R help - May 2012 - Pasting with Quotes

Hello useRs!

So, I have a random question.  I'm trying to build a character string, then
evaluate it.  I think an example would be the easiest way to explain:


    kern.vec = c("rbfdot","polydot")
    for( j in 1:length( kern.vec ) )
    {
      formula    = paste("ksvm( ind ~ . ,
data=d.temp[,c(ind_col,dep_cols)], kernel =",kern.vec[j],",
prob.model=T )")
      svm    = eval( parse( text=formula ) )
     ...
    }


The problem I always seem to have is that in the formula, I need to have
quotes around "rbfdot" (for example).  But, when I paste the
expression
together, it removes the quotes.  Is there a better way to do this (or at
least a way around this problem)?  My method seems a bit kludgy :)

Thanks for all your help!

Josh

	[[alternative HTML version deleted]]

What's the "big picture" of what you're trying to do?
eval(parse(text
= )) is often a less than optimal idea.

Some guesses:

Are you trying to construct a formula object (in the strict sense of
something that you pass to a modeling function)?

Maybe lazy evaluation of the deparse(substitute(x)) flavor might help here?

Michael

On Sat, May 5, 2012 at 2:42 PM, Josh Browning
<rockclimber112358 at gmail.com> wrote:
> Hello useRs!
>
> So, I have a random question. ?I'm trying to build a character string,
then
> evaluate it. ?I think an example would be the easiest way to explain:
>
>
> ? ?kern.vec = c("rbfdot","polydot")
> ? ?for( j in 1:length( kern.vec ) )
> ? ?{
> ? ? ?formula ? ?= paste("ksvm( ind ~ . ,
> data=d.temp[,c(ind_col,dep_cols)], kernel =",kern.vec[j],",
prob.model=T )")
> ? ? ?svm ? ?= eval( parse( text=formula ) )
> ? ? ...
> ? ?}
>
>
> The problem I always seem to have is that in the formula, I need to have
> quotes around "rbfdot" (for example). ?But, when I paste the
expression
> together, it removes the quotes. ?Is there a better way to do this (or at
> least a way around this problem)? ?My method seems a bit kludgy :)
>
> Thanks for all your help!
>
> Josh
>
> ? ? ? ?[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

On May 5, 2012, at 2:42 PM, Josh Browning wrote:
> Hello useRs!
>
> So, I have a random question.  I'm trying to build a character  
> string, then
> evaluate it.
Actually you are trying to build a language object , a call or an  
expression. You might have gotten further with:

do.call(ksvm, list(  ... named arguments ...)
>  I think an example would be the easiest way to explain:
>
>
>    kern.vec = c("rbfdot","polydot")
>    for( j in 1:length( kern.vec ) )
>    {
>      formula    = paste("ksvm( ind ~ . ,
> data=d.temp[,c(ind_col,dep_cols)], kernel =",kern.vec[j],",  
> prob.model=T )")
>      svm    = eval( parse( text=formula ) )
>     ...
>    }
>
>
> The problem I always seem to have is that in the formula, I need to  
> have
> quotes around "rbfdot" (for example).  But, when I paste the  
> expression
> together, it removes the quotes.  Is there a better way to do this  
> (or at
> least a way around this problem)?  My method seems a bit kludgy :)
Perhaps looking at either:

?substitute
?bquote

kern.vec = c("rbfdot","polydot")
    for( j in 1:length( kern.vec ) )
    {
      formula    = bquote(expression( ksvm( ind ~ . ,  
data=d.temp[,c(ind_col,dep_cols)], kernel =.(kern.vec[j] ) ,  
prob.model=T ) ))
      print(formula)
    }

expression(ksvm(ind ~ ., data = d.temp[, c(ind_col, dep_cols)],
     kernel = "rbfdot", prob.model = T))
expression(ksvm(ind ~ ., data = d.temp[, c(ind_col, dep_cols)],
     kernel = "polydot", prob.model = T))

Notice that the values for kern.vec are 'character' which is what you  
passed them (and what you seem to be requesting. If you wanted the  
values of those named objects you might try get(<name>). (I'm not a  
user of whatever package has `ksvm` in it, so I'm not aware of whether  
'rbfdot' is supposed to be a character value as a parameter or if  
those are named objects, and I'm not running out to identify the  
package and then to locate a working example to test the eval-result.  
Those are details you should have provided.)
>

-- 

David Winsemius, MD
West Hartford, CT