Getting strings recognized as variable names in R

Related: Strings as variable references in R
Possibly related: Concatenate expressions to subset a dataframe

I've simplified the question per the comment request. Here goes with some example data.

dat <- data.frame(num=1:10,sq=(1:10)^2,cu=(1:10)^3)
set1 <- subset(dat,num>5)
set2 <- subset(dat,num<=5)

Now, I'd like to make a bubble plot from these. I have a more complicated data set with 3+ colors and complicated subsets, but I do something like this:

symbols(set1$sq,set1$cu,circles=set1$num,bg="red")
symbols(set2$sq,set2$cu,circles=set2$num,bg="blue",add=T)

I'd like to do a for loop like this:

colors <- c("red","blue")
sets <- c("set1","set2")
vars <- c("sq","cu","num")

for (i in 1:length(sets)) {
   symbols(sets[[i]][,sq],sets[[i]][,cu],circles=sets[[i]][,num],
   bg=colors[[i]],add=T)
}

I know you can have a variable evaluated to specify the column (like var="cu"; set1[,var]; I want to know how to get a variable to specify the data.frame itself (and another to evaluate the column).

Update: Ran across this post on r-bloggers which has this example:

x <- 42
eval(parse(text = "x"))
[1] 42

I'm able to do something like this now:

eval(parse(text=paste(set[[1]],"$",var1,sep="")))

In fiddling with this, I'm finding it interesting that the following are not equivalent:

vars <- data.frame("var1","var2")
eval(parse(text=paste(set[[1]],"$",var1,sep="")))
eval(parse(text=paste(set[[1]],"[,vars[[1]]]",sep="")))

I actually have to do this:

eval(parse(text=paste(set[[1]],"[,as.character(vars[[1]])]",sep="")))

Update2: The above works to output values... but not in trying to plot. I can't do:

for (i in 1:length(set)) {
symbols(eval(parse(text=paste(set[[i]],"$",var1,sep=""))),
       eval(parse(text=paste(set[[i]],"$",var2,sep=""))),
       circles=paste(set[[i]],".","circles",sep=""),
       fg="white",bg=colors[[i]],add=T)
}

I get invalid symbol coordinates. I checked the class of set[[1]] and it's a factor. If I do is.numeric(as.numeric(set[[1]])) I get TRUE. Even if I add that above prior to the eval statement, I still get the error. Oddly, though, I can do this:

set.xvars <- as.numeric(eval(parse(text=paste(set[[i]],"$",var1,sep=""))))
set.yvars <- as.numeric(eval(parse(text=paste(set[[i]],"$",var2,sep=""))))
symbols(xvars,yvars,circles=data$var3)

Why different behavior when stored as a variable vs. executed within the symbol function?

You found one answer, i.e. eval(parse()) . You can also investigate do.call() which is often simpler to implement. Keep in mind the useful as.name() tool as well, for converting strings to variable names.

The basic answer to the question in the title is eval(as.symbol(variable_name_as_string)) as Josh O'Brien uses. e.g.

var.name = "x"
assign(var.name, 5)
eval(as.symbol(var.name)) # outputs 5

Or more simply:

get(var.name) # 5

Getting strings recognized as variable names in R

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