On 01/01/2014 07:36 PM, William Dunlap wrote:
2. However, Bill (and Henrik) raised the question of replacing '1' with
'1L'; I understand the meaning of that, but does it matter (in practice)?
On 12/22/2013 06:57 PM, William Dunlap wrote:
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].
It depends what your practice involves.
seq_len(n)[-1], 2:n, and seq_len(n-1)+1L all produce an integer vector (if
0<n<2^31 or so).
seq_len(n-1)+1 produces a numeric (double precision floating point) vector.
Integers and numerics have different properties which might affect your results,
but in many cases you will not care. Integers use 4 bytes of memory, numerics
8.
Integers have 32 bits of precision, numerics 52. Integers range from -2^31+1
to 2^31-1
and arithmetic which would give a result outside of that range results in NA
(with a warning).
Numerics range from c. -2^1023 (c. -10^308) to c. 2^1023 (c. 10^308) and
arithmetic
which would give a result outside of that range results in +-Inf.
Thanks Bill; I am aware of these details since my days as a successful
Fortran programmer;)! I guess this background is what makes me worry
about the apparently careless choice of variable type that I can get
away with in R.
If you prefer a sequence to be numeric, then use as.numeric(seq_len(n)),
as.numeric(seq_len(n))[-1],
or seq_len(n)+1 when making it. If you prefer integers, then use seq_len(n),
seq_len(n)[-1], or seq_len(n)+1L.
If you don't care, do whatever seems easiest at the time.
if (x > 1){
for (x in 2:x){
...
is the easiest, most effective, and most easy-to-understand.
The dangerous part of that idiom is what you do in the 'else' part of the 'if'
statement.
Do both clauses make objects with the same names and types? I mildly prefer
avoiding
if statements because it makes reasoning about the results of the code more
complicated.
I get your point; I however never have an 'else' part in this context
(if x == 1 I'm already done).
Another observation: The 'while (x < n) x <- x + 1' approach is very
slow compared to a for loop.
Göran Broström
Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
-----Original Message-----
From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On
Behalf
Of Göran Broström
Sent: Tuesday, December 31, 2013 4:10 PM
To: r-help@R-project.org
Subject: Re: [R] seq_len and loops
Thanks for the answers from Duncan, Bill, Gabor, and Henrik. You
convinced me that
1. The solution
if (x > 1){
for (x in 2:x){
...
is the easiest, most effective, and most easy-to-understand.
2. However, Bill (and Henrik) raised the question of replacing '1' with
'1L'; I understand the meaning of that, but does it matter (in practice)?
3. Noone commented upon
i <- 1
while (i < x){
i <- i + 1
...
}
I suppose that it means that it is the best solution.
Thanks, and Happy New Year 2014!
Göran
On 12/22/2013 06:57 PM, William Dunlap wrote:
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-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On
Behalf
Of Duncan Murdoch
Sent: Saturday, December 21, 2013 3:52 PM
To: Göran Broström; R-help@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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______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
