[R] changing character to a vector name

[Gabor Grothendieck]

Laura Holt <lauraholt_983 <at> hotmail.com> writes:

.> I would like to generate a vector/variable name from within a loop to be 
.> passed to a table
.> function.
.> 
.> This is what I have so far:
.> >assign("p1",paste("raw3.df$",rw2$V1[3],sep=""))
.> >p1
.> [1] "raw3.df$CITIZEN"
.> >
.> Essentially, I want to use the raw3.df$CITIZEN along with another value to 
.> generate a table.

You can use eval and parse to parse and execute 
arbitrary R commands that you construct from strings.  The exec
function below is a one statement convenience function
that pastes its arguments together and executes them this way in
its parent's environment.  (Sometimes when one gets complex
requirements like this if one backs up a bit one discovers
that the requirements can be simplified and you might
consider whether that is the case here in which case you
might be able to disregard all this.)

R> ### create some test data ###
R> data(iris)
R> irish <- head(iris)
R> irish
  Sepal.Length Sepal.Width Petal.Length Petal.Width Species
1          5.1         3.5          1.4         0.2  setosa
2          4.9         3.0          1.4         0.2  setosa
3          4.7         3.2          1.3         0.2  setosa
4          4.6         3.1          1.5         0.2  setosa
5          5.0         3.6          1.4         0.2  setosa
6          5.4         3.9          1.7         0.4  setosa

R> ### this is the actual processing ###
R> exec <- function(...) eval.parent(parse(text = paste(..., collapse = "")))
R> exec("irish$Sepal.Length", "<-", "irish$Sepal.Length", "+1")
R> irish
  Sepal.Length Sepal.Width Petal.Length Petal.Width Species
1          6.1         3.5          1.4         0.2  setosa
2          5.9         3.0          1.4         0.2  setosa
3          5.7         3.2          1.3         0.2  setosa
4          5.6         3.1          1.5         0.2  setosa
5          6.0         3.6          1.4         0.2  setosa
6          6.4         3.9          1.7         0.4  setosa