[R] help with calculation from dataframe with multiple entries per sample

[David Winsemius]

On Sep 17, 2012, at 5:00 PM, David Winsemius wrote:

> 
> On Sep 17, 2012, at 4:15 PM, Julie Lee-Yaw wrote:
> 
>> 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))
>> 
> Please tell me where you learned that as.data.frame(cbind(.)) construction.
> 
>> (
>>  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
> 
> mydata$gain2.3 <- with( mydata, ave( Mass , Time, FUN=function(x) diff(x[2],x[3]) ) )

OOOPpps  .... the code above was a failed attempt.
>> 
>>  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


... the code below should "work".

>> 
> 
>> mydata$gain2.3 <- with( mydata, ave( Mass , Sample, FUN=function(x) (x[3]-x[2]) ) )
>> mydata
>  Sample Time Mass gain2.3
> 1      1    1  3.0     0.3
> 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. 
> 
> -- 
> David Winsemius, MD
> Alameda, CA, USA
> 
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David Winsemius, MD
Alameda, CA, USA