seq_len and loops

for (i in seq_len(x - 1) + 1)

should be efficient and safe.

Oops, not safe when x is 0.

-----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: G?ran Brostr?m; 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, G?ran Brostr?m 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