Hi all,
I apologize for this probably stupid question, but I really can't figure it
out.
I have a dataframe like this:

group <- c(rep('A', 8), rep('B', 15), rep('C', 6))
time <- c(rep(seq(1:4), 2), rep(seq(1:5), 3), rep(seq(1:3), 2))
value <- runif (29, 1, 10)
dfx <- data.frame (group, time, value)

I want to calculate mean and standard deviation for all values that belong
to the same group and the same time and end up with a dataframe with the
columns time, group, mean and sd that contains the calculated values for
every group at every time point only once (12).
What is the most elegant way to do this? Oh, and I would like to avoid
renaming columns (like the _X1/_X2 created by casting with multiple
functions), if possible.
I am sure that this is pretty basic, but I have already wasted a ridiculous
amount of time on this.

Thanks,

Kai

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