Different NaNs can hash to different values and so be treated differently by duplicated(), unique(), match(). Any IEEE double value with exponent bits all 1 is NaN (except the one that is NA). AFAICT all arithmetic and mathematical operations in R produce the same NaN, but this is not guaranteed by any standard. read.spss() will produce different NaNs when reading missing values in at least some circumstances. -thomas Thomas Lumley Asst. Professor, Biostatistics tlumley@u.washington.edu University of Washington, Seattle -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- r-devel mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html Send "info", "help", or "[un]subscribe" (in the "body", not the subject !) To: r-devel-request@stat.math.ethz.ch _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._