Apply function only if columns respect condition

I’m new to R and i’m trying to learn fundamentals. I’m facing a small problem for which i can’t find a solution online.

What i want to do: write a function to lower case for all the columns in my data.frame if they respect a condition (class = factor)

This code works, but for all my columns :

lower = function(x) {data.frame(tolower(as.matrix(x)))}

I need more something like this, but it doesn’t work:

lower = function (x) { for (i in 1:length(x)) {
 if (class(i)=="factor") {
   data.frame(tolower(as.matrix(x))) 
 }
}}

x is my data.frame

Thank you

Y.P