R help - Sep 2009 - list of symbols to substitution

Hi,

I'm trying to use a list of symbols as one of the values to be
substituted in a substitute expression, but I can't figure out how to
write the correct expression for my problem.  Let me illustrate a
simple example of what I'm trying to do.  The following code snippet
will evaluate to '5':

symname <- 'foo'
foo <- 5
expr <- substitute(c(expr), list(expr = as.symbol(symname)))
eval(expr)

I would like the next similar example to evaluate to the list('5',
'6', '7'), but instead it evaluates to the list(foo, bar, baz)
where
the type of foo, bar, and baz are all symbols.  For the purposes of
finding a solution assume that the length and contents of symnames are
unknown ahead of time (symname is not a constant, assume it is some
parameter to a function).

symnames <- c('foo', 'bar', 'baz')
foo <- 5
bar <- 6
baz <- 7
expr <- substitute(expr, list(expr = lapply(symnames, as.symbol)))
eval(expr)

Thanks!
--Michael

Instead of eval(expr), use lapply(expr,eval)

HTH
Schalk Heunis


On Wed, Sep 16, 2009 at 5:50 AM, Michael Spiegel <
michael.m.spiegel@gmail.com> wrote:
> Hi,
>
> I'm trying to use a list of symbols as one of the values to be
> substituted in a substitute expression, but I can't figure out how to
> write the correct expression for my problem.  Let me illustrate a
> simple example of what I'm trying to do.  The following code snippet
> will evaluate to '5':
>
> symname <- 'foo'
> foo <- 5
> expr <- substitute(c(expr), list(expr = as.symbol(symname)))
> eval(expr)
>
> I would like the next similar example to evaluate to the list('5',
> '6', '7'), but instead it evaluates to the list(foo, bar,
baz) where
> the type of foo, bar, and baz are all symbols.  For the purposes of
> finding a solution assume that the length and contents of symnames are
> unknown ahead of time (symname is not a constant, assume it is some
> parameter to a function).
>
> symnames <- c('foo', 'bar', 'baz')
> foo <- 5
> bar <- 6
> baz <- 7
> expr <- substitute(expr, list(expr = lapply(symnames, as.symbol)))
> eval(expr)
>
> Thanks!
> --Michael
>
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