R help - Apr 2011 - appending to a vector

Which one is more efficient?

x2=c()
for (i in 1:length(x)) {
 x2=c(x2,func(x[i]))
}

or 

x2=x
for (i in 1:length(x)) {
 x2=func(x[i])
}

where func is any function?

Dirk 

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Hi Dirk,

On Thu, Apr 14, 2011 at 4:59 AM, dirknbr <dirknbr at gmail.com>
wrote:
> Which one is more efficient?
>
> x2=c()
> for (i in 1:length(x)) {
> ?x2=c(x2,func(x[i]))
> }
>
> or
>
> x2=x
> for (i in 1:length(x)) {
> ?x2=func(x[i])
> }
>
> where func is any function?
This one. Creating a [vector|matrix|dataframe] of its full final size is
more efficient than growing it incrementally.

But this is likely to be even more efficient, and is shorter and easier
to read:

x2 <- sapply(x, func)

Sarah
-- 
Sarah Goslee
http://www.functionaldiversity.org

Neither is the fastest method. The best way would be to vectorize func so 
that it accepts and returns a vector. Many builtin R functions do this and 
say so in their documentation.

The slower way would be to use one of the apply functions, such as:

?lapply
x2 <- lapply(x, func)

If you must use a for loop, your second example is faster since it does 
not rewrite x2 each iteration.

An excellent reference on this subject and many other R pitfalls is 'The R 
Inferno'

http://lib.stat.cmu.edu/s/Spoetry/Tutor/R_inferno.pdf 
--------------------------------------
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Technician - USGS Leetown Science Center
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(304) 724-4480
"Is the room still a room when its empty? Does the room,
 the thing itself have purpose? Or do we, what's the word... imbue it."
     - Jubal Early, Firefly

r-help-bounces at r-project.org wrote on 04/14/2011 04:59:04 AM:
> [image removed] 
> 
> [R] appending to a vector
> 
> dirknbr 
> 
> to:
> 
> r-help
> 
> 04/14/2011 11:32 AM
> 
> Sent by:
> 
> r-help-bounces at r-project.org
> 
> Which one is more efficient?
> 
> x2=c()
> for (i in 1:length(x)) {
>  x2=c(x2,func(x[i]))
> }
> 
> or 
> 
> x2=x
> for (i in 1:length(x)) {
>  x2=func(x[i])
> }
> 
> where func is any function?
> 
> Dirk 
> 
> --
> View this message in context: http://r.789695.n4.nabble.com/
> appending-to-a-vector-tp3449109p3449109.html
> Sent from the R help mailing list archive at Nabble.com.
> 
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