R convert data to factor will corrupt all other data.frame columns

Question

I have a data.frame, all columns are numeric. I want to convert one integer column to factor, but doing so will convert all other columns to class character. Is there anyway to just convert one column to factor?

The example is from Converting variables to factors in R:

myData <- data.frame(A=rep(1:2, 3), B=rep(1:3, 2), Pulse=20:25)
myData$A <-as.factor(myData$A)

The result

apply(myData,2,class)
#           A           B       Pulse 
# "character" "character" "character"

sessionInfo()

R version 3.1.2 (2014-10-31) 
Platform: x86_64-apple-darwin13.4.0 (64-bit) 

locale: [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8  
attached base packages: 
[1] splines stats graphics grDevices utils datasets methods base ... 

str(myData$A)
# Factor w/ 2 levels "1","2": 1 2 1 2 1 2

That seems unlikely. Does the code you provided actually reproduce the problem for you? — MrFlick
The only time I know of this happening is if myData is a matrix or array or vector, not a dataframe. are you sure class(myData) is a dataframe (for whichever data is causing you the problem)? — mathematical.coffee
class(myData) is data.frame. apply(myData,2,class) produces A B Pulse "character" "character" "character" — Echo
@Pascal sessionInfo() gives R version 3.1.2 (2014-10-31) Platform: x86_64-apple-darwin13.4.0 (64-bit) locale: [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8 attached base packages: [1] splines stats graphics grDevices utils datasets methods base ... — Echo

MichaelVE MichaelVE · Accepted Answer · 2015-07-15T04:55:02

Your code actually works when I test it.

This is my output from str(myData):

    'data.frame':   6 obs. of  3 variables:
 $ A    : Factor w/ 2 levels "1","2": 1 2 1 2 1 2
 $ B    : int  1 2 3 1 2 3
 $ Pulse: int  20 21 22 23 24 25

Your issue is because, as ?apply states:

‘apply’ attempts to coerce to an array via ‘as.matrix’ if it is two-dimensional (e.g., a data frame)

This is done before executing the function on each column. And when you run as.matrix(myData) you end up with everything forced to one class, in this case character data:

is.character(as.matrix(myData))
#[1] TRUE

R convert data to factor will corrupt all other data.frame columns

1 Answers