Calculate the mean by group

Question

I have a large data frame that looks similar to this:

df <- data.frame(dive = factor(sample(c("dive1","dive2"), 10, replace=TRUE)),
            


        

Answer 1

2015 update with dplyr:

df %>% group_by(dive) %>% summarise(percentage = mean(speed))
Source: local data frame [2 x 2]

   dive percentage
1 dive1  0.4777462
2 dive2  0.6726483