[R] Error: Cannot convert Dictionary Array of type dictionary<values=large_string, indices=uint32, ordered=0> to R

Describe the bug, including details regarding any error messages, version, and platform.

ChunkedArray objects of type "DictionnaryType" have the method as_vector (as do ChunkedArray objects of other types) but calling that method ends with an error.

I am using the R package polars to create a problematic ChunkedArray. This is how I discovered the issue and I did not find how to create a similar ChunkedArray using the R package arrow alone:

> require("arrow"); require("polars")
> pl_df <- polars::as_polars_df(data.frame(foo = factor(c("x", "y", "x"))))
> ar_df <- arrow::as_arrow_table(pl_df)
> ar_df$foo
ChunkedArray
<dictionary<values=large_string, indices=uint32>>
[

  -- dictionary:
    [
      "x",
      "y"
    ]
  -- indices:
    [
      0,
      1,
      0
    ]
]
> ar_df$foo$as_vector()
Error: Cannot convert Dictionary Array of type `dictionary<values=large_string, indices=uint32, ordered=0>` to R

arrow version: 12.0.0.9000
R version: 4.2.3

Component(s)

R

I found this in the documentation, where the conversion of a ChunkedArray of type dictionnary to an R vector is working: https://arrow.apache.org/docs/r/articles/data_types.html

The issue seems to be when values is of type large_string.

@lgautier

Hi @lgautier, thanks for the report. It looks like the issue is that the conversion routine this uses doesn't support uint32 indices:

arrow/r/src/array_to_vector.cpp

Lines 593 to 603 in c170af4

    
           switch (indices_type.id()) { 
        
             case Type::UINT8: 
        
             case Type::INT8: 
        
             case Type::UINT16: 
        
             case Type::INT16: 
        
             case Type::INT32: 
        
               // TODO: also add int64, uint32, uint64 downcasts, if possible 
        
               break; 
        
             default: 
        
               cpp11::stop("Cannot convert Dictionary Array of type `%s` to R", 
        
                           dict_type.ToString().c_str());

As a workaround until this feature is added, you should be able to cast() to a similar type but with int32 indices so long as your indices can be contained within an int32:

ar_df$foo$cast(dictionary(index_type = int32(), value_type = large_utf8()))$as_vector()

The pure arrow reproducer for this is,

Array$create(factor(c("x", "y", "z")))$cast(dictionary(index_type = uint32(), value_type = large_utf8()))

This bug or one with the same error message also seems to prevent R Arrow from reading Parquet files with Categorical or Enum columns created in Python Polars.

Reprex tested with Python Polars 1.31.0 and R Arrow 20.0.0.2:

# In Python:
import polars as pl

data = pl.DataFrame({"x": ["a", "b"]}, schema={"x": pl.Categorical})
data.write_parquet("test.parquet")

# In R:
library("arrow")
read_parquet("test.parquet")

Error message:

Error: Cannot convert Dictionary Array of type `dictionary<values=string, indices=uint32, ordered=0>` to R

GH-39603: [R] Error: Cannot convert Dictionary Array of type dictiona…

Issue resolved by pull request 49710
#49710

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[R] Error: Cannot convert Dictionary Array of type `dictionary<values=large_string, indices=uint32, ordered=0>` to R #39603

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	switch (indices_type.id()) {
	case Type::UINT8:
	case Type::INT8:
	case Type::UINT16:
	case Type::INT16:
	case Type::INT32:
	// TODO: also add int64, uint32, uint64 downcasts, if possible
	break;
	default:
	cpp11::stop("Cannot convert Dictionary Array of type `%s` to R",
	dict_type.ToString().c_str());

[R] Error: Cannot convert Dictionary Array of type dictionary<values=large_string, indices=uint32, ordered=0> to R #39603

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lgautier commented on Jan 14, 2024

amoeba commented on Jan 18, 2024

nick-ulle commented on Jul 22, 2025

thisisnic commented on Jun 10, 2026

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[R] Error: Cannot convert Dictionary Array of type `dictionary<values=large_string, indices=uint32, ordered=0>` to R #39603