R help - Sep 2012 - 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

	[[alternative HTML version deleted]]

On Sep 16, 2012, at 9:04 PM, Bryan Keller wrote:
> Is it possible to use "paste" to write out an expression and
evaluate it?
Of course.

?as.expression
?parse
?eval
> 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 = "+"))
Instead of noquote, you might want to look at bquote.
> 
> Is there a way to tell R that I want to evaluate the text, not just print
> it out as a character?
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)

-- 
David Winsemius, MD
Alameda, CA, USA

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

??? [[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.

Bryan,

Try this.

char <- paste("X", 1:2, sep="", collapse="+")
eval(parse(text=char))

Jean


Bryan Keller <bsbkeller@gmail.com> wrote on 09/16/2012 11:04:19
PM:
> 
> 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
	[[alternative HTML version deleted]]

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

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


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



-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com