[R] Get object name inside lapply

[peter dalgaard]

On 25 Feb 2016, at 22:43 , Ben Tupper <ben.bighair at gmail.com> wrote:

> Hi,
> 
> Using your example (note I called the list 'z')...
> 
> z <-list(a = seq(1:5), b = seq(10:20))
> 
> I picture lapply as extracting each element of z like this z[[i]] - the `[[` extracts the ith value from the context of residing in a list - hence it's name is 'lost' in the new context.  That's different than z[i] which extracts a list of elements.  Try..
> 
> z[['a']]  vs. z['a']
> 

Exactly. lapply applies the function to each _element_ in turn, not to a list containing one element. To further understand why the names do not carry onto elements, contemplate things like 

z <- list(a=c(b=1))
names(z[[1]])


> As an alternative and depending upon what you really want to do, you could iterate through the names of the list, and pass the list as a parameter.  
> 
> r <- lapply(names(z),
>    function(nm, dat = NULL){
>        sprintf("%s has %i elements", nm, length(dat[[nm]]) )
>    },
>    dat = z)
> r
> [[1]]
> [1] "a has 5 elements"
> 
> [[2]]
> [1] "b has 11 elements"
> 


Or, 

> mapply(z,names(z), FUN=function(e,n) sprintf("%s has %i elements", n, length(e))) 
                  a                   b 
 "a has 5 elements" "b has 11 elements" 

and variation thereof.

-pd



> Ben
> 
>> On Feb 25, 2016, at 4:27 PM, Mohammad Tanvir Ahamed via R-help <r-help at r-project.org> wrote:
>> 
>> Hello, 
>> 
>> I want to get object name of a list inside lapply 
>> 
>>> c<-list(a=seq(1:5),b=seq(10:20)) 
>>> lapply(c,names) 
>> $a 
>> NULL 
>> 
>> $b 
>> NULL 
>> 
>> Why NULL ? 
>> 
>> but i am expecting the names of object . Any help will be appreciated . 
>> 
>> I want to grab the names of object inside lapply for further process. 
>> 
>> Thanks . 
>> 
>> 
>> Tanvir Ahamed 
>> Göteborg, Sweden  |  mashranga at yahoo.com
>> 
>> ______________________________________________
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>> and provide commented, minimal, self-contained, reproducible code.
> 
> ______________________________________________
> 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.

-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
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Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com