Mean by factor by level

Question

Maybe this is simple but I can\'t find answer on web. I have problem with mean calculation by factors by level. My data looks typicaly:

factor, value
a,1
a,2
b,1         


        

Answer 1

Just for fun posting the data.table solution although you probably should do what @lukeA suggested

library(data.table) 
A <- setDT(df)[factor == "a", mean(value)]
## [1] 1.5