data.table alternative for dplyr mutate?

Question

I'm learning R and I'm not sure if it makes sense to standardise on dplyr or data.table. Dplyr has really nice syntax, but as far as I understand it copies data frame on each operation, which is (or could be) a drawback.

One thing that I can't figure out is alternative for mutate.

if I have

df %>% group_by(foo) %>% mutate(
    bar  = cumsum(baz),
    q    = bar * 3.14)

I could do sth like

df[,c("bar"):=list(cumsum(baz)),by=foo]
df$q <- df$bar*3.14

Is there a better way of doing this in data.table?

Answer 1

You may do just this:

# some test data:
df <- data.table(baz = 1:10, foo = c(rep(1, 5), rep(2, 5)))

df[, bar := cumsum(baz), by = foo]
df[, q := bar*3.14]

While being in two lines, it is very readable and easy to write.