[R] How to extract sublist from a list?

[Joshua Wiley]

On Thu, Aug 4, 2011 at 1:40 AM, Ashim Kapoor <ashimkapoor at gmail.com> wrote:
>
>
> On Thu, Aug 4, 2011 at 1:02 PM, Joshua Wiley <jwiley.psych at gmail.com> wrote:
>>
>> On Thu, Aug 4, 2011 at 12:12 AM, Ashim Kapoor <ashimkapoor at gmail.com>
>> wrote:
>> >> How would we do this problem looping over seq(1:2) ?
>>
>> Because this goes to an email list serv, it is good practice to quote
>> the original problem.  I have no idea what "this" is.
>>
>> >>
>> >>
>> > To extend the example in the corresponding nabble post : -
>> >  sub1<-list(x="a",y="ab")
>> >  sub2<-list(x="c",y="ad")
>> >  lst<-list(sub1=sub1,sub2=sub2)
>> >  for ( t in seq(1:2) )  print(lst[[t]]$y)
>> >
>> > So I can print out the sub1$y/sub2$y but it's not clear how to extract
>> > them.
>>
>> Well, to extract them, just drop the call to print. You could use them
>> directly in the loop or could store them in new variables.
>>
>
>> j<- for ( t in seq(1:2) )  lst[[t]]$y
>> j
> NULL
>
> Why is j NULL  ?

You are confusing how for loops work, please read the documentation for ?for

>
>>
>> ## note seq(1:2) is redundant with simply 1:2
>> or (t in 1:2) print(nchar(lst[[t]]$y))
>>
>> I am guess, though, that what you might be hoping to do is extract
>> specific elements from a list and store the extract elements in a new
>> list.
>>
>> lapply(1:2, function(i) lst[[i]]["y"])
>> ## or compare
>> lapply(1:2, function(i) lst[[i]][["y"]])
>>
>> >
>> > My original was different though.
>> >
>> > How would  say:-
>> >
>> > for ( t in seq(1:2) ) sub"t"$y
>> >
>> > Where sub"t" evaluates to sub1 or sub 2?
>>
>> if you actually want "sub1", or "sub2":
>>
>> ## note that I am wrapping in print() not so that it works
>> ## but so that you can see it at the console
>> for (t in 1:2) print(paste("sub", t, sep = ''))
>>
>> from which we can surmise that the following should work:
>>
>> for (t in 1:2) print(lst[[paste("sub", t, sep = '')]])
>>
>> which trivially extends to:
>>
>> for (t in 1:2) print(lst[[paste("sub", t, sep = '')]]$y)
>>
>> or perhaps more appropriately
>>
>> for (t in 1:2) print(lst[[paste("sub", t, sep = '')]][["y"]])
>>
>> If you just need to go one level down for *all* elements of your list
>>
>> lapply(lst, `[[`, "y")
>> ## or if you are only retrieving a single value
>> sapply(lst, `[[`, "y")
>>
>> Hope this helps,
>>
>>
>> Josh
>>
>> >
>> > Many thanks.
>> > Ashim
>> >
>> >
>> >> On Thu, Aug 4, 2011 at 10:59 AM, Richard Ma
>> >> <xuanlong.ma at uts.edu.au>wrote:
>> >>
>> >>> Thank you so much GlenB!
>> >>>
>> >>> I got it done using your method.
>> >>>
>> >>> I'm just curious how did you get this idea? Cause for me, this looks
>> >>> so
>> >>> tricky....
>> >>>
>> >>> Cheers,
>> >>> Richard
>> >>>
>> >>> -----
>> >>> I'm a PhD student interested in Remote Sensing and R Programming.
>> >>> --
>> >>> View this message in context:
>> >>>
>> >>> http://r.789695.n4.nabble.com/How-to-extract-sublist-from-a-list-tp3717451p3717713.html
>> >>> Sent from the R help mailing list archive at Nabble.com.
>> >>>
>> >>> ______________________________________________
>> >>> 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.
>> >>>
>> >>
>> >>
>> >
>> >        [[alternative HTML version deleted]]
>> >
>> > ______________________________________________
>> > 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.
>> >
>>
>>
>>
>> --
>> Joshua Wiley
>> Ph.D. Student, Health Psychology
>> Programmer Analyst II, ATS Statistical Consulting Group
>> University of California, Los Angeles
>> https://joshuawiley.com/