ARROW-5625: [R] convert Array of struct type to data frame columns

[romainfrancois]

struct arrays become data frame columns, i.e.

library(arrow, warn.conflicts=FALSE)
library(tibble)
tf<- tempfile()
writeLines(' { "hello": 3.5, "world": false, "yo": "thing", "nuf": {} } { "hello": 3.25, "world": null, "nuf": null } { "hello": 3.125, "world": null, "yo": "\u5fcd", "nuf": { "ps": 78.0, "house": "Gryffindor"} } { "hello": 0.0, "world": true, "yo": null, "nuf": { "ps": 90.0, "house": "Slytherin" } }', tf)
tab1<- read_json_arrow(tf, as_tibble=FALSE)
array<-tab1$column(3)$data()$chunk(0)
array$field(0)
#> arrow::Array #> [#> null,#> null,#> 78,#> 90#> ]array$as_vector()
#> ps house#> 1 NA <NA>#> 2 NA <NA>#> 3 78 Gryffindor#> 4 90 Slytherin
as.data.frame(tab1)
#> # A tibble: 4 x 4#> hello world yo nuf$ps $house #> <dbl> <lgl> <chr> <dbl> <chr> #> 1 3.5 FALSE thing NA <NA> #> 2 3.25 NA <NA> NA <NA> #> 3 3.12 NA 忍 78 Gryffindor#> 4 0 TRUE <NA> 90 Slytherin

^{Created on 2019-06-17 by the reprex package (v0.3.0.9000)}

[romainfrancois]

This is similar to #4575, @fsaintjacques you might want to adopt this one too.

[fsaintjacques]

What about omitting fields, e.g. {"hello":"hi"}.

[romainfrancois]

library(arrow, warn.conflicts=FALSE)
library(tibble)
tf<- tempfile()
writeLines(' { "hello": 3.5, "world": false, "yo": "thing" } { "hello": 2.3}', tf)
read_json_arrow(tf, as_tibble=TRUE)
#> # A tibble: 2 x 3#> hello world yo #> <dbl> <lgl> <chr>#> 1 3.5 FALSE thing#> 2 2.3 NA <NA>

Maybe this read_json_arrow() needs more tests.

library(arrow, warn.conflicts=FALSE)
library(tibble)
tf<- tempfile()
writeLines(' { "hello": 3.5, "world": false, "yo": "thing" } { "hello": "hi"}', tf)
read_json_arrow(tf, as_tibble=TRUE)
#> Error in json___TableReader__Read(self): Invalid: Empty JSON file

but it's not really this pull request territory. I'm using this at the moment because we currently don't have an R way to create list arrays or struct arrays, i.e. we need to be able to go the other direction with e.g.

library(arrow, warn.conflicts=FALSE)
library(tibble)
array(list(1:3, 4:5), type= list_of(int32()))
#> Error in Array__from_vector(x, type): NotImplemented: type not implemented

which is the purpose of this one: https://issues.apache.org/jira/browse/ARROW-3809?filter=12344983

[fsaintjacques]

This is an error of mine, I meant {"hello": 1.2} not a bad type. But good if it uncovered an error :)

[codecov-io]

Codecov Report

Merging #4593 into master will decrease coverage by 12.76%.
The diff coverage is 92.47%.

@@ Coverage Diff @@## master #4593 +/- ##
==========================================
- Coverage 88.57% 75.8% -12.77% 
==========================================
Files 860 56 -804 Lines 108022 3278 -104744 Branches 1253 0 -1253 ==========================================
- Hits 95678 2485 -93193 + Misses 12065 793 -11272 + Partials 279 0 -279

Impacted Files	Coverage Δ
r/src/arrow_types.h	96% <ø> (ø)	⬆️
r/src/datatype.cpp	74.48% <100%> (+1.08%)	⬆️
r/src/symbols.cpp	54.54% <100%> (+4.54%)	⬆️
r/src/array.cpp	79.16% <100%> (+4.8%)	⬆️
r/R/ChunkedArray.R	100% <100%> (ø)	⬆️
r/R/arrowExports.R	73.73% <100%> (+0.61%)	⬆️
r/R/Struct.R	100% <100%> (ø)	⬆️
r/src/arrowExports.cpp	73.9% <100%> (+0.67%)	⬆️
r/R/array.R	75% <60%> (+3%)	⬆️
r/src/array__to_vector.cpp	76.63% <86.48%> (+1.16%)	⬆️
... and 804 more

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