[R] creating a variable using concatenation

[David Winsemius]

On Mar 30, 2010, at 9:24 PM, zubin wrote:

> A general problem i run into, i know there must be a simple solution.
>
> I like to create a variable by appending a 1 for example, (i need to
> loop later on from 1 to X, thus the reason for this).   So i assign  
> the
> variable vplot with this value, however it has quotes and when i use  
> it
> in a barplot, it throws an error.  but the tcenter$X1 does exist,  
> its an
> element of a data frame.  So if i type directly it works, but i like  
> to
> do this programmatically, as i have to generate a bunch of these plots
> and need to loop.
>
> So how do i concatenate to create a variable, then reference that
> variable in a function call?
>
>
> R> x <- data.frame(1,2,3,4,5,6,7,8)
> R> x
>  X1 X2 X3 X4 X5 X6 X7 X8
> 1  1  2  3  4  5  6  7  8
> R> x$X1
> [1] 1
>
> R> i=1
> R> toplot <- paste("x$X",i,sep="")
> R> toplot
> [1] "x$X1"
>
Yes, except consider what happens when you offer  "x$X1" to the  
interpreter?

[1] "x$X1"

The interpreter does not take it any further because that character  
object is atomic, at least I hope that is the correct terminology. You  
would need to wrap it up in eval(parse(text= "x$X1") or  
eval(parse(text=toplot)) to get it processed of as a language object.


> okay lets test:
>
> R> plot(x$X1)
> -it works i see the plot
>
> however this DOES not work
>
> R> plot(toplot)

What about plot(x[1]) or plot(x[[1]]) or plot(x[i]) or plot(x[[i]])?  
Those should all work.
>
> Error in plot.window(...) : need finite 'ylim' values
> In addition: Warning messages:
> 1: In xy.coords(x, y, xlabel, ylabel, log) : NAs introduced by  
> coercion
> 2: In min(x) : no non-missing arguments to min; returning Inf
> 3: In max(x) : no non-missing arguments to max; returning -Inf
>
>
> Thus, that's my problem, i know it must be simple -  the variable is  
> equal to x$X1 but it does not work in a function call?  i tried many  
> functions - always some type of error.

I think we may need to see what you are planning on doing with this  
knowledge, to know what sort of knowledge to offer.

David Winsemius, MD
West Hartford, CT