R help - Apr 2004 - expression with running index

Hi,

I need a list of expression of the form expression(b[i]), where i is a
running index.  So for example, if i <- -1:2, I would like to have a
list equivalent to

list(expression(b[-1]), expression(b[0]), expression(b[1]), expression(b[2]))

"i" might be a character vector (like c("f", "g",
"h"))

Could somebody help me out by writing a function that produces the
list above for a given string in place of "b" and a vector of
subscripts?

Sorry if this has been discussed before, I tried searching the
archives but "expression" as a keyword gives too many results on
different subjects.

Thanks,

Tamas

-- 
Tam??s K. Papp
E-mail: tpapp at axelero.hu
Please try to send only (latin-2) plain text, not HTML or other garbage.

Tamas Papp wrote:
> Hi,
> 
> I need a list of expression of the form expression(b[i]), where i is a
> running index.  So for example, if i <- -1:2, I would like to have a
> list equivalent to
> 
> list(expression(b[-1]), expression(b[0]), expression(b[1]),
expression(b[2]))
> 
> "i" might be a character vector (like c("f",
"g", "h"))
> 
> Could somebody help me out by writing a function that produces the
> list above for a given string in place of "b" and a vector of
subscripts?
> 
> Sorry if this has been discussed before, I tried searching the
> archives but "expression" as a keyword gives too many results on
> different subjects.
> 
> Thanks,
> 
> Tamas
> 
For example:

  lapply(as.double(-1:2), function(x) substitute(b[i], i = list(i=x)))

Uwe Ligges

lapply(-1:2, function(i) substitute(expression(b[i]), list(i=i)))

would be a good start. (Note that what it gives is

[[1]]
expression(b[as.integer(-1)])

which is not what you asked for but is what I think you intended.

Then we can elaborate this to 

f <- function(ind, vec)
    lapply(ind, 
           function(i, vec) substitute(expression(vec[i]), 
                                       list(i=i, vec=vec)), 
           vec=as.name(vec))

f(-1:2, "b") and f(c("f", "g", "h"),
"b") both work

Also, I am not sure you need the expression() in there, as without it you 
have a language call which will almost certainly do.


On Sun, 25 Apr 2004, Tamas Papp wrote:
> Hi,
> 
> I need a list of expression of the form expression(b[i]), where i is a
> running index.  So for example, if i <- -1:2, I would like to have a
> list equivalent to
> 
> list(expression(b[-1]), expression(b[0]), expression(b[1]),
expression(b[2]))
> 
> "i" might be a character vector (like c("f",
"g", "h"))
> 
> Could somebody help me out by writing a function that produces the
> list above for a given string in place of "b" and a vector of
subscripts?
> 
> Sorry if this has been discussed before, I tried searching the
> archives but "expression" as a keyword gives too many results on
> different subjects.
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595