# [R] some question about vector[-NULL]

PO SU rhelpmaillist at 163.com
Thu Sep 11 12:04:58 CEST 2014

```
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: desolator88 at 163.com
Majored in Statistics from SJTU

At 2014-09-11 05:59:42, "PO SU" <rhelpmaillist at 163.com> 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: desolator88 at 163.com
>Majored in Statistics from SJTU
>
>
>
>
>At 2014-09-11 05:38:12, "Duncan Murdoch" <murdoch.duncan at gmail.com> 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: desolator88 at 163.com
>>> Majored in Statistics from SJTU
>>>
>>>
>>>
>>>
>>> At 2014-09-11 01:58:46, "Duncan Murdoch" <murdoch.duncan at gmail.com> 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 <rhelpmaillist at 163.com> 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: desolator88 at 163.com
>>>>>> Majored in Statistics from SJTU
>>>>>>
>>>>>>
>>>>>>
>>>>>> At 2014-09-10 06:45:59, "Duncan Murdoch" <murdoch.duncan at gmail.com> 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
>>>>>>>
>>>>>> ______________________________________________
>>>>>> R-help at r-project.org mailing list
>>>>>> https://stat.ethz.ch/mailman/listinfo/r-help