R help - Apr 2005 - assign to an element of a vector

I am trying to find a way to assign values to elements of a vector
that will be defined by a user. So I don't have the name of the vector
and cannot hard code the assignment in advance. In the example below I
have to get() the vector using its name. When I try to assign to an
element I get an error:
> a <- c(1,2,3)
> get('a')[1] <- 0
Error: Target of assignment expands to non-language object

Any suggestions?

FS

how about

assign( 'a', { z <- get('a'); z[1] <- 0; z } )


-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Fernando Saldanha
Sent: Wednesday, April 27, 2005 3:22 PM
To: Submissions to R help
Subject: [R] assign to an element of a vector


I am trying to find a way to assign values to elements of a vector that will
be defined by a user. So I don't have the name of the vector and cannot hard
code the assignment in advance. In the example below I have to get() the
vector using its name. When I try to assign to an element I get an error:
> a <- c(1,2,3)
> get('a')[1] <- 0
Error: Target of assignment expands to non-language object

Any suggestions?

FS

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You did not explain the full context of what you are trying to do.  Perhaps
this could help:
> varName <- as.name("bahbah")
> varName
bahbah
> substitute(a[1] <- 0, list(a=varName))
bahbah[1] <- 0

So you could perhaps eval() this expression.

Andy
> From: Fernando Saldanha
> 
> I am trying to find a way to assign values to elements of a vector
> that will be defined by a user. So I don't have the name of the vector
> and cannot hard code the assignment in advance. In the example below I
> have to get() the vector using its name. When I try to assign to an
> element I get an error:
> 
> > a <- c(1,2,3)
> > get('a')[1] <- 0
> Error: Target of assignment expands to non-language object
> 
> Any suggestions?
> 
> FS
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
> 
> 
>

Fernando Saldanha [fsaldan1 at gmail.com] wrote:
> I am trying to find a way to assign values to elements of a vector
> that will be defined by a user.
> > a <- c(1,2,3)
> > get('a')[1] <- 0
> Error: Target of assignment expands to non-language object

Try this function:
g.assign <- function(i, pos=1, ..., value) {
  x <- if (pos > 0) get(i, pos) else get(i, , parent.frame())
  x[...] <- value
  if (pos > 0) assign(i, x, pos) else assign(i, x, , parent.frame())
}

Your example becomes:
R> a <- c(1,2,3)
R> g.assign("a", pos=1, 1, value=0)

Note you use a positive <pos> (usually pos=1) for "global"
variables,
and
pos=0 for "local" variables inside a function.  The <...>
construction
allows
this function to work for arrays too, but the downside is that you must
type
"value=" explicitly.

-- David Brahm (brahm at alum.mit.edu)