[R] Sharing levels across multiple factor vectors

[Jeff Brown]

Hi,

I've got a data frame with multiple factor columns, but they should share
the same set of labels, such as this tiny example:

df <- data.frame (
	a = factor( c( "bob", "alice", "bob" ) ),
	b = factor( c( "kenny", "alice", "alice" ) )
);

In my data, though, the strings are enormous.  I would like to replace them
with integers, which would take up so much less space that I could actually
understand the screen display.  Converting a single factor column to
integers is easy: You can just use as.numeric().  But sharing the labels
across two vectors has eluded me.  

Here's what I tried:  First, collect all the levels into a single vector:

> allLevels <- unique( c( levels( df$a ), levels( df$b ) ) )
> allLevels
[1] "alice"    "bob"      "kenny"

Now change the "levels" attribute of each vector:

> for (c in colnames(df)) levels( df[,c] ) <- allLevels
> data.frame (	
+ 	as.numeric( a ),
+ 	as.numeric( b )
+ )
  as.numeric.a. as.numeric.b.
1             2             2
2             1             1
3             2             1

What happened to Kenny?

Thanks in advance,
Jeff
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