Create Equivalent Methods for Existing Datatable Functions

[samukweku]

This is a follow-up from the discussion here on adding methods to FExpr.

The method chaining format would look like this DT[:, f[:].rowsum().sum()], which would be an equivalent for DT[:, dt.rowsum(dt.sum(f[:]))]

Methods:

• [ ] time methods
• [ ] str methods
• [ ] math methods
• [ ] cut
• [ ] qcut
• [ ] cov
• [ ] corr

[samukweku]

@st-pasha , @oleksiyskononenko If you would be kind to show me how to pull this off for one of the existing functions (e.g sum). I tried using the same idea as for len, since I was not changing any C++ code, but it did not reflect when I restarted the python console

[st-pasha]

Hi @samukweku -- please see PR #2949 which implements the sum function. Most of it is self-explanatory: tests, documentation, etc. Most of the changes to C++ are in lines 331-345 of "fexpr.cc".

[samukweku]

@st-pasha Thank you very much! I'll go through them tonight. Again, thank you!!

[oleksiyskononenko]

@samukweku Do you want to create a list of methods in this issue that we need to implement for the existing dt functions? So that we keep track as to what is implemented and what is left.

[samukweku]

@oleksiyskononenko we also need to make method chaining work on dt.expr as well; at the moment it does not work on dt.expr objects