[R] Calculate mean/var by ID

[liujb]

Hello,

I have a data set that looks like this. 
ID    value
111     5
111     6
111     2
178     7
178     3
138     3
138     8
138     7
138     6
.
.
.

I'd like to calculate the mean and var for each object identified by the ID.
I can in theory just loop through the whole thing..., but is there a easier
way/command which let me calculate the mean/var by ID?

Thanks,
Julia
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