[R] Can you turn an input into a variable name?

[Greg Snow]

This is in part answered by FAQ 7.21.

The most important part of that answer is at the bottom where it says
that it is usually better to use a list.

It may be safer to use a list for your case so that other important
variables do not become masked (hidden by the global variables you
just created).

Here is one way to do it:

txtcon <- textConnection("color=green
shape=circle")

textlines<-readLines(txtcon)
tmp <- strsplit(textlines, "=")

mylist <- list()
for(i in tmp) {
  mylist[[ i[1] ]] <- i[2]
}

or another option:

mylist[ sapply(tmp, `[`, 1)] <- sapply(tmp, `[`, 2)

If you know that you will not need a regular expression then you can
just use the `sep` argument to `read.table`:

txtcon <- textConnection("color=green
shape=circle")

tmp2 <- read.table(txtcon, sep='=', stringsAsFactors = FALSE)
mylist <- list()
mylist[ tmp2[,1] ] <- tmp2[,2]

Now you can use functions like `with`, `within`, `evalq`, etc. to work
with the elements of the list as if they were variables:

> with(mylist, ls() )
[1] "color" "shape"
> with(mylist, color)
[1] "green"
> mylist$shape
[1] "circle"
> vname <- 'color'
> mylist[[vname]]
[1] "green"
> evalq(paste('it is a', color, shape), mylist)
[1] "it is a green circle"

If you need regular expressions to find your names and values (extra
spaces, lines without '=', pairs embedded in longer sentences, etc.)
then look at the `regmatches` function or possibly the gsubfn package
(or other tools).


On Fri, Oct 4, 2019 at 12:59 AM April Ettington
<aprilettington using gmail.com> wrote:
>
> Let's say I am parsing a file with a list of parameters followed by an
> equal sign, and their corresponding values, eg:
>
> color=green
> shape=circle
>
> and I want to use this information to create a variable called color with
> the value 'green' and a variable shape with the value 'circle'.  However, I
> also want my code to be able to do this *when it doesn't know up front what
> the parameter names will be.  *So, if the file also included "age=7", it
> should still make a variable called age with the value '7', even though
> 'age' doesn't specifically appear anywhere in my code.  Is there a way to
> do this?
>
> Thank you,
> April
>
>         [[alternative HTML version deleted]]
>
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-- 
Gregory (Greg) L. Snow Ph.D.
538280 using gmail.com