[R] detecting measurement of specific id in column in R

[John Kane]

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    On Thursday, November 22, 2018, 11:15:00 a.m. EST, Romy Rehschuh via R-help <r-help using r-project.org> wrote:  
 
 Dear all,

I hope this is the right way to ask questions.
I have a problem with R regarding the detection of the measurement of a
specific sample_id (see example file attached). I have to substract the
"IN" values (means the air which goes into the chambers) from the values of
"d13C", "ppm_CO2" and "ppm_13CO2" for every single chamber (=sample ID).
The "IN" values have to be the ones which were measured* before *the
measurements of the single chambers in time. I measured "IN" once and then
up to 10 chambers in a row to safe time, then "IN" again, but it can change.
Therefore, searching for the closest "IN" does not work.

Do you have any suggestions? Would it be possible to write a loop for this?
I would really much appreciate your help!

Best, Vicci
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