Hi, to count vector elements with some property, the standard idiom seems to be length(which): --8<---------------cut here---------------start------------->8--- x <- c(1,1,0,0,0) count.0 <- length(which(x == 0)) --8<---------------cut here---------------end--------------->8--- however, this approach allocates and discards 2 vectors: a logical vector of length=length(x) and an integer vector in which. is there a cheaper alternative? Thanks! -- Sam Steingold (http://sds.podval.org/) on Ubuntu 12.04 (precise) X 11.0.11103000 http://www.childpsy.net/ http://iris.org.il http://honestreporting.com http://jihadwatch.org http://pmw.org.il http://www.PetitionOnline.com/tap12009/ War doesn't determine who's right, just who's left.
Hi Sam, Here is one alternative, which is at least faster: system.time(count.0 <- length(which(x == 0))) system.time(count.1 <- sum(x == 0)) all.equal(count.0, count.1) Best, Ista On Fri, Jan 4, 2013 at 10:30 AM, Sam Steingold <sds at gnu.org> wrote:> Hi, > to count vector elements with some property, the standard idiom seems to > be length(which): > --8<---------------cut here---------------start------------->8--- > x <- c(1,1,0,0,0) > count.0 <- length(which(x == 0)) > --8<---------------cut here---------------end--------------->8--- > however, this approach allocates and discards 2 vectors: a logical > vector of length=length(x) and an integer vector in which. > is there a cheaper alternative? > Thanks! > > -- > Sam Steingold (http://sds.podval.org/) on Ubuntu 12.04 (precise) X 11.0.11103000 > http://www.childpsy.net/ http://iris.org.il http://honestreporting.com > http://jihadwatch.org http://pmw.org.il http://www.PetitionOnline.com/tap12009/ > War doesn't determine who's right, just who's left. > > ______________________________________________ > R-help at 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.
What is the concern if it works? you can also do sum(x==0) Is performance a concern? How often are you going to do it and what other parts of your script also take longer? Why are you concerned about allocating/discarding two vectors? On Fri, Jan 4, 2013 at 10:30 AM, Sam Steingold <sds at gnu.org> wrote:> Hi, > to count vector elements with some property, the standard idiom seems to > be length(which): > --8<---------------cut here---------------start------------->8--- > x <- c(1,1,0,0,0) > count.0 <- length(which(x == 0)) > --8<---------------cut here---------------end--------------->8--- > however, this approach allocates and discards 2 vectors: a logical > vector of length=length(x) and an integer vector in which. > is there a cheaper alternative? > Thanks! > > -- > Sam Steingold (http://sds.podval.org/) on Ubuntu 12.04 (precise) X 11.0.11103000 > http://www.childpsy.net/ http://iris.org.il http://honestreporting.com > http://jihadwatch.org http://pmw.org.il http://www.PetitionOnline.com/tap12009/ > War doesn't determine who's right, just who's left. > > ______________________________________________ > R-help at 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.-- Jim Holtman Data Munger Guru What is the problem that you are trying to solve? Tell me what you want to do, not how you want to do it.
My 2 cents: AFAIK both which and length are from C compiled code: http://cran.r-project.org/doc/manuals/r-release/R-ints.html#g_t_002eInternal-vs-_002ePrimitive so they must be quite efficient ie .Primitive and .Internal. Probably combination of this with a pattern in C would be more memory efficient to count patterns, but would that make sense? Because in general if you look for a pattern in a vector, you need to know where it is, hence which operation, at least for debugging/testing purposes... On 4 January 2013 16:30, Sam Steingold <sds at gnu.org> wrote:> Hi, > to count vector elements with some property, the standard idiom seems to > be length(which): > --8<---------------cut here---------------start------------->8--- > x <- c(1,1,0,0,0) > count.0 <- length(which(x == 0)) > --8<---------------cut here---------------end--------------->8--- > however, this approach allocates and discards 2 vectors: a logical > vector of length=length(x) and an integer vector in which. > is there a cheaper alternative? > Thanks! > > -- > Sam Steingold (http://sds.podval.org/) on Ubuntu 12.04 (precise) X 11.0.11103000 > http://www.childpsy.net/ http://iris.org.il http://honestreporting.com > http://jihadwatch.org http://pmw.org.il http://www.PetitionOnline.com/tap12009/ > War doesn't determine who's right, just who's left. > > ______________________________________________ > R-help at 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.
On Jan 4, 2013, at 7:30 AM, Sam Steingold wrote:
Hi,
to count vector elements with some property, the standard idiom seems to
be length(which):
--8<---------------cut here---------------start------------->8---
x <- c(1,1,0,0,0)
count.0 <- length(which(x == 0))
--8<---------------cut here---------------end--------------->8---
however, this approach allocates and discards 2 vectors: a logical
vector of length=length(x) and an integer vector in which.
is there a cheaper alternative?
I don't know if it is "cheaper", but the way I "learned to
count" was:
sum(x==8, na.rm=TRUE)
--
David Winsemius
Alameda, CA, USA