guidance and eventually try it out.

Btw, thanks a lot for creating, supporting, and making polars freely available.

[deanm0000]

The numpy function https://numpy.org/doc/stable/reference/generated/numpy.expm1.html#numpy.expm1

Is a ufunc so you can use it directly such as df.with_columns(np.expm1(pl.col('values')))

You can monkey patch a wrapper to Expr namespace so you can use it like other polars methods

pl.Expr.expm1 = lambda col : (
np.expm1(col)
)

Then you could just do

df.with_columns(pl.col('values').expm1())

[trendelkampschroer]

Awesome, thanks a lot @deanm0000.

[trendelkampschroer]

I am re-opening this because of #13557, i.e. in a group_by context a polars.Expression cannot simply be replaced by a numpy 'ufunc'.

[deanm0000]

#14135