R: group-wise min or max

Question

There are so many posts on how to get the group-wise min or max with SQL. But how do you do it in R?

Let\'s say, you have got the following data frame

Answer 1

df is your data.frame -

library(data.table)

setDT(df) # convert to data.table in place

df[, value[which.min(t)], by = ID]

Output -

> df[, value[which.min(t)], by = ID]
   ID V1
1:  a  3
2:  b  2