R: Calculate the mean value of a variable by unique values of another variable in a dataframe?

Question

I am new to R. I have a data frame that looks like this:

Pupil ID     State      GPA
1            FL         3.9
2            TX         3.2
3            NY         2.2
4            AK         3.0
5            CO         2.4

... etc. What I would like to do is create a new data frame that looks like this:

State        Mean GPA     Number of pupils 
AL           2.91         23
AK           3.23         24

and so on. In other words, I'd like to find the unique values for state, and calculate the mean GPA for each one and number of pupils for each one.

Is this possible in R? I know I can do table(data$State) to get the unique states and number of pupils, but I don't know how to calculate the mean for the unique values of state.

Answer 1

One of very many ways to do this:

x <- read.table(header=T, text="Pupil.ID     State      GPA
1            FL         3.9
2            TX         3.2
3            NY         2.2
4            AK         3.0
5            CO         2.4")

aggregate(GPA~State, data=x, FUN=function(x) c(mean=mean(x), count=length(x)))
##   State GPA.mean GPA.count
## 1    AK      3.0       1.0
## 2    CO      2.4       1.0
## 3    FL      3.9       1.0
## 4    NY      2.2       1.0
## 5    TX      3.2       1.0