scoping

[Martyn Plummer]

On 26-Jun-98 Paul Gilbert wrote:
>Can Luke Tierney's recent S-new post on scoping be included in the R FAQ?

Could this be cross-posted?

>I'm just going through my package and eliminating a few instances of "eval" as
>I
>hadn't realized the dangers. I noticed that one way I've used eval is for
>prompting, as in
>
>n <- eval(parse(prompt="Enter the number of singular values to use: "))
>
>I presume this is not too dangerous, but this has never really seemed like the
>right way to do this. Can someone suggest a better alternative?

If you just want to read in a number, I think it's a lot safer to read
the input as character data with "readline" and then coerce it to numeric.
I wrote a utility function called "read.and.check" which does this. It
also checks that the user input satisfies certain conditions (mode,
lower and upper limit for numeric data, or set of allowed values), looping
until it gets an acceptable answer. It's in the coda package, with a help page.

Martyn
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