Applying nlminb to subsets of data (by index or label) and store what the program returns as a new data frame - data-structures

Question

I was wondering if anyone could kindly help me with this seemingly easy task. I'm using nlminb to conduct optimization and compute some statistics by index. Here's an example from nlminb help.
> x <- rnbinom(100, mu = 10, size = 10)
> hdev <- function(par) {
+ -sum(dnbinom(x, mu = par[1], size = par[2], log = TRUE))
+ }
> nlminb(c(9, 12), hdev)
$par
[1] 9.730000 5.954936
$objective
[1] 297.2074
$convergence
[1] 0
$message
[1] "relative convergence (4)"
$iterations
[1] 10
$evaluations
function gradient
12 27
Suppose I generate random variables x, y, and z where z acts as an index (from 1 to 3).
> x <- rnbinom(100, mu = 10, size = 10)
> y <- rnbinom(100, mu = 10, size = 10)
> z <- rep(1:3, length=100)
> A <- cbind(x,y,z)
> hdev <- function(par) {
+ -sum(dnbinom(x+y, mu = par[1], size = par[2], log = TRUE))}
How can I apply nlminb(c(9, 12), hdev) to the data set by index z? In other words, I would like to compute nlminb(c(9, 12), hdev) for z=1, z=2, and z=3 separately. I tried by(A, z, function(A) nlminb(c(9,12), hdev)) and sparseby(A, z, function(A) nlminb(c(9,12), hdev)), but they return exactly the same values for each value of z.
I would like to turn each output into a new data frame so that it will become a 3X2 matrix.
[1] Z1_ANSWER_1 Z1_ANSWER_2
[2] Z2_ANSWER_1 Z2_ANSWER_2
[3] Z3_ANSWER_1 Z3_ANSWER_2
Since nlminb returns the summary of statistics, I needed to use CASEZ1<-nlminb$par, CASEZ2<-nlminb$par, CASEZ3<-nlminb$par and then use cbind to combine them. However, I would like to automate this process as the real data I'm working on has a lot more categories than z presented here.
If I'm not making myself clear, please let me know. I'll see if I can replicate the actual data set and functions I'm working on (I just don't have them on this computer).
Thank you very much in advance.

Answer 1

Let me try an approach
x <- rnbinom(100, mu = 10, size = 10)
y <- rnbinom(100, mu = 10, size = 10)
z <- rep(1:3, length=100)
A <- as.data.frame(cbind(x,y,z))
At first load the plyr library
library(plyr)
The following code returns the results for each z
dlply(A, .(z), function(x) {
hdev <- function(par, mydata) {-sum(dnbinom(mydata, mu = par[1], size = par[2], log = TRUE))}
nlminb(c(9, 12), hdev, mydata=t(as.vector(x[1] + as.vector(x[2]))))
}
)
Now, with this one you will get a 3x2 dataframe with the $par results
ddply(A, .(z), function(x) {
hdev <- function(par, mydata) {-sum(dnbinom(mydata, mu = par[1], size = par[2], log = TRUE))}
res <- nlminb(c(9, 12), hdev, mydata=t(as.vector(x[1] + as.vector(x[2]))))
return(res$par)
}
)