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Printing of an expression type column does not work when the expression wraps to a new line #3011

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RichardRedding opened this issue Aug 24, 2018 · 6 comments
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Printing of an expression type column does not work when the expression wraps to a new line #3011

RichardRedding opened this issue Aug 24, 2018 · 6 comments
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non-atomic column e.g. list columns, S4 vector columns

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@RichardRedding
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RichardRedding commented Aug 24, 2018 •
edited by MichaelChirico
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Printing of an expression type column does not work when the expression wraps to a new line

Works:

data.table(e = expression(1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12))

Doesn't:

data.table(e = expression(1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13))

At least on my computer, outside of a data.table, the first expression would print all on one line and the second would be split over two.

***I have just noticed when doing rbindlist that it says expression type columns are not supported
***Therefore, maybe the issue is that I was able to create a data.table with an expression type column at all

P.S. I was unable to download the development version of data.table as per the wiki:
data.table::update.dev.pkg(repo="https://gitlab.com/Rdatatable/data.table") Error: Line starting '<!DOCTYPE html> ...' is malformed!

Instead I did:
devtools::install_github("Rdatatable/data.table")
@jangorecki
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jangorecki commented Aug 24, 2018 •
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as for the upgrade call, the url was incorrect, I already corrected it in wiki. data.table::update.dev.pkg(repo="https://Rdatatable.gitlab.io/data.table")
if you can compile from source (you have Rtools, etc) you can just call data.table::update.dev.pkg().

@MichaelChirico
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I have just noticed when doing rbindlist that it says expression type columns are not supported

data.table has no issue with storing expressions in a column (should be a list column I guess?), the error is coming from rbindlist which doesn't support stitching together such columns... you'll have to make a workaround for this for now.

@MichaelChirico
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As to your main issue, on my machine:

data.table(e = expression(1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13)

is an error:

Error in dimnames(x) <- dn :
length of 'dimnames' [1] not equal to array extent

with traceback():

3: `rownames<-`(`*tmp*`, value = paste0(format(rn, right = TRUE, 
       scientific = FALSE), ":")) at print.data.table.R#73
2: print.data.table(x)
1: function (x, ...) 
   UseMethod("print")(x)

So there's something wrong in print.data.table...

Ideally you'll be able to use #3414 to customize a print method for expression columns, but it probably shouldn't be erroring in the meantime.

@MichaelChirico
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I see... it's breaking because the output wraps to a new line but print.data.table expects only one line since the table is 1-row:

expression(1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13)
# expression(1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 
#     13)

I'll fix this in #3414

MichaelChirico added a commit that referenced this issue May 4, 2019
@MichaelChirico
Closes #3011 -- adds a method to print expression columns
bf591d6 
reword
@MichaelChirico
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For the record, I dove into this a bit; print is using print.default, internally PrintExpression I think is using R_print.cutoff to truncate but I don't think we have access to that, so there's no way to prevent the wrapping on the expression directly AFAICT

@jangorecki jangorecki added the non-atomic column e.g. list columns, S4 vector columns label Apr 5, 2020
@MichaelChirico MichaelChirico mentioned this issue May 19, 2020
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@jangorecki
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jangorecki commented May 24, 2020 •
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As of now the most recent conclusion about expression type as a data.table column, is to keep it inside a list. Expression in general is a vector of language objects. The problem is that whenever any of its input is just atomic scalar, then its type is not language anymore.

sapply(expression(-1,1), is.language)
# [1]  TRUE FALSE

so because it can mix types, it behaves more like a list than a vector.
Because it behave like a list, it then make sense to not duplicate the work we have already for handling lists.
Unless of course someone can show a case where wrapping expression into list impose an overhead, then it make sense to consider supporting that directly.
Closing that for now, yet your feedback is very welcome, ideally in #4040.

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