How to match already-calculated means to the original data set? - sql

Question

I am now learning R. I feel that there is a very easy succinct answer to my problem, but I am having trouble solving it myself.
I have a large data set. One column contains various 'categories'. I aggregated these categories to get the mean for each one. So, right now, my aggregated table looks like this:
Category __ Average
A ________ a
B ________ b
C ________ c
etc...
I want now to take these average and combine it as another column onto my original data.
So, I want it to look something like this:
Categories _____ Averages
B _____________ b
A______________a
B______________b
C______________c
B______________b
C______________c
In other words, I want to match each category with its corresponding mean. I have tried variations of merge(), match(), and different apply functions. The fact that my aggregated table is so much smaller than my original data is causing some problems.
Is there a specific function I can use for this simple problem? Thanks in advance.

Answer 1

In base R:
data <- data.frame(Category=c(rep("A",3), rep("B",4), rep("C",2)), Value=1:9)
> data
Category Value
1 A 1
2 A 2
3 A 3
4 B 4
5 B 5
6 B 6
7 B 7
8 C 8
9 C 9
> avg <- lapply(split(data$Value, data$Category), mean)
$A
[1] 2
$B
[1] 5.5
$C
[1] 8.5
> data$Averages <- avg[data$Category]
> data
Category Value Averages
1 A 1 2
2 A 2 2
3 A 3 2
4 B 4 5.5
5 B 5 5.5
6 B 6 5.5
7 B 7 5.5
8 C 8 8.5
9 C 9 8.5
You can use plyr, data.table, etc. more efficiently for larger datasets.