[R] crazy loop error.
gunter.berton at gene.com
Tue Jan 25 18:03:12 CET 2011
Well, I'm not Prof. Ripley, but the answer is: Look at the code.
seq_len, seq.int, and seq_along call Primitives, which are implemented
in C, and therefore MUCH faster than seq(), which is implemented as
pure R code (and is also a generic, so requires method dispatch).
Though for small n (up to a few thousand, say), it probably doesn't
make much difference.(Here, to be corrected by Prof. Ripley is
On Tue, Jan 25, 2011 at 2:22 AM, Ivan Calandra
<ivan.calandra at uni-hamburg.de> wrote:
> Mr Ripley,
> May I ask why seq_len() and seq_along() are better than seq()?
> Le 1/25/2011 09:58, Prof Brian Ripley a écrit :
>> On Tue, 25 Jan 2011, Petr Savicky wrote:
>>> On Mon, Jan 24, 2011 at 11:18:35PM +0100, Roy Mathew wrote:
>>>> Thanks for the reply Erik, As you mentioned, grouping consecutive
>>>> of 'a' was my idea.
>>>> I am unaware of any R'ish way to do it. It would be nice if someone in
>>>> community knows this.
>>>> The error resulting in the NA was pretty easy to fix, and my loop works,
>>>> the results are still wrong (new script below).
>>>> Ideally it should print single "hello" for the single letters and
>>>> grouped '3
>>>> hellos' for the fives, grouped '2 hellos' for the sixes etc.
>>>> Based on the run results, if the value of n is being tracked, it changes
>>>> quite unpredictably.
>>>> Can someone explain how the value of n changes from end of the loop to
>>>> top without anything being done to it?
>>> A for-loop in R is different from a for-loop in C. It is similar
>>> to foreach loop in Perl. If v is a vector, then
>>> for (n in v)
>>> first creates the vector v and then always performs length(v) iterations.
>>> Before iteration i, n is assigned v[i] even if n is changed in the
>>> previous iteration.
>> And also if v is changed during the loop.
>>> If you want to control the loop variable during execution, it is possible
>>> to use a while loop, where you have full control. While loop may be
>>> also if v has a very large length, since, for example
>>> for (n in 1:1000000)
>>> creates a vector of length 1000000 in memory.
>>> It should also be noted that the for-loop
>>> for (n in 1:k)
>>> performs 2 iterations, if k is 0, since 1:0 is a vector of length 2.
>>> If k may be 0, then it is better to use
>>> for (n in seq(length=k))
>>> since seq(length=0) has length 0.
>> Since you keep mentioning that, it is actually much better to use
>> seq_len(k) (and seq_along(x) instead of your earlier recommendation of
>> seq(along=x)). And if you are using seq() in other cases in programs,
>> consider seq.int() instead.
>>> Hope this helps.
>>> Petr Savicky.
> Ivan CALANDRA
> PhD Student
> University of Hamburg
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Genentech Nonclinical Biostatistics
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