R help - Jun 2017 - is.null(mylist[1]) and is.null(mylist$a) returns different values

Hi

I have a list :

mylist <- list( a = NULL, b = 1, c = 2 )
> mylist[1]
$a
NULL
> is.null(mylist[1])
[1] FALSE
> is.null(mylist$a)
[1] TRUE

why? I need to use mylist[1]

Hello,

You have to be aware that mylist[1] and mylist[[1]] are different things.


class(mylist[1])
[1] "list"
class(mylist[[1]])
[1] "NULL"


Apparently you want/need the latter:

is.null(mylist[[1]])
[1] TRUE


Hope this helps,

Rui Barradas

Em 15-06-2017 16:33, ce escreveu:
> Hi
>
> I have a list :
>
> mylist <- list( a = NULL, b = 1, c = 2 )
>
>> mylist[1]
> $a
> NULL
>
>> is.null(mylist[1])
> [1] FALSE
>
>> is.null(mylist$a)
> [1] TRUE
>
> why? I need to use mylist[1]
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.
>

Hi,

Try
> is.null(mylist[[1]])
[1] TRUE

Notice the double square brackets.

From: ?`[`
"The most important distinction between [, [[ and $ is that the [ can
select more than one element whereas the other two select a single
element."

On Thu, Jun 15, 2017 at 11:33 AM, ce <zadig_1 at excite.com>
wrote:
> Hi
>
> I have a list :
>
> mylist <- list( a = NULL, b = 1, c = 2 )
>
>> mylist[1]
> $a
> NULL
>
>> is.null(mylist[1])
> [1] FALSE
>
>> is.null(mylist$a)
> [1] TRUE
>
> why? I need to use mylist[1]
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.


-- 
Huzefa Khalil
PhD Candidate,
Department of Political Science,
University of Michigan

I think look at the manual

Is.null(mylist[[1]])

Will work

-----Original Message-----
From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of ce
Sent: 15 June 2017 16:33
To: r-help at r-project.org
Subject: [R] is.null(mylist[1]) and is.null(mylist$a) returns different values

Hi

I have a list :

mylist <- list( a = NULL, b = 1, c = 2 )
> mylist[1]
$a
NULL
> is.null(mylist[1])
[1] FALSE
> is.null(mylist$a)
[1] TRUE

why? I need to use mylist[1]
______________________________________________
R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
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.

I find that the str function is more helpful for understanding the difference
between a null list and a list containing a null list than the implicit print
function call that the interpreter invokes when you enter an expression at the
console.

str( mylist[1] )

-- 
Sent from my phone. Please excuse my brevity.

On June 15, 2017 8:39:47 AM PDT, Huzefa Khalil <huzefa.khalil at
umich.edu> wrote:
>Hi,
>
>Try
>
>> is.null(mylist[[1]])
>[1] TRUE
>
>Notice the double square brackets.
>
>From: ?`[`
>"The most important distinction between [, [[ and $ is that the [ can
>select more than one element whereas the other two select a single
>element."
>
>On Thu, Jun 15, 2017 at 11:33 AM, ce <zadig_1 at excite.com> wrote:
>> Hi
>>
>> I have a list :
>>
>> mylist <- list( a = NULL, b = 1, c = 2 )
>>
>>> mylist[1]
>> $a
>> NULL
>>
>>> is.null(mylist[1])
>> [1] FALSE
>>
>>> is.null(mylist$a)
>> [1] TRUE
>>
>> why? I need to use mylist[1]
>> ______________________________________________
>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> 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.