Normalizing selection of dataframe columns with dplyr

Question

I have a data.frame with variables var1 var2 (both strings) and variables x, y, and z. I would like

Answer 1

It could be a problem with the attributes or grouping variable. We can reset the dataset without external attributes by converting to data.frame and then do the mutate_at

df_ %>% 
   as.data.frame %>%
   mutate_at(vars(x, y, z), funs(norm = ./.[1]))

Answer 2

Building on @akrun's answer, you can also use the first() function from dplyr:

df_ %>%
        mutate_at(vars(c("x", "y", "z")), funs(norm = ./first(.)))