Hello,
Try the following.
sp <- split(mydata, mydata$Sample)
do.call(rbind, lapply(sp, function(x){x$Gain <- x$Mass[3] - x$Mass[2]; x}))
Hope this helps,
Rui Barradas
Em 18-09-2012 00:15, Julie Lee-Yaw escreveu:
> Hi
>
> I have a dataframe similar to:
>
>> Sample<-c(1,1,1,2,2,2,3,3,3)
>> Time<-c(1,2,3,1,2,3,1,2,3)
>> Mass<-c(3,3.1,3.4,4,4.3,4.4,3,3.2,3.5)
>> mydata<-as.data.frame(cbind(Sample,Time,Mass))
>
> Sample Time Mass
> 1 1 1 3.0
> 2 1 2 3.1
> 3 1 3 3.4
> 4 2 1 4.0
> 5 2 2 4.3
> 6 2 3 4.4
> 7 3 1 3.0
> 8 3 2 3.2
> 9 3 3 3.5
>
> where for each sample, I've measured mass at different points in time.
>
> I now want to calculate the difference between Mass at Time 2 and 3 for each
> unique Sample and store this as a new variable called "Gain2-3". So in my
> example three values of 0.3,0.1,0.3 would be calculated for my three unique
> samples and these values would be repeated in the table according to Sample.
> I am thus expecting:
>
>> mydata #after adding new variable
> Sample Time MassGain2-3
> 1 1 1 3.00.3
> 2 1 2 3.1 0.3
> 3 1 3 3.4 0.3
> 4 2 1 4.0 0.1
> 5 2 2 4.3 0.1
> 6 2 3 4.4 0.1
> 7 3 1 3.0 0.3
> 8 3 2 3.2 0.3
> 9 3 3 3.5 0.3
>
> Does anyone have any suggestions as to how to do this? I've looked at the
> various apply functions but I can't seem to make anything work. I'm fairly
> new to R and would appreciate specific suggestions.
>
> Thanks!
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>
>
>
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