# [R] matrix calculation

David Winsemius dwinsemius at comcast.net
Sat Dec 10 15:54:07 CET 2011

```On Dec 10, 2011, at 9:13 AM, R. Michael Weylandt wrote:

> 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")] )

`tapply` is not documented to work on dataframes as a first argument.
I think you may want to use aggregate or the split/lapply approach.

--
David.

>
> 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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>> https://stat.ethz.ch/mailman/listinfo/r-help
>> and provide commented, minimal, self-contained, reproducible code.
>
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