help with calculation from dataframe with multiple entries per sample

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Julie -
    Since the apply functions operate on one row at a time, they
can't do what you want.  I think the easiest way to solve your 
problem is to reshape the data set, and merge it back with the 
original:
dd = data.frame(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))
rdd = reshape(dd,timevar='Time',idvar='Sample',direction='wide')
rdd$"Gain2-3" = rdd$Mass.3 - rdd$Mass.2
merge(dd,subset(rdd,select=c('Sample',"Gain2-3")))
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

You may want to avoid using special characters like dashes in variable
names.

Hope this helps.

 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu

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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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]) ) )
  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

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

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

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
David Winsemius, MD
Alameda, CA, USA
Or diff(x[2:3])

Rui Barradas
Em 18-09-2012 01:05, David Winsemius escreveu:
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

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
David Winsemius, MD
Alameda, CA, USA

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
HI,
Try this:
?mydata$Gain<-rep(tapply(mydata$Mass,mydata$Sample,FUN=function(x) (x[3]-x[2])),each=length(unique(mydata$Sample)))
?mydata
#? Sample Time Mass Gain
#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
A.K.

----- Original Message -----
From: Julie Lee-Yaw <julleeyaw at yahoo.ca>
To: "r-help at r-project.org" <r-help at r-project.org>
Cc: 
Sent: Monday, September 17, 2012 7:15 PM
Subject: [R] help with calculation from dataframe with multiple entries per sample

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!
??? [[alternative HTML version deleted]]

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
HI,
Modified version of my earlier solution:
res1<-tapply(mydata$Mass,mydata$Sample,FUN=function(x) (x[3]-x[2]))
res2<-data.frame(Sample=names(res1),Gain2_3=res1)
?merge(mydata,res2)

#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
A.K.

----- Original Message -----
From: Julie Lee-Yaw <julleeyaw at yahoo.ca>
To: "r-help at r-project.org" <r-help at r-project.org>
Cc: 
Sent: Monday, September 17, 2012 7:15 PM
Subject: [R] help with calculation from dataframe with multiple entries per sample

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!
??? [[alternative HTML version deleted]]

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

HI,
Try this:
 mydata$Gain<-rep(tapply(mydata$Mass,mydata$Sample,FUN=function(x) (x[3]-x[2])),each=length(unique(mydata$Sample)))
 mydata
#  Sample Time Mass Gain
#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
A.K.
That is going to fail as soon as there are an uneven number of rows for one value of Sample.
Sample<-c(1,1,1,2,2,2,3,3,3,3)
Time<-c(1,2,3,1,2,3,1,2,3, 4)
Mass<-c(3,3.1,3.4,4,4.3,4.4,3,3.2,3.5, 3.7)
mydata<-as.data.frame(cbind(Sample,Time,Mass))
 mydata$Gain<-rep(tapply(mydata$Mass,mydata$Sample,FUN=function(x) (x[3]-x[2])),each=length(unique(mydata$Sample)))
Error in `$<-.data.frame`(`*tmp*`, "Gain", value = c(0.3, 0.3, 0.3, 0.100000000000001,  : 
  replacement has 9 rows, data has 10

----- Original Message -----
From: Julie Lee-Yaw <julleeyaw at yahoo.ca>
To: "r-help at r-project.org" <r-help at r-project.org>
Cc: 
Sent: Monday, September 17, 2012 7:15 PM
Subject: [R] help with calculation from dataframe with multiple entries per sample

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!
    [[alternative HTML version deleted]]

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

David Winsemius, MD
Alameda, CA, USA
The following works even when the input data frame has its rows
scrambled.  It does not currently check that there is exactly one entry
in each sample for Time==2 and Time==3.

within(mydata, 
            `Gain2-3` <- ave(seq_along(Sample),
                                          Sample,
                                          FUN=function(i) {
                                             L2 <- Time[i]==2
                                             L3 <- Time[i]==3
                                             Mass[i][L3] - Mass[i][L2] }))

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
Of arun
Sent: Monday, September 17, 2012 8:12 PM
To: Julie Lee-Yaw
Cc: R help
Subject: Re: [R] help with calculation from dataframe with multiple entries per sample

HI,
Modified version of my earlier solution:
res1<-tapply(mydata$Mass,mydata$Sample,FUN=function(x) (x[3]-x[2]))
res2<-data.frame(Sample=names(res1),Gain2_3=res1)
?merge(mydata,res2)

#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
A.K.

----- Original Message -----
From: Julie Lee-Yaw <julleeyaw at yahoo.ca>
To: "r-help at r-project.org" <r-help at r-project.org>
Cc:
Sent: Monday, September 17, 2012 7:15 PM
Subject: [R] help with calculation from dataframe with multiple entries per sample

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!
??? [[alternative HTML version deleted]]

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______________________________________________
R-help at r-project.org mailing list
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
Thanks everyone for the help! I pulled together a bunch of your suggestions
to get the result that I needed. I'm posting my final code below. Probably
not the most efficient way of doing things but gets the job done in a way
that a newbie can understand!

##Here again is the example dataset

Sample<-c(1,1,1,2,2,2,3,3,3)
Mass<-c(3,3.1,3.4,4,4.3,4.4,3,3.2,3.5)
Time<-c(1,2,3,1,2,3,1,2,3)
mydata<-as.data.frame(cbind(Sample,Time,Mass))

## I split the dataset by Sample and then calculate the difference between
mass at time 3 and mass at time 2 for each Sample; then use the merge
function to attach this data to my initial dataset

sp<-split(mydata,mydata$Sample)
y<-rbind(lapply(sp,function(x){Gain<-x$Mass[x$Time==3]-x$Mass[x$Time==2]}))

## note here that as I modification to some of the suggestions posted, I
wanted a way to specifically call "mass at time 3" etc. for each sample
rather than relying on the position of such data within each split/Sample
(hence allowing me to deal with samples that may have the Time/Mass data
input in a different order

# some massaging of the results
u<-t(y)
s<-data.frame(Sample=row.names(u),Gain2_3=u)
fulldata<-merge(mydata,s)

## as I wished to export the data in the end using write.csv, I had to
covert "list" data into "numeric" in the final dataframe

fulldata$Gain<-as.numeric(fulldata$Gain2_3) 
fulldata$Gain2_3<-NULL

Thanks again everyone!

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