[
https://issues.apache.org/jira/browse/ARROW-6830?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16947927#comment-16947927
]
Neal Richardson commented on ARROW-6830:
----------------------------------------
Looking at [https://github.com/apache/arrow/blob/master/r/R/read-table.R],
{{read_arrow}} returns a data frame while {{read_table}} keeps the data in an
Arrow Table. Tables have a {{$select()}} method (which is [how
{{read_csv_arrow}} implements
{{col_select}}|https://github.com/apache/arrow/blob/master/r/R/csv.R#L124]),
and you can more naturally access that through the usual {{[}} method. So IIUC
what you're trying to do,
{code:r}
tab <- read_table(data_rbfr)
as.data.frame(tab[, 6])
{code}
and of course you could reference that column by name instead of position.
If you wanted to add {{col_select}} to {{read_arrow()}}, I'd recommend
following the model of {{read_csv_arrow}}, which sounds pretty straightforward.
Happy to review a pull request if you submit it.
> Question / Feature Request- Select Subset of Columns in read_arrow
> ------------------------------------------------------------------
>
> Key: ARROW-6830
> URL: https://issues.apache.org/jira/browse/ARROW-6830
> Project: Apache Arrow
> Issue Type: New Feature
> Components: C++, R
> Reporter: Anthony Abate
> Priority: Minor
>
> *Note:* Not sure if this is a limitation of the R library or the underlying
> C++ code:
> I have a ~30 gig arrow file with almost 1000 columns - it has 12,000 record
> batches of varying row sizes
> 1. Is it possible at to use *read_arrow* to filter out columns? (similar to
> how *read_feather* has a (col_select =... )
> 2. Or is it possible using *RecordBatchFileReader* to filter columns?
>
> The only thing I seem to be able to do (please confirm if this is my only
> option) is loop over all record batches, select a single column at a time,
> and construct the data I need to pull out manually. ie like the following:
> {code:java}
> for(i in 0:data_rbfr$num_record_batches) {
> rbn <- data_rbfr$get_batch(i)
>
> if (i == 0)
> {
> merged <- as.data.frame(rbn$column(5)$as_vector())
> }
> else
> {
> dfn <- as.data.frame(rbn$column(5)$as_vector())
> merged <- rbind(merged,dfn)
> }
>
> print(paste(i, nrow(merged)))
> } {code}
>
>
--
This message was sent by Atlassian Jira
(v8.3.4#803005)
