[Request] convert DT to xts efficiently

[jangorecki]

Proposed feature request:

As data.table is used in finance I wonder if we could convert to xts by reference?
Assumptions:

• first column contain POSIXct or Date
• because xts is a matrix all other columns needs to be numeric or integer (others would be dropped)
• setDT should also accept xts object to ensure the following is TRUE: setXTS(setDT(xts_obj)) == xts_obj

This may enhance the integration of data.table with already well developed functionalities around xts.

[arunsrinivasan]

xts object is a matrix, isn't it? If so, can't convert a matrix to data.table by reference IIUC. The underlying data structure isn't the same. A data.frame is a vector of column pointers - rows of each column is contiguous in memory, but not the columns themselves. But a matrix is just a vector with attributes - entire vector is contiguous in memory.

[jangorecki]

Thanks. You are right. The matrix is just a dimensioned vector. So by reference cannot be done. Closing as I don't see any alternative for this.

non by-reference wrappers for xts-data.table conversion available here: https://github.com/jangorecki/dwtools/blob/b8981b5658509333378f6325d5b1a0c71bf9da9a/R/xts.R

[mattdowle]

Jan - very nice answer : http://stackoverflow.com/a/27356884/403310
Reopening and changed title to consider including this in data.table as per Dirk's comment?

[jangorecki]

I can prepare PR to data.table.
Just some things to consider:

1. roxygen did not produce expected NAMESPACE entries in my case, currently it produces:

S3method(as.data.table,xts)
export(as.xts.data.table)

I'm not sure but this might be related to lack of as.data.table.default method in data.table (related #969).
2. changing arg name from keep.rownames to keep.index?
3. handling case when the xts object already contain column named index, maybe the simple warning about duplicate colnames in resulting dt?
4. GSee comment from SO:

The as.data.table.xts function converts the index to character and the as.xts.data.table function does not allow for xts objects that are not numeric (e.g. an all character xts)

Didn't check this one yet.
5. prepare xts index by reference. I believe I could use setattr instead rownames on the DF right before as.xts, here.

[gsee]

I suppose, it's really as.data.frame.xts that loses the class of the index when it converts it to rownames. Since there are some choices to be made, simple is probably better -- i.e. leave it up to the user to convert back to a timeBased class.

[jangorecki]

I've address 2-5.
Find recent dev version of code in:

• both side converters: xts.R
• testthat script: test-xts.R
• examples in as_data_table_xts_examples.R, as_xts_data_table_examples.R

It has now the advantage over data.frame because it retains the class of index after conversion.
When completed I will submit PR, to 1.9.7 hopefully :)
I still have problem with point 1, tried few combinations to keep xts in suggests and do not attach it on search path. After compiling in package it needs to by used as:

as.data.table(my_xts)
as.xts.data.table(my_dt)

Seems to not be related to lack of as.data.table.default but suggested package xts.
Info from: http://r-pkgs.had.co.nz/namespace.html

A method for a generic in a suggested package. Namespace directives must refer to available functions, so they can not reference suggested packages.

looks like ti will not be handled nice using xts in suggests.

[jangorecki]

I've pushed it as a new DT branch: https://github.com/jangorecki/data.table/tree/as.xts
Tests are ready (as testthat at the moment) but not yet merged into tests.Rraw.
Am I correct they should be transformed into test(num,x,y) tests?
I will PR it to 1.9.7.

[arunsrinivasan]

@jangorecki looks great! Looks like you've .Rd files now. If they've to be merged to .Rraw, then yes, they have to be converted. I can help with that.

But if you'd like to keep the file as-is, then maybe @lianos can help? He has setup tests for S4-classes using testthat framework.

I'll push a as.data.table.default method. xts should be in suggests and should the examples be wrapped with \dontrun{}?

[lianos]

Yes, can help w/ tests if you want to keep w/ testthat -- I'm not actually sure if these are being run anymore? We can migrate this to the new suggested layout for testthat tests if so ...