[R] some question about vector[-NULL]

Duncan Murdoch murdoch.duncan at gmail.com
Thu Sep 11 12:48:44 CEST 2014 

On 11/09/2014, 6:04 AM, PO SU wrote:
> 
> 
> 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)

If a is a long vector, that is a very expensive operation, at least as
expensive as constructing a logical vector of the same length as a.

> 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.

Understanding R memory use is hard:  be sure to profile it before you
decide on the efficiency of one operation over another.

Duncan Murdoch

> 
> 
> 
> 
> --
> 
> 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
>>>>>>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>>>>>> and provide commented, minimal, self-contained, reproducible code.
>>>>>
>>>

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