How does one aggregate and summarize data quickly?

Question

I have a dataset whose headers look like so:

PID Time Site Rep Count

I want sum the Count by Rep for each P

Answer 1

You should look at the package data.table for faster aggregation operations on large data frames. For your problem, the solution would look like:

library(data.table)
data_t = data.table(data_tab)
ans = data_t[,list(A = sum(count), B = mean(count)), by = 'PID,Time,Site']