R help - Oct 2010 - transfer string to expression

Dear all:

when I use parse() there is some problems. Below is an example:

b0<-1
b1<-1
x<-1
str2expr<-function(x){eval(parse(text=x))}
test1<-"b0+b1*sqrt(x)"
test2<-"b0+b1"
str2expr(test1)
str2expr(test2)

it can work well for test2 but not for test1.

Could you tell me how to fix this problem or is there other more stable
method to transfer an string to expression?

best regards

Elong

	[[alternative HTML version deleted]]

On 31/10/2010 3:22 PM, Yilong Zhang wrote:
> Dear all:
>
> when I use parse() there is some problems. Below is an example:
>
> b0<-1
> b1<-1
> x<-1
> str2expr<-function(x){eval(parse(text=x))}
> test1<-"b0+b1*sqrt(x)"
> test2<-"b0+b1"
> str2expr(test1)
> str2expr(test2)
>
> it can work well for test2 but not for test1.
>
> Could you tell me how to fix this problem or is there other more stable
> method to transfer an string to expression?
You are evaluating those expressions in the evaluation frame of your 
function, where "x" is the expression.  Just specify the environment
in
which you want to evaluate them, e.g.

str2expr<-function(x){eval(parse(text=x), envir=parent.frame() )}

Duncan Murdoch
>
> best regards
>
> Elong
>
> 	[[alternative HTML version deleted]]
>
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> and provide commented, minimal, self-contained, reproducible code.

On Oct 31, 2010, at 3:22 PM, Yilong Zhang wrote:
> Dear all:
>
> when I use parse() there is some problems. Below is an example:
>
> b0<-1
> b1<-1
> x<-1
> str2expr<-function(x){eval(parse(text=x))}
> test1<-"b0+b1*sqrt(x)"
> test2<-"b0+b1"
> str2expr(test1)
> str2expr(test2)
I don't think the scoping rules are up to the task of keeping the x's  
distinct:

 > b0<-1
 > b1<-1
 > xx<-1
 > str2expr<-function(x){eval(parse(text=x))}
 > test1<-"b0+b1*sqrt(xx)"
 > test2<-"b0+b1"
 > str2expr(test1)
[1] 2
 > str2expr(test2)
[1] 2
>
> it can work well for test2 but not for test1.
>
> Could you tell me how to fix this problem or is there other more  
> stable
> method to transfer an string to expression?
I generally try to keep the variables distinct so my brain can handle  
the various levels. So I would probably not have used x in that  
manner. I think it made the interpreter attempt to take the sqrt of  
"b0+b1*sqrt(x)", which gave you the "Error in sqrt(x) :
Non-numeric
argument to mathematical function"

And apropos is this:
 > fortune("If the answer is parse()")

If the answer is parse() you should usually rethink the question.
    -- Thomas Lumley
       R-help (February 2005)

-- 
David Winsemius, MD
West Hartford, CT