?apply

as in:

answer<-apply(yourframe,2,function(x)x[EXP==1 & DOSE==1])

Note that there are slicker ways to do this call.

Note also for that for the particular case of column means, you have a

built-in much faster alternative:

answer<-colMeans(yourframe[EXP==1 & DOSE==1,],na.rm=TRUE)

Also note that the == construction may be problematic if EXP or DOSE are not

factors, say.

Finally, you need to start reading the R docs to learn about this yourself.

"An Introduction to R" is a good place to start.

Oh... and your attempt below will certainly not work. After you read the

basic docs, you'll see why not.

-- Bert Gunter

Genentech Non-Clinical Statistics

South San Francisco, CA

> -----Original Message-----

> From:

[hidden email]
> [mailto:

[hidden email]] On Behalf Of Kevin Middleton

> Sent: Friday, February 03, 2006 1:41 PM

> To:

[hidden email]
> Subject: [R] Using string vectors as for loop arguments

>

> Is there a mechanism to interate through a vector of strings? Say I

> have a data frame of 50 variables (VAR1 to VAR50), each with 100

> measurements along with some coding factors (EXP and DOSE). I

> want to

> calculate the mean of a subset of each of VAR1 to VAR 50 (selecting

> by EXP and DOSE). Rather than just copy/pasting the same code 50

> times, I thought of using a for loop to go through a vector of VAR1

> to VAR50 (VAR.LIST below). Each iteration would be the variable name

> for which the mean would be calculated.

>

> Trying this, I get an error that the argument is not numeric or

> logical. So I assume that for loops with vectors of strings are

> disallowed. What I am wondering is whether there a way to mimic this

> sort of procedure?

>

> The code looked something like this:

>

> VAR.LIST<-paste(c("VAR"), 1:50, sep="")

>

> for (i in VAR.LIST){

> mean(i[EXP==1 & DOSE==1], na.rm=T)

> }

>

>

>

> Thanks

> Kevin Middleton

>

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