apply() not working when checking column class in a data.frame

Question

I have a dataframe. I want to inspect the class of each column.

x1 = rep(1:4, times=5)
x2 = factor(rep(letters[1:4], times=5))
xdat = data.frame         


        

Answer 1

You could use

sapply(xdat, class)
#     x1        x2 
# "integer"  "factor" 

using apply would coerce the output to matrix and matrix can hold only a single 'class'. If there are 'character' columns, the result would be a single 'character' class. To understand this check

 str(apply(xdat, 2, I))
 #chr [1:20, 1:2] "1" "2" "3" "4" "1" "2" "3" "4" "1" ...
 #- attr(*, "dimnames")=List of 2
 # ..$ : NULL
 # ..$ : chr [1:2] "x1" "x2"

Now, if we check

 str(lapply(xdat, I))
 #List of 2
 #$ x1:Class 'AsIs'  int [1:20] 1 2 3 4 1 2 3 4 1 2 ...
 #$ x2: Factor w/ 4 levels "a","b","c","d": 1 2 3 4 1 2 3 4 1 2 ...