[R] lapply vs. for (was: Incrementing a counter in lapply)

Gregor Gorjanc gregor.gorjanc at gmail.com
Wed Mar 15 22:39:54 CET 2006

> From: Thomas Lumley
>> On Tue, 14 Mar 2006, John McHenry wrote:
>> > Thanks, Gabor & Thomas.
>> >
>> > Apologies, but I used an example that obfuscated the question that I
>> > wanted to ask.
>> >
>> > I really wanted to know how to have extra arguments in 
>> functions that
>> > would allow, per the example code, for something like a 
>> counter to be 
>> > incremented. Thomas's suggestion of using mapply 
>> (reproduced below with 
>> > corrections) is probably closest.
>> It is probably worth pointing out here that the R 
>> documentation does not 
>> specify the order in which lapply() does the computation.
>> If you could work out how to increment a counter (and you could, with 
>> sufficient effort), it would not necessarily work, because the 'i'th 
>> evaluation would not necessarily be of the 'i'th element.
>> [lapply() does in fact start at the beginning, go on until it 
>> gets to the 
>> end, and then stop, but this isn't documented.   Suppose R became 
>> multithreaded, for example....]
> The corollary, it seems to me, is that sometimes it's better to leave the
> good old for loop alone.  It's not always profitable to turn for loops into
> some *apply construct.  The trick is learning to know when to do it and when
> not to.

Can someone share some of this tricks with me? Up to now I have always
done things with for loop. Just recently I started to pay attention to
*apply* constructs and I already wanted to start implementing them
instead of good old for, but then a stroke of lightning came from this
thread. Based on words from Thomas, lapply should not be used for tasks
where order is critical. Did I get this clear enough. Additionally, I
have read notes (I lost link, but was posted on R-help, I think) from
Thomas on R and he mentioned that it is commonly assumed that *apply* (I
do not remember which one of *apply*) is faster than loop, but that this
is not true. Any additional pointers to literature?

Lep pozdrav / With regards,
    Gregor Gorjanc

University of Ljubljana     PhD student
Biotechnical Faculty
Zootechnical Department     URI: http://www.bfro.uni-lj.si/MR/ggorjan
Groblje 3                   mail: gregor.gorjanc <at> bfro.uni-lj.si

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"One must learn by doing the thing; for though you think you know it,
 you have no certainty until you try." Sophocles ~ 450 B.C.

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