[R] crazy loop error.

[Prof Brian Ripley]

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 elements
>> of 'a' was my idea.
>> I am unaware of any R'ish way to do it. It would be nice if someone in the
>> community knows this.
>>
>> The error resulting in the NA was pretty easy to fix, and my loop works, but
>> 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 the
>> top without anything being done to it?
>
> Hi.
>
> 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 better
> 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.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595