[jira] [Created] (ARROW-17606) [C++] Cast float to decimal truncates

["Neal Richardson (Jira)" <ji...@apache.org>]

Neal Richardson created ARROW-17606:
---------------------------------------

             Summary: [C++] Cast float to decimal truncates
                 Key: ARROW-17606
                 URL: https://issues.apache.org/jira/browse/ARROW-17606
             Project: Apache Arrow
          Issue Type: Bug
          Components: C++
            Reporter: Neal Richardson
             Fix For: 10.0.0


Even with all of the cast options to allow truncation and overflow set to False, cast from double to decimal truncates. 

Short reproducer in R:

{code}
library(arrow, warn.conflicts = FALSE)

a <- Scalar$create(sqrt(2))
a
#> Scalar
#> 1.4142135623730951

a$cast(decimal(5, 1))
#> Scalar
#> 1.4
{code}

Longer version that shows the cast options:

{code}
library(arrow, warn.conflicts = FALSE)
library(dplyr, warn.conflicts = FALSE)

tab <- arrow_table(a = sqrt(2))
tab
#> Table
#> 1 rows x 1 columns
#> $a <double>

tab$a
#> ChunkedArray
#> <double>
#> [
#>   [
#>     1.4142135623730951
#>   ]
#> ]

# See that all of the cast options are false:
tab %>% 
  mutate(b = cast(a, decimal(5, 1)))
#> Table (query)
#> a: double
#> b: decimal128(5, 1) (cast(a, {to_type=decimal128(5, 1), allow_int_overflow=false, allow_time_truncate=false, allow_time_overflow=false, allow_decimal_truncate=false, allow_float_truncate=false, allow_invalid_utf8=false}))
#> 
#> See $.data for the source Arrow object

new_tab <- tab %>% 
  mutate(b = cast(a, decimal(5, 1))) %>% 
  compute()
new_tab
#> Table
#> 1 rows x 2 columns
#> $a <double>
#> $b <decimal128(5, 1)>

new_tab$b
#> ChunkedArray
#> <decimal128(5, 1)>
#> [
#>   [
#>     1.4
#>   ]
#> ]
{code}



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