R help - May 2002 - Using merge can convert character variables to factor

I am not sure if this is a bug or a feature, but I could not find it
documented. In certain circumstances, using merge can convert a character
variable to factor. Consider a simple example:
> x <- data.frame(a = 1:4)
> y <- data.frame(b = LETTERS[1:3])
> z <- merge(x, y, by = 0)
> unlist(lapply(z, data.class))
Row.names         a         b 
 "factor" "integer"  "factor" 

So far, so good. b should be a factor since it is a factor in y.
> is.factor(y$b)
[1] TRUE

Changing b to be a charcter variable works as well.
> y$b <- as.character(y$b)
> z <- merge(x, y, by = 0)
> unlist(lapply(z, data.class))
  Row.names           a           b 
   "factor"   "integer" "character" 

But when we change the merge to include all of the x rows in the resulting
dataframe, we get:
> z <- merge(x, y, by = 0, all.x = TRUE)
> unlist(lapply(z, data.class))
Row.names         a         b 
 "factor" "integer"  "factor"
> 
I think that b should still be a character variable in this case.

If this is a bug, please let me know and I would be happy to submit it.
> R.version
         _                   
platform sparc-sun-solaris2.6
arch     sparc               
os       solaris2.6          
system   sparc, solaris2.6   
status                       
major    1                   
minor    5.0                 
year     2002                
month    04                  
day      29                  
language R                   

Thanks,

Dave Kane
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On Thu, 16 May 2002, David Kane  <David Kane wrote:
> I am not sure if this is a bug or a feature, but I could not find it
> documented. In certain circumstances, using merge can convert a character
> variable to factor. Consider a simple example:
>
> > x <- data.frame(a = 1:4)
> > y <- data.frame(b = LETTERS[1:3])
> > z <- merge(x, y, by = 0)
> > unlist(lapply(z, data.class))
> Row.names         a         b
>  "factor" "integer"  "factor"
>
> So far, so good. b should be a factor since it is a factor in y.
>
> > is.factor(y$b)
> [1] TRUE
>
> Changing b to be a charcter variable works as well.
>
> > y$b <- as.character(y$b)
> > z <- merge(x, y, by = 0)
> > unlist(lapply(z, data.class))
>   Row.names           a           b
>    "factor"   "integer" "character"
>
> But when we change the merge to include all of the x rows in the resulting
> dataframe, we get:
>
> > z <- merge(x, y, by = 0, all.x = TRUE)
> > unlist(lapply(z, data.class))
> Row.names         a         b
>  "factor" "integer"  "factor"
sapply would be better, BTW.
> I think that b should still be a character variable in this case.
Well, b is character in x and factor in y. Your example is inconsistent:
perhaps it should complain at you?

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595


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