Actually, i thought the way: a<-1:3 b<-NULL or 2 a[-b] will not work if b is NULL
A<-c(a,1) B<-c(b,length(A)) A[-B] will get the same result as if b is NULL get a, if b is 2 get a[-2] I think it works well in considering memory use or efficiency or code tidy. -- PO SU mail: [email protected] Majored in Statistics from SJTU At 2014-09-11 05:59:42, "PO SU" <[email protected]> wrote: > >Orignally i don't want to do the if ( length) check because i know that in a >10000 loops, after may be 10 or 20 or 100 loops , "i" will not be empty. >so i mean , in the left loops, i would always check something not needed to >check which i would not like to do. > > > >-- > >PO SU >mail: [email protected] >Majored in Statistics from SJTU > > > > >At 2014-09-11 05:38:12, "Duncan Murdoch" <[email protected]> wrote: >>On 10/09/2014, 9:53 PM, PO SU wrote: >>> >>> Tks, i think using logical index is a way, but to do that, i have to keep a >>> vector as long as the original vector. that's, to exclude position 1 and 3 >>> from >>> a<-1:5 >>> I have to let b<-c(F,T,F,T,T) and exec a[b], not a[-c(1,3)]. which c(1,3) >>> is much shorter than b if a is a long vector. that's, b would be >>> c(F,T,F,T,T,T,T,......,T) >> >>If you know that you want to omit elements 1 and 3, then negative >>integer indexing is safe. You'll never need to explicitly type out the >>whole logical vector. >> >>The only problem with it is when you construct the indices by tests, and >>sometimes nothing matches the tests, so you end up with an empty vector >>or NULL. In that case you can just as easily construct the logical >>vector. >> >>If your vectors are so long that you are worried about the cost of >>constructing a logical vector that long, then it's probably worthwhile >>checking the length of the integer index explicitly, i.e. instead of >> >>a <- a[ -i ] >> >>use >> >>if (any(i > 0)) a <- a[ -i ] >> >>Duncan Murdoch >> >> >>> I thought a way , >>> let d<-c(a,1) >>> that d<-c(1,2,3,4,5,1) >>> and initialize the index vector iv to length(d). that is iv<-6. >>> then, d[-iv] is always equal a[- i ] , whether i is NULL or not. >>> Because if i is NULL ,then iv is 6, if i is 2.then iv is c(2,6) and so >>> on....... >>> >>> >>> >>> >>> >>> >>> >>> -- >>> >>> PO SU >>> mail: [email protected] >>> Majored in Statistics from SJTU >>> >>> >>> >>> >>> At 2014-09-11 01:58:46, "Duncan Murdoch" <[email protected]> wrote: >>>> On 10/09/2014 12:20 PM, William Dunlap wrote: >>>>> 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). >>>> >>>> I think the problem with integer(0) and NULL is the same: a[-i] doesn't >>>> act as expected (leaving out all the elements of i, i.e. nothing) if i >>>> is either of those. The solution is to use logical indexing, not >>>> negative numerical indexing. >>>> >>>> Duncan Murdoch >>>>> >>>>> 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.
