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, rip...@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
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