R check if a variable is a factor

Question

I am trying to write a loop which can apply to a dataframe. The loop will basically check each variable in the dataframe and tell me which variable is a factor.

An example:

test1<-c("red","red","blue","yellow")
test1<-as.factor(test1)
test2<-c(1,2,3,4)

variable<-c("test1","test2")
count<-2

for (i in 1:count)
{
        if (is.factor(paste(variable[i]))==TRUE) 
        { 
           print("This is a factor")
        }
}

test1 variable is supposed to be a factor and therefore the sentence "This is a factor" should be printed. However nothing happened. I wonder why?

Solution 1

Try using sapply or lapply instead of you for-loop as follows:

variable <- data.frame(test1,test2)
sapply(variable,is.factor) # returns a vector
test1 test2 
 TRUE FALSE

lapply(variable,is.factor) # returns a list
$test1
[1] TRUE

$test2
[1] FALSE

Or you can easily use your function instead of is.factor

Edit: (thanks to Ananda Mahto) vapply is even faster than sapply

require(microbenchmark)
microbenchmark(
  vapply(variable,is.factor, c(is.factor=FALSE)),
  sapply(variable,is.factor),
  lapply(variable,is.factor),
  times = 10000
)
Unit: microseconds
                                              expr    min     lq median     uq      max neval
 vapply(variable, is.factor, c(is.factor = FALSE)) 12.248 13.829 14.618 15.409  959.698 10000
                       sapply(variable, is.factor) 31.608 35.560 36.350 37.534 1159.618 10000
                       lapply(variable, is.factor)  9.877 11.458 11.853 12.644  935.597 10000

Solution 2

This is an evaluation problem. The character arrays "test1" or "test2" are not factors.

> is.factor(get(variable[1]))
[1] TRUE
> is.factor(get(variable[2]))
[1] FALSE

Comments

[A5C1D2H2I1M1N2O1R2T1]

Why are you neglecting vapply :-)

[Rentrop]

Thanks! Never used vapply before. But i am going to in the future