On 10/09/2014, 3:21 AM, PO SU wrote: > > Dear expeRts, > I have some programming questions about NULL in R.There are listed as > follows: > 1. I find i can't let a list have a element NULL: > a<-list() > a$ress<-1 > a$res<-NULL > a > str(a)
You can do it using a <- list(ress = 1, res = NULL) > How can i know i have a named element but it is NULL, not just get > a$xxxx,a$iiii,a$oooo there all get NULL That's a little harder. There are a few ways: "res" %in% names(a) & is.null(a[["res"]]) or identical(a["res"], list(res = NULL)) or is.null(a[[2]]) should all work. Generally because of the special handling needed, it's a bad idea to try to store NULL in a list. > 2.The most important thing: > a<-1:10 > b<-NULL or 1 > a<-c(a,b) will work so i don't need to know whether b is null or not,but: > a[-NULL] can't work!! i just need a[-NULL]==a , how can i reach this purpose? Using !, and a logical test, e.g. a[!nullentry(a)] where nullentry() is a function based on one of the tests above, but applied to all entries. Duncan Murdoch ______________________________________________ [email protected] 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.
