what does the "!!!" mean in R code

General
1

I noticed a R function, and below is part of code of function. May some one help to to understand why the author uses "rlang::syms(groupvars)" and "group_by(!!!groupvars)" in the code? what do the "!!!" and "!!" mean in code?

Thanks

....
groupvars <- rlang::syms(groupvars)
measurevar <- rlang::sym(measurevar)

datac <- data %>%
dplyr::group_by(!!!groupvars) %>%
dplyr::summarise(
N = length2(!!measurevar, na.rm = na.rm),
sd = sd (!!measurevar, na.rm = na.rm),
!!measurevar := mean (!!measurevar, na.rm = na.rm),
...

2

As part of quasiquotation. See the unquoting section:

https://adv-r.hadley.nz/quasiquotation.html#unquoting

Though you can now do this with cury-curly well now:

3

Thanks a lot. I still need to learn.

4

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