[GitHub] [arrow] jonkeane commented on pull request #13070: ARROW-15804: [R] Update as.Date() to support several tryFormats

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jonkeane commented on PR #13070:
URL: https://github.com/apache/arrow/pull/13070#issuecomment-1124050785

   > Do you have any ideas on this? I"m struggling a bit to replicate the base behaviour (subsetting the column ...).
   
   What have you tried so far? This _will_ be tricky + it's totally possible that it is not feasible to emulate this behavior with the way we build these. 
   
   > On the returning NA. It depends. We don't align with the behaviour of as.Date() only when all formats fail to parse and optional = FALSE (which is the default) , in which case we align with as.Date(optional = TRUE). It's a decent, but again, not perfect overlap. 
   
   Since we have the option to either emit an `NA` or error on parsing fail, could we capture this behavior?  
   
   
   Tangentially, the example you use works, though it's a bit confusing IMO `%y` fails because it's year-without-century as opposed to `%Y`. And the NA at the beginning also made me think that was what you were relying on for failing the guessing, but that's doesn't actually make the parsing fail (which I got from reading the source of `as.Date.character`...). Here's a slightly more minimal | possibly less confusing example of that same phenomenon: 
   
   ```
   > x <- c("2022-01-01", "2022/01/01")
   > as.Date(x, tryFormats = c("%Y--", "%m--"), optional = FALSE)
   Error in charToDate(x) : 
     character string is not in a standard unambiguous format
   > as.Date(x, tryFormats = c("%Y--", "%m--"), optional = TRUE)
   [1] NA NA
   ```
   


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