Fastest summarise on a conditional subset
Solution 1:
Unclear to me if you specifically want the summaries with t still grouped. This base R approach ignores the grouping.
a$b1 <- mean(a$x[a$s == 0])
a$b2 <- mean(a$x[a$s == 1])
Or a dplyr approach that keeps the grouping of t:
a %>%
summarize(b1 = mean(x[s==0]),
b2 = mean(x[s==1]))