R help - Jun 2007 - non permanent change of vector values

Hi All,

I have the following problem: I have a vector

x = rep(0,15)
x[1:2] = 1
x
  [1] 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0

I need to be able to call that vector 'x' so that if condition
'A' is true, only
the first value is kept 'as is' and all the others are put to 0

if(A == T)

function(x) with x returning 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0

and if 'A' is false the second value is kept 'as is' and all the
others are put to 0

if(A == F)

function(x) with x returning 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0

BUT, and that's the rub, I need x to changed in a *non permanent* way, so
that
at the end x is still

x
  [1] 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0

(that is because condition 'A' might be called again and could be
different in
it's T/F state from previous calls).

Any ideas?

Cheers,

Fede

-- 
Federico C. F. Calboli
Department of Epidemiology and Public Health
Imperial College, St Mary's Campus
Norfolk Place, London W2 1PG

Tel  +44 (0)20 7594 1602     Fax (+44) 020 7594 3193

f.calboli [.a.t] imperial.ac.uk
f.calboli [.a.t] gmail.com

On 20/06/2007 6:46 AM, Federico Calboli wrote:
> Hi All,
> 
> I have the following problem: I have a vector
> 
> x = rep(0,15)
> x[1:2] = 1
> x
>   [1] 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0
> 
> I need to be able to call that vector 'x' so that if condition
'A' is true, only
> the first value is kept 'as is' and all the others are put to 0
> 
> if(A == T)
> 
> function(x) with x returning 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> 
> and if 'A' is false the second value is kept 'as is' and
all the others are put to 0
> 
> if(A == F)
> 
> function(x) with x returning 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0
> 
> BUT, and that's the rub, I need x to changed in a *non permanent* way,
so that
> at the end x is still
> 
> x
>   [1] 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0
> 
> (that is because condition 'A' might be called again and could be
different in
> it's T/F state from previous calls).
Simply make a function that does what you want:

modifyx <- function(x, A) {
   if (A) x[-1] <- 0
   else x[-2] <- 0
   x
}

then call your function by passing modifyx(x, A) instead of just x.

You don't need to put A or x in the argument list of the function, but 
it probably makes sense to do so.

Duncan Murdoch