On 01/09/2010 11:09 AM, Tal Galili wrote:
Hello all,
A friend recently brought to my attention that vector assignment actually
recreates the entire vector on which the assignment is performed.
So for example, the code:
x[10]<- NA # The original call (short version)
Is really doing this:
x<- replace(x, list=10, values=NA) # The original call (long version)
# assigning a whole new vector to x
Which is actually doing this:
x<- `[<-`(x, list=10, values=NA) # The actual call
Assuming this can be explained reasonably to the lay man, my question is,
why is it done this way ?
Your friend misled you. The `[<-` function is primitive. It acts as
though it does what you describe, but it is free to do internal
optimizations, and in many cases it does. The replace() function is a
regular R-level function so it has much less freedom and is likely to be
a lot less efficient.
For example, in evaluating the expression x[10] <- NA, in most cases R
knows that the original vector x will never be needed again, so it won't
be duplicated. But in evaluating
replace(x, list=10, values=NA)
R can't be sure, so it would make a duplicate copy.
You can see the difference in the following code:
> x <- 1:1000
> tracemem(x)
[1] "<0x0547a6c0>"
> x[10] <- NA
> x <- replace(x, list=10, values=NA)
tracemem[0x0547a6c0 -> 0x0488a768]: replace
Only the second version caused x to be duplicated.
One example that looks as though it is doing unnecessary duplication is
this:
> x[10] <- 3
tracemem[0x0488a768 -> 0x04881260]:
tracemem[0x04881260 -> 0x05613368]:
I can see that one duplication is necessary (x is being changed from
type integer to type double), but why two?
Duncan Murdoch
Why won't it just change the relevant pointer in memory?
On small vectors it makes no difference.
But on big vectors this might be (so I suspect) costly (in terms of time).
I'm curious for your responses on the subject.
Best,
Tal
----------------Contact
Details:-------------------------------------------------------
Contact me: tal.gal...@gmail.com | 972-52-7275845
Read me: www.talgalili.com (Hebrew) | www.biostatistics.co.il (Hebrew) |
www.r-statistics.com (English)
----------------------------------------------------------------------------------------------
[[alternative HTML version deleted]]
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
