[R] How to repeat code snippet for several variables in a data frame?

[Sean O'Riordain]

Sander,

consider writing a function to do your plotting, then pass in the
dataframe name so... something along the lines of...

# create a function which takes two arguments
# mydf - a dataframe of a particular format... eg. the abc column is number 4
# i the column we want to aggregate and plot this time around...
myplotfn <- function(mydf,i) {
  # body of the function
  # extract the relevant column - i
  colval <- mydf[3+i]
  tmp <- aggregate(colval, list(
    mydf$bins, mydf$groups),
    mean)
  # grab the name of the column we're working on; eg. "abc"
  colnam.r <- names(mydf)[3+i]
  colnames(tmp) <- c("bins", "groups", colnam.r)
  tmp
  #pltName <- paste("line_datGrassChem_", colnam.r, ".eps", sep="")
  #postscript(pltName)
    labs <- c("-15/-9","-9/-6","-6/-3","-3/0")
    sps <- trellis.par.get("superpose.symbol")
    sps$pch <- 1:4
      trellis.par.set("superpose.symbol", sps)
      # create the proper formula for plotting something like "abc ~ bins"
      myformula <- as.formula(paste(colnam.r, "~ bins"))
      xyplot( myformula, data = tmp,
      groups = groups, type = list("p", "l"),
      scales = list(x = list(labels=labs)),
      layout = c(1,1),
      key = list(columns = 4,
        text = list(paste(unique(tmp$groups))),
        points = Rows(sps, 1:4)
        )
    )
  #dev.off()
}

# then call it by 
myplotfn(test,1)
myplotfn(test,2)
myplotfn(test,3)

obviously this can be put in a loop :-)

So how did I figure out how to do this? well in the introduction
manual it talks about functions... the tricky bit was the substitution
of abc into the relevant places...

I didn't know how to convert the string formula "abc ~ bins" into a
formula that I could plot, so first off, I looked at ?plot, in there
it mentioned ?plot.formula and in there it mentioned the class formula
so I said ?formula where it mentioned as.formula()... bingo... this
allowed me to say
myformula <- as.formula("abc ~ bins")

So I made this into a "learning experience" for me :-)

I'm relatively new to R... so giving myself little problems like this
is a good way of learning R... and hopefully helping somebody else!
:-)

I'm pretty sure that there is a better way of doing this in R - but
this works :-)

cheers
Sean




On 15/08/05, Sander Oom <slist at oomvanlieshout.net> wrote:
> Dear all,
> 
> I have a data frame containing the results of an experiment. Like this:
> 
> a<-seq(1,4,by=1)
> b<-seq(1,2,by=1)
> test<-expand.grid(b,a,a)
> colnames(test)<-c("replicates","bins", "groups")
> test$abc <- rnorm(32)
> test$def <- rnorm(32)
> test$ghi <- rnorm(32)
> test
> 
> The following code snippet aggregates the data for one variable and then
> draws a plot:
> 
> tmp <- aggregate(test$abc, list(
>    test$bins, test$groups),
>    mean)
> colnames(tmp) <- c("bins", "groups", "abc")
> tmp
> #pltName <- paste("line_datGrassChem_", "abc", ".eps", sep="")
> #postscript(pltName)
>    labs <- c("-15/-9","-9/-6","-6/-3","-3/0")
>    sps <- trellis.par.get("superpose.symbol")
>    sps$pch <- 1:4
>    trellis.par.set("superpose.symbol", sps)
>    xyplot( abc ~ bins, data = tmp,
>      groups = groups, type = list("p", "l"),
>      scales = list(x = list(labels=labs)),
>      layout = c(1,1),
>      key = list(columns = 4,
>        text = list(paste(unique(tmp$groups))),
>        points = Rows(sps, 1:4)
>        )
>    )
> #dev.off()
> 
> How can I wrap R code around this code snippet, such that I can repeat
> the same code snippet for all other variables in the data frame (i.e.
> def, ghi, etc.).
> 
> Thanks for your suggestions!
> 
> Sander.
> 
> --
> --------------------------------------------
> Dr Sander P. Oom
> Animal, Plant and Environmental Sciences,
> University of the Witwatersrand
> Private Bag 3, Wits 2050, South Africa
> Tel (work)      +27 (0)11 717 64 04
> Tel (home)      +27 (0)18 297 44 51
> Fax             +27 (0)18 299 24 64
> Email   sander at oomvanlieshout.net
> Web     www.oomvanlieshout.net/sander
> 
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