R help - Oct 1999 - sum elements of dataframe

I have the following problem.

Suppose I save the data from an experiment in the following way. There are
21 pairs of std and cf. Each pairing is an experimental condition, and 4
data values are saved. The conditions are run (and data saved) in random
order, though blocked such that all conditions are run once, then all in
random order a second time, and so on (let's say 3 or 4 repetitions)

What I want to do is to sum the data values of the repetitions.  For
example, suppose I have
std     cf      hit     miss    fa      cr
10      20      50      40      31      92
...
10	20	40	35	42	87
...
10	20	35	45	35	95

Then I want to sum the hit, miss, fa, and cr columns to get
10	20	125	120	108	274
totals for this condition.

So I will do
data<-read.table("data.dat",header=TRUE)
But then how do I sum data$hit etc across the reps?

If there is an alternative way to save the data to file(s) that makes this
task easier that is fine. E.g. I could save each condition to its own
file. But then I have to read in 21 files.

Thanks very much for any help.

Bill Simpson

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You want to use aggregate():

data<-read.table("data.dat",header=TRUE)
aggregate(data[,-(1:2)], by=list(std=data$std, cf=data$cf), sum)

On 13-Oct-99 Bill Simpson wrote:
> I have the following problem.
> 
> Suppose I save the data from an experiment in the following way. There are
> 21 pairs of std and cf. Each pairing is an experimental condition, and 4
> data values are saved. The conditions are run (and data saved) in random
> order, though blocked such that all conditions are run once, then all in
> random order a second time, and so on (let's say 3 or 4 repetitions)
> 
> What I want to do is to sum the data values of the repetitions.  For
> example, suppose I have
> std     cf      hit     miss    fa      cr
> 10      20      50      40      31      92
> ...
> 10    20      40      35      42      87
> ...
> 10    20      35      45      35      95
> 
> Then I want to sum the hit, miss, fa, and cr columns to get
> 10    20      125     120     108     274
> totals for this condition.
> 
> So I will do
> data<-read.table("data.dat",header=TRUE)
> But then how do I sum data$hit etc across the reps?
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Send "info", "help", or "[un]subscribe"
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stat.math.ethz.ch
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