[R] Evaluate variable

[Douglas Bates]

You probably want to use substitute() to construct your formula and be
careful of the distinction between character strings and names

> substitute(foo ~ bar, list(foo = as.name("y"), bar = as.name("x")))
y ~ x

On Fri, Apr 9, 2010 at 2:06 PM, Nic Rivers <njrivers at sfu.ca> wrote:
> Dear R-users:
>
> I would like to create a system of regression equations of length n, where
> the variables are drawn from a data frame.  The result I would like is given
> by the variable named "system" in the code below.  However, when I use a
> loop to create the system of equations, I cannot seem to get the expressions
> to evaluate immediately, and so the variable "system2" does not replicate
> the result I would like.  Is there a way to force the evaluation of the loop
> variable "i" (I have tried eval() and force() without success)?
>
> Thanks for your help,
>
> Nic Rivers
> Simon Fraser University
>
>
> dat <-
> data.frame(lhs.1=rnorm(100,1,1),lhs.2=rnorm(100,2,1),rhs.1=rnorm(100,3,1),rhs.2=rnorm(100,4,1))
>
> lhs <- c("lhs.1","lhs.2")
> rhs <- c("rhs.1","rhs.2")
>
> n <- 2 # number of equations
> r <- vector("list",length=n)
>
> # create system of equations manually
> r[["1"]] <- dat[lhs[1]] ~ dat[rhs[1]]
> r[["2"]] <- dat[lhs[2]] ~ dat[rhs[2]]
> system <- c(r[["1"]],r[["2"]])
>
> # create system of equations with a loop
> system2 <- list()
> for (i in 1:n) {
>        r[[i]] <- dat[lhs[i]] ~ dat[rhs[i]]
>        system2 <- append(system2,r[i])
>        }
>
> system
> system2
>
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