Explicitly parse part of R code in a script

Question

My usecase is as follows:

Display GUI to collect some R expressions
Generate a single source file containing my simulation framework, intermixed with the R expressions provided by the user
Allow the user to generate the single source file. If any of the expressions specified by the user contain errors (syntax or runtime), I want to display these in a user-friendly way.

In short, I want to do something as follows:

myExpression <- "3 * 5" myExprParsed <- tryCatch( parse(myExpression), .... )

My question is very simple: does R have some construct like \Q \E in regex, fully quoting a text string from beginning to end ? It is not unlikely that myExpression will contain the " character, thereby introducing a syntax error. I do not want to write the text contents to a separate text file and use source().

=Example=

myExpression <- " XXXXXXX "

where XXXXXX is paste("my random value is ", runif(3)) would amount to

myExpression <- " paste("my random value is ", runif(3)) "

which would give a syntax error. I want something like

myExpression <- verbatim@ paste("my random value is ", runif(3)) @

Dont understand your question. What has a regex got to do with it? Why would having quotes input by your users be a problem? They're being stored in strings, which isn't a problem. — Spacedman
– Spacedman, Commented Oct 17, 2012 at 9:04

Spacedman · Accepted Answer · 2012-10-17 09:07:46Z

Get some input from the user - you'd just be getting values from text boxes in your GUI, I guess. I'll use scan:

> dowhat = scan(what="") 1: x="hello world" 3: Read 2 items

We have quoted strings:

> dowhat [1] "x=\"hello" "world\""

We can parse it:

> parse(text=dowhat) expression(x="hello world")

And there's a line break because scan() splits things by lines. Not really a problem, and fixable if you care.

Now let's actually run it:

> eval(parse(text=dowhat))

And now we should have an x:

> x [1] "hello\nworld" >

?

So I will have to import the code from somewhere: either user input, or a file, or ... (it seems that I cannot just enter blocks of code like I can in e.g. XML using [CDATA )

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