appending to a vector

Hi Dirk,

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 
--------------------------------------
Jonathan P. Daily
Technician - USGS Leetown Science Center
11649 Leetown Road
Kearneysville WV, 25430
(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."
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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 

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