Is there a way to make this R for loop work

I'm looking for an easy way to loop through my data and print out certain columns based on a condition. This for-loop ought to find the column within the data that meets the if condition and then print out these columns. This is what I have tried.

for (var in names(df)) {
    if(length (unique(df[var])) == 2) {
       print (var)
      }
}