[datatable-help] "Within" Reversed column order?

[Yike Lu]

Thinking through this, the reasons I use within:
1) Avoid chained [:=]
2) Better semantics than transform (i.e, avoid commas, have one large 
expression block)

However, if i do:

X=within(X, {...}) #versus
transform(X)

is transform then more efficient?

I'm all for keeping consistency with data.frame, and I assume that's the 
real reason these functions are here.

Perhaps this is really a case where we want an efficient multi-update 
instead of a properly ordered within?
Or perhaps an overload on transform that allows {expr}.

On 5/28/2012 6:09 AM, Matthew Dowle wrote:
> Not sure why but it seems to be consistent with base data.frame :
>
>> DF = data.frame(a=1:3,b=4:6)
>> within(DF,{c=a*b;d=a+b})
>    a b d  c
> 1 1 4 5  4
> 2 2 5 7 10
> 3 3 6 9 18
> I don't mind it being raised as a bug in data.table and we'll fix it and
> and add to FAQ 2.17 as another difference to data.frame. Or maybe it
> would be clearer to remove within.data.table(). Do we need it?
>
> Matthew