Constructing variables within a loop and using them

Question

I'm trying to loop over variables (I'm still struggling to get my head around the apply() family, one day soon) and construct the variable names that I wish to obtain the labels for.

A simplified example is perhaps easiest...

library(Hmisc)
t <- data.frame(matrix(1:100, 10))
label(t$X1)  <- "This is my first variable"
label(t$X2)  <- "This is my second variable"
label(t$X3)  <- "This is my third variable"
label(t$X4)  <- "This is my fourth variable"
label(t$X5)  <- "This is my fifth variable"
label(t$X6)  <- "This is my sixth variable"
label(t$X7)  <- "This is my seventh variable"
label(t$X8)  <- "This is my eighth variable"
label(t$X9)  <- "This is my ninth variable"
label(t$X10) <- "This is my tenth variable"
for(x in 1:10){
  my.label <- label(paste("t$X", x, sep=""))
  print(my.label)
}

When run this gives....

[1] ""
[1] ""
[1] ""
[1] ""
[1] ""
[1] ""
[1] ""
[1] ""
[1] ""
[1] ""

When I'm expecting to see

[1] "This is my first variable"
[1] "This is my second variable"
[1] "This is my third variable"
[1] "This is my fourth variable"
[1] "This is my fifth variable"
[1] "This is my sixth variable"
[1] "This is my seventh variable"
[1] "This is my eighth variable"
[1] "This is my ninth variable"
[1] "This is my tenth variable"

I know the paste() function is working correctly because...

for(x in 1:10){
  print(paste("t$X", x, sep=""))
}
[1] "X$1"
[1] "X$2"
[1] "X$3"
[1] "X$4"
[1] "X$5"
[1] "X$6"
[1] "X$7"
[1] "X$8"
[1] "X$9"
[1] "X$10"

I'm stumped, I've tried placing the paste() within eval() as in...

for(x in 1:10){
  my.label <- label(eval(paste("t$X", x, sep="")))
  print(my.label)
}

...but no joy. It seems such a simple thing, and I've tried searching for solutions but clearly aren't describing it properly with phrases I'm trying hence asking here.

Insights and pointers very much appreciated.

Thanks,

slackline

EDIT : The above is a simplified example to illustrate what I am trying to achieve which is a bit more complex, currently my code looks like...

for(type1 in c("bouldering", "routes")){
  if(type1 == "bouldering"){
    part   <- c("indoors", "outdoors")
    xlabel <- "Grade (Fontainebleau)"
  }
  else if(type1 == "routes"){
    part <- c("onsight", "redpoint")
    xlabel <- "Grade (French Sports)"
  }
  for(type2 in part){
    for(training in c("pullup.rep", "pullup.weight", "hang.time", "hang.size", "bench.press", "bench.press.scaled", "dead.lift", "dead.lift.scaled", "front.lever", "height", "weight", "bmi")){
      ### Obtain the current variables label for using in the graph
      ylabel <- label(paste("clean.data", training, sep="$"))
      ### Paste the bouldering/routes together with indoors/
      ### outdoors or onsight/redpoint so variables and files can be constructed
      file.stub <- paste(type1, type2, sep="-")
      metric    <- paste(type1, type2, sep=".")
      file.out  <- paste("latex/figures/", gsub("\\.", "-", training) , "-", file.stub, ".png", sep="")
      png(file.out, width=1024, height=768)
      t <- qplot(metric, training,
                 data     = clean.data,
                 geom     = "boxplot",
                 fill     = factor(metric),
                 xlab     = xlabel,
                 ylab     = ylabel)
      t + opts(legend.position = "none")
      dev.off()
    }
  }
}

So currently I don't get a label, and I don't get graphs because the commands (label() and qplot()) don't know that I'm referring to column names with the data frame clean.data

Answer 1

Hey this is kind of a 'hack', but try this:

for (i in 1:10)    {
my.label <- label(t[,names(t)==paste("X", i, sep="")])
print(my.label)
}

So instead of turning the pasted material into a data frame column call, you turn the column name into a character string. It worked for me when I tried it.

Answer 2

This will work:

for(x in 1:10){
  my.label <- label(t[paste("X", x, sep="")])
  print(my.label)
}

This would be simpler:

label(t)