Best way to preallocate numeric NA array?
Douglas Bates wrote:
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.
Thanks--good questions. For any code, I'd order the requirements like this: 1) Correct 2) Readable 3) Space efficient 4) Time efficient Compromises are sometimes necessary. R is such an odd language that it really helps readability to settle on easily recognizable idioms. That's true in any language where there's more than one way to do things but I find it especially true in R. I agree that the efficiency of this operation only matters with very large vectors or very many repetitions.