[R] some question about vector[-NULL]

[PIKAL Petr]

Hi

I am puzzled about what do you want?

You was discouraged using $ operator due to partial matching, however you still use it.

In your example you do not have named element being NULL. Just check it yourself.

> a<-list()
> a$ress<-1
> a$res<-NULL
> a
$ress
[1] 1

> str(a)
List of 1
 $ ress: num 1

For working with lists lapply/sapply is the preferred way.

> a<-vector("list",3)
> a[[2]]<-1:10
> a
[[1]]
NULL

[[2]]
 [1]  1  2  3  4  5  6  7  8  9 10

[[3]]
NULL

> is.null(a)
[1] FALSE
> lapply(a, is.null)
[[1]]
[1] TRUE

[[2]]
[1] FALSE

[[3]]
[1] TRUE


What do you expect from your second example? a is numeric vector b is NULL, c(a,b) is vector.

> a[-NULL]
Error in -NULL : invalid argument to unary operator

gives an error.

OK, let me polish my crystal ball.
You want to get rid of all list elements which are null?

a[unlist(lapply(a, function(x) !is.null(x)))]

Regards
Petr

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of PO SU
> Sent: Wednesday, September 10, 2014 9:21 AM
> To: R. Help
> Subject: [R] some question about vector[-NULL]
>
>
> 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)
>
> 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
> 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?
>
>
>
>
>
>
>
>
>
>
>
> --
>
> PO SU
> mail: desolator88 at 163.com
> Majored in Statistics from SJTU
> ______________________________________________
> 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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