# [R] matrix calculation

R. Michael Weylandt michael.weylandt at gmail.com
Sat Dec 10 15:13:53 CET 2011

```Perhaps something like this (untested) -- it's going to depend on the
exact structure of your data so if this doesn't work, please use
dput() to send a plain text representation:

tapply(data, data\$animal, function(d) d[, c("A01", "A02")] - d[d\$time
== "d0", c("A01", "A02")] )

In short, take "data" split it into groups by "data\$animal" and then
apply the function which consists of subtracting the "A01" and "A02"
columns by the row that has time == "d0"

Michael

On Fri, Dec 9, 2011 at 1:44 PM, Junyu Lee <junyu0813 at gmail.com> wrote:
> Hello,
>
> I have a matrix
>
> animal            time              A01                 A02
>  A                  d0               -5.4                     2.7
>  A                  d112            4.6                     5.9
>  A                  d224             3.9                    6.3
>  B                  d0                7.1                     5.6
>  B                  d112            1.5                     3.2
>  B                  d224             2.9                    3.6
>  C                  d112            3.7                     5.8
>  C                  d0                7.1                     NA
>  C                 d224            4.2                       5.7
>
> I have three animal A, B, C. Each animal have two measurements(A01 and A02)
> at three different time points (d0, d112 and d224).  I'd like to calculate:
>
> animal A:
>
> A01 at d112 (4.6) - A01 at d0 (-5.4)
> A01 at d224 (3.9) - A01 at d0 (-5.4)
>
> A02 at d112 (5.9) - A02 at d0 (2.7)
> A02 at d224 (6.3) - A02 at d0 (2.7)
>
> Same for animal B and C
>
> I really appreciate your help.
>
> Junyu
>
>        [[alternative HTML version deleted]]
>
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```