[R] seq_len and loops

[William Dunlap]

> > for (i in seq_len(x - 1) + 1)
> >
> > should be efficient and safe.
> 
> Oops, not safe when x is 0.

Also, the '+ 1' should be '+ 1L' to get the same answer as
seq_len(x)[-1].

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of Duncan Murdoch
> Sent: Saturday, December 21, 2013 3:52 PM
> To: Göran Broström; R-help at r-project.org
> Subject: Re: [R] seq_len and loops
> 
> On 13-12-21 6:50 PM, Duncan Murdoch wrote:
> > On 13-12-21 5:57 PM, Göran Broström wrote:
> >> I was recently reminded on this list that
> >>
> >> "Using 1:ncol() is bad practice (seq_len is designed for that purpose)"
> >> (Ripley)
> >>
> >> This triggers the following question: What is "good practice" for
> >> 2:ncol(x)? (This is not a joke; in a recursive situation it often makes
> >> sense to perform the calculation for the start value i = 1, then
> >> continue with a loop over the rest, "the Fortran way";)
> >>
> >> I usually use
> >>
> >> if (ncol(x) > 1)
> >>        for (i in 2:ncol(x)){
> >>           ....
> >>
> >> but I can think of
> >>
> >> for (i in seq_len(x - 1)){
> >>        I <- i + 1
> >>       ....
> >>
> >> and
> >>
> >> i <- 1
> >> while (i < ncol(x)){
> >>        i <- i + 1
> >>        ....
> >>
> >> What is "good practice" (efficient and safe)?
> >
> > for (i in seq_len(x - 1) + 1)
> >
> > should be efficient and safe.
> 
> Oops, not safe when x is 0.
> 
>  >
> A little less efficient, but clearer would be
> >
> > for (i in seq_len(x)[-1])
> >
> > Duncan Murdoch
> >
> 
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