Hello all,

An elementary question that I am sure can be easily cracked by an R
enthusiast.  Let's say I have multiple scores (y) on subjects (x.sample).
Some subjects have a few more scores than others.  Can somebody suggest some
code that will select the first score for each subject?

For example, the following code generates scores for 5 subjects:

> x <- c(1:5)
> x.sample <- sample(x, 20, replace = TRUE)
> x.sample <- sort(x.sample)
> y <- rnorm(20)
> z <- cbind(x.sample, y)
> z

      x.sample          y
 [1,]        1 -1.2006469
 [2,]        1  0.7615261
 [3,]        1 -0.1287516
 [4,]        1 - 1.1796474
 [5,]        1 -1.2902519
 [6,]        2 -0.1614918
 [7,]        2 -0.1464773
 [8,]        2 -0.8875417
 [9,]        2  0.3062891
[10,]        2  0.4398530
[11,]        3 -0.5717729
[12,]        3 - 0.2938118
[13,]        4 -0.2398887
[14,]        4  0.8425419
[15,]        4  2.5269801
[16,]        4 -0.3643613
[17,]        5  1.1690564
[18,]        5 -0.7644521
[19,]        5  1.4178982
[20,]        5 - 0.8198921

I am only interested in extracting the first score (y) for each unique
subject (x.sample).  So, I would like to generate the following output.

        x.sample       y
[1,]    1                  -1.2006469
[2,]    2                  -0.1614918
[3,]    3                  -0.5717729
[4,]    4                  -0.2398887
[5,]    5                   1.1690564

Any assistance would be greatly appreciated.

Regards,
Brian

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