turn some variables in `data.frame` into `factor` keeping the data.frame structure R

Question

I'm trying to turn some of the variables in my data.frame into factor while preserving the data.frame structure. I follow the suggestions HERE but I don't get my desired output, any fix?

d <- data.frame(a = c(1,2, 3, 5), b = c(2,3, 4, 2), e = c(3,4,5,1), f = rep(c("long", "short"), 2))
factor.name <- names(d)[-4]

d[] <- lapply(seq_along(factor.name), function(i) as.factor(d[factor.name[i]]))

Answer 1

Subset the data with 'factor.name', pass that in lapply and update the columns

d[factor.name] <- lapply(d[factor.name], factor)

Answer 2

library(dplyr)
d <-
  d %>% 
  mutate_at(vars(a, b, e), as.factor)