[Rd] POSIXlt matching bug

From: McGehee, Robert <Robert.McGehee_at_geodecapital.com>
Date: Tue, 29 Jun 2010 10:46:06 -0400


I came across the below mis-feature/bug using match with POSIXlt objects (from strptime) in R 2.11.1 (though this appears to be an old issue).

> x <- as.POSIXlt(Sys.Date())
> table <- as.POSIXlt(Sys.Date()+0:5)
> length(x)

[1] 1
> x %in% table # I expect TRUE

[1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> match(x, table) # I expect 1

[1] NA NA NA NA NA NA NA NA NA

This behavior seemed more plausible when the length of a POSIXlt object was 9 (back in the day), however since the length was redefined, the length of x no longer matches the length of the match function output, as specified by the ?match documentation: "A vector of the same length as 'x'".

I would normally suggest that we add a POSIXlt method for match that converts x into POSIXct or character first. However, match does not appear to be generic. Below is a possible rewrite of match that appears to work as desired.

match <- function(x, table, nomatch = NA_integer_, incomparables = NULL)

    .Internal(match(if(is.factor(x)||inherits(x, "POSIXlt")) as.character(x) else x,

                    if(is.factor(table)||inherits(table, "POSIXlt"))
as.character(table) else table,
                    nomatch, incomparables))

That said, I understand some people may be very sensitive to the speed of the match function, and may prefer a simple change to the ?match documentation noting this (odd) behavior for POSIXlt.

Thanks, Robert

R.version

               _                            
platform       x86_64-unknown-linux-gnu     
arch           x86_64                       
os             linux-gnu                    
system         x86_64, linux-gnu            
status                                      
major          2                            
minor          11.1                         
year           2010                         
month          05                           
day            31                           
svn rev        52157                        
language       R                            
version.string R version 2.11.1 (2010-05-31)

Robert McGehee, CFA
Geode Capital Management, LLC
One Post Office Square, 28th Floor | Boston, MA | 02109 Tel: 617/392-8396 Fax:617/476-6389
mailto:robert.mcgehee_at_geodecapital.com

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