[R] Selecting values

[Matthew Keller]

Is this easier?

x.index <- duplicated(x.sample)==FALSE
cbind(x.sample[x.index],y[x.index])


- Matt

On 9/28/07, Marc Schwartz <marc_schwartz at comcast.net> wrote:
> On Fri, 2007-09-28 at 17:48 -0400, Brian Perron wrote:
> > 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
>
> See ?split, ?sapply and ?unique.
>
> Then try this:
>
> > cbind(unique(z[, 1]), sapply(split(z[, 2], z[, 1]), "[", 1))
>   [,1]       [,2]
> 1    1 -1.2006469
> 2    2 -0.1614918
> 3    3 -0.5717729
> 4    4 -0.2398887
> 5    5  1.1690564
>
>
> The key part of that is:
>
> > split(z[, 2], z[, 1])
> $`1`
> [1] -1.2006469  0.7615261 -0.1287516 -1.1796474 -1.2902519
>
> $`2`
> [1] -0.1614918 -0.1464773 -0.8875417  0.3062891  0.4398530
>
> $`3`
> [1] -0.5717729 -0.2938118
>
> $`4`
> [1] -0.2398887  0.8425419  2.5269801 -0.3643613
>
> $`5`
> [1]  1.1690564 -0.7644521  1.4178982 -0.8198921
>
>
> which splits 'z' by the values in the first column.
>
> Then we use sapply() to go through the list and subset the first element
> in each vector:
>
> > sapply(split(z[, 2], z[, 1]), "[", 1)
>          1          2          3          4          5
> -1.2006469 -0.1614918 -0.5717729 -0.2398887  1.1690564
>
>
> Then we cbind() that result to the unique values in the first column.
>
> HTH,
>
> Marc Schwartz
>
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>


-- 
Matthew C Keller
Postdoctoral Fellow
Virginia Institute for Psychiatric and Behavioral Genetics