[R] within: order of newly added variables

[Peter Ehlers]

Heinrich,

You could create your own function mywithin()
by inserting a couple of rev()'s in within.data.frame().

In within.data.frame(), replace the two commented lines
with those immediately following:

mywithin <-
function (data, expr, ...)
{
     parent <- parent.frame()
#    e <- evalq(environment(), data, parent)
     e <- evalq(environment(), rev(data), parent)
     eval(substitute(expr), e)
#    l <- as.list(e)
     l <- rev(as.list(e))
     l <- l[!sapply(l, is.null)]
     nD <- length(del <- setdiff(names(data), (nl <- names(l))))
     data[nl] <- l
     if (nD)
         data[del] <- if (nD == 1)
             NULL
         else vector("list", nD)
     data
}

Peter Ehlers

RINNER Heinrich wrote:
> Dear R community,
> 
> I am using function 'within' in R.2.9.1 to add variables to an existing data.frame. This works wonderful, except for one minor point: The new variables are added to the data in reverse order.
> 
> For example:
> x <- data.frame(a = 1:3, b = 4:6)
> y <- within(x, {
>      c = a^2
>      d = b^2
>      e = c+d
>      }
> )
> gives
>   a b  e  d c
> 1 1 4 17 16 1
> 2 2 5 29 25 4
> 3 3 6 45 36 9
> 
> Just what I want; except that I would prefer the columns to be in order a,b,c,d,e instead.
> 
> I could use transform ("transform(x, c=a^2, d=b^2, e=c+d)"), which preserves the specified order of variables, but that won't work here because unfortunately it doesn't find object 'd' (same with "within(x, {e = c+d; d = b^2; c = a^2})", by the way).
> 
> Of course in my toy example I can easily do something like y[, c(1:2,5:3)] afterwards, but I'd like to ask if maybe there is a simple way to make 'within' preserve the order of specified variables (or maybe someone can shed light on why this is not possible?).
> 
> Thanks,
> Heinrich.
> 
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