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Using paste to create and evaluate a variable expression

6 messages · Bryan Keller, David Winsemius, arun +2 more

David Winsemius
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On Sep 16, 2012, at 9:04 PM, Bryan Keller wrote:

            

      
      
      
    

        
Of course.

?as.expression
?parse
?eval

            

      
      
      
    

        
Instead of noquote, you might want to look at bquote.

            

      
      
      
    

        
Evaluate in what context? It looks as though you are building a formula.

parse(text= paste(paste("X", 1, sep = ""), paste("X", 2, sep = ""), sep = "+") )
expression(X1+X2)

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      arun
    

    

      
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Hi,
Try this:
expr1<-parse(text=paste(paste0("X",1:2),collapse="+"))
?eval(expr1)
#[1] 7 7 7 7 7 7
A.K.




----- Original Message -----
From: Bryan Keller <bsbkeller at gmail.com>
To: r-help at r-project.org
Cc: 
Sent: Monday, September 17, 2012 12:04 AM
Subject: [R] Using paste to create and evaluate a variable expression

Is it possible to use "paste" to write out an expression and evaluate it?
Suppose I want to add two vectors X1 and X2, defined as follows:

X1 <- 1:6
X2 <- 6:1

If I write the following it looks like what I want but is a character:
noquote(paste(paste("X", 1, sep = ""), paste("X", 2, sep = ""), sep = "+"))

Is there a way to tell R that I want to evaluate the text, not just print
it out as a character?

Bryan

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      Peter Dalgaard
    

    

      
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On Sep 17, 2012, at 06:04 , Bryan Keller wrote:

        

            

      
      
      
    

        
You need to parse() it first, then eval()uate. (Read the respective help pages for details.)


However:

            

      
      
      
    

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