Can you make your example a bit more concrete? E.g., is your 'index vector' A an integer vector? If so, integer(0), an integer vector with no elements, would be a more reasonable return value than NULL, an object of class NULL with length 0, for the 'not found' case and you could check for that case by asking if length(A)==0.
Show us typical inputs and expected outputs for your function (i.e., the problem you want to solve). Bill Dunlap TIBCO Software wdunlap tibco.com On Wed, Sep 10, 2014 at 8:53 AM, PO SU <[email protected]> wrote: > > Tks for your > > a <- list(ress = 1, res = NULL) > And in my second question, let me explain it : > Actually i have two vectors in global enviroment, called A and B .A is > initialized to NULL which used to record some index in B. > Then i would run a function F, and each time, i would get a index value or > NULL. that's, D<-F(B). D would be NULL or some index position in B. > But in the function F, though input is B, i would exclude the index value > from B recorded in A. That's : > F<-function( B ) { > B<-B[-A] > some processing... > res<-NULL or some new index not included in A > return(res) > } > so in a loop, > A<-NULL > for( i in 1:100000) { > D<-F(B) > A<-c(A,D) > } > I never know whether D is a NULL or a different index compared with indexes > already recorded in A. > Actually, A<-c(A,D) work well, i never worry about whether D is NULL or a > real index, but in the function F, B<-B[-A] won't work. > so i hope that, e.g. > a<-1:3 > a[-NULL] wouldn't trigger an error but return a. > Because, if i wrote function like the following: > > F<-function( B ) { > if( is.null(A)) > B<-B > else > B<-B[-A] > some processing... > res<-NULL or some new index not included in A > return(res) > } > May be after 5 or 10 loops, A would already not NULL, so the added if ..else > statement would be repeated in left 9999 loops which i would not like to see. > > > > > > -- > > PO SU > mail: [email protected] > Majored in Statistics from SJTU > > > > At 2014-09-10 06:45:59, "Duncan Murdoch" <[email protected]> wrote: >>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. ______________________________________________ [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.
