R: replacing column values in data.table with condition over loop

Question

library(data.table)
dt <- data.table(colx=c(\'a\',\'a\', \'b\',\'a\'), coly=c(\'b\',\'a\', \'b\',\'a\'),colz=c(\'a\',\'b\', \'b\',\'a\'))
dt

i wa