[
https://issues.apache.org/jira/browse/ARROW-17886?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17611061#comment-17611061
]
Dewey Dunnington commented on ARROW-17886:
------------------------------------------
It hasn't been implemented yet, but we're probably going to include this at
least internally to support additional tidyselect helpers (see ARROW-12778). In
the meantime, you may be able to use this workaround:
{code:R}
library(arrow, warn.conflicts = FALSE)
#> Some features are not enabled in this build of Arrow. Run `arrow_info()` for
more information.
simulate_data_frame <- function(schema) {
arrays <- lapply(schema$fields, function(field) concat_arrays(type =
field$type))
vectors <- lapply(
arrays,
function(array) tryCatch(
as.vector(array),
error = function(...) vctrs::unspecified()
)
)
names(vectors) <- names(schema)
tibble::new_tibble(vectors, nrow = 0)
}
simulate_data_frame(schema(col1 = int32(), col2 = string()))
#> # A tibble: 0 × 2
#> # … with 2 variables: col1 <int>, col2 <chr>
{code}
> [R] Convert schema to the corresponding ptype (zero-row data frame)?
> --------------------------------------------------------------------
>
> Key: ARROW-17886
> URL: https://issues.apache.org/jira/browse/ARROW-17886
> Project: Apache Arrow
> Issue Type: Improvement
> Components: R
> Reporter: Kirill Müller
> Priority: Minor
>
> When fetching data e.g. from a RecordBatchReader, I would like to know, ahead
> of time, what the data will look like after it's converted to a data frame. I
> have found a way using utils::head(0), but I'm not sure if it's efficient in
> all scenarios.
> My use case is the Arrow extension to DBI, in particular the default
> implementation for drivers that don't speak Arrow yet. I'd like to know which
> types the columns should have on the database. I can already infer this from
> the corresponding R types, but those existing drivers don't know about Arrow
> types.
> Should we support as.data.frame() for schema objects? The semantics would be
> to return a zero-row data frame with correct column names and types.
> library(arrow)
> #> Some features are not enabled in this build of Arrow. Run `arrow_info()`
> for more information.
> #>
> #> Attaching package: 'arrow'
> #> The following object is masked from 'package:utils':
> #>
> #> timestamp
> data <- data.frame(
> a = 1:3,
> b = 2.5,
> c = "three",
> stringsAsFactors = FALSE
> )
> data$d <- blob::blob(as.raw(1:10))
> tbl <- arrow::as_arrow_table(data)
> rbr <- arrow::as_record_batch_reader(tbl)
> tibble::as_tibble(head(rbr, 0))
> #> # A tibble: 0 × 4
> #> # … with 4 variables: a <int>, b <dbl>, c <chr>, d <blob>
> rbr$read_table()
> #> Table
> #> 3 rows x 4 columns
> #> $a <int32>
> #> $b <double>
> #> $c <string>
> #> $d <<blob[0]>>
> #>
> #> See $metadata for additional Schema metadata
--
This message was sent by Atlassian Jira
(v8.20.10#820010)
