Best way to preallocate numeric NA array?
Hi
There is one issue which I encountered recently with this type of
behaviour,
mat<-matrix(NA,5,4)
fix(mat)
put some number in any cell and close fix
mat
col1 col2 col3 col4
[1,] NA NA NA NA
[2,] NA NA NA NA
[3,] NA NA NA NA
[4,] NA NA NA NA
[5,] NA NA NA NA
No value is put into mat. There is easy workaround, but it can be
frustrating if somebody tries to find why the value is not inputed. I know
that it is not preferred way to fill a matrix but if you have such small
matrix and it has only few non NA values this could be used.
Maybe on help page could be some kind of explanation:
"Fix can not convert a type(mode) of its argument and therefore it is
possible to input only values which match type of x."
or something like that
Regards
Petr
r-help-bounces at r-project.org napsal dne 26.11.2009 17:22:45:
On Thu, Nov 26, 2009 at 10:03 AM, Rob Steele <freenx.10.robsteele at xoxy.net> wrote:
These are the ways that occur to me. ## This produces a logical vector, which will get converted to a
numeric
## vector the first time a number is assigned to it. That seems ## wasteful. x <- rep(NA, n) ## This does the conversion ahead of time but it's still creating a ## logical vector first, which seems wasteful. x <- as.numeric(rep(NA, n)) ## This avoids type conversion but still involves two assignments for ## each element in the vector. x <- numeric(n) x[] <- NA ## This seems reasonable. x <- rep(as.numeric(NA), n) Comments?
My intuition would be to go with the third method (allocate a numeric vector then assign NA to its contents) but I haven't tested the different. In fact, it would be difficult to see differences in, for example, execution time unless n was very large. This brings up a different question which is, why do you want to consider this? Are you striving for readability, for speed, for low memory footprint, for "efficiency" in some other way? When we were programming in S on machines with 1 mips processors and a couple of megabytes of memory, such considerations were important. I'm not sure they are quite as important now.
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