[R] Indexing a list with a list

[Greg Hirson]

Jim,

Thank you for your help. I had a feeling it would be an apply function 
that could help.

Kind regards,

Greg

jim holtman wrote:
> Your example has one more element in 'ind' than 'example' and that is 
> what causes the error, but this should be close.
>  
> > #sample data
> > set.seed(100)
> > example = list(letters[1:10], letters[1:10], letters[1:10])
> > ind = list(as.logical(sample(0:1, 10, rep=TRUE)),
> +            as.logical(sample(0:1, 10, rep=TRUE)),
> +            as.logical(sample(0:1, 10, rep=TRUE)),
> +            as.logical(sample(0:1, 10, rep=TRUE)))
> >           
> > mapply(function(a,b)a[b], example, ind)
> [[1]]
> [1] "c" "g" "i"
>
> [[2]]
> [1] "a" "b" "e" "f" "j"
>
> [[3]]
> [1] "a" "b" "c" "d" "g" "h" "i"
>
> [[4]]
> [1] "b" "d" "e" "f" "h" "i"
>
> Warning message:
> In mapply(function(a, b) a[b], example, ind) :
>   longer argument not a multiple of length of shorter
> >
>
>
> On Fri, Jun 26, 2009 at 2:27 PM, Greg Hirson <ghirson at ucdavis.edu 
> <mailto:ghirson at ucdavis.edu>> wrote:
>
>     Dear list,
>
>     I have two lists: one with data and one with TRUE/FALSE values for
>     data I want to further analyze (see example below). I have been
>     able to use a for loop to extract the data that I want to keep,
>     but think that there probably exists a way to do it without a
>     loop. Any ideas?
>
>     #sample data
>     set.seed(100)
>     example = list(letters[1:10], letters[1:10], letters[1:10])
>     ind = list(as.logical(sample(0:1, 10, rep=TRUE)),
>     as.logical(sample(0:1, 10, rep=TRUE)), as.logical(sample(0:1, 10,
>     rep=TRUE)), as.logical(sample(0:1, 10, rep=TRUE)))
>
>     #loop method
>     for (i in 1:length(example)) print(example[[i]][ind[[i]]])
>
>     #result
>     [1] "c" "g" "i"
>     [1] "a" "b" "e" "f" "j"
>     [1] "a" "b" "c" "d" "g" "h" "i"
>
>
>     Thank you for the help.
>
>     Greg
>
>     -- 
>     Greg Hirson
>     ghirson at ucdavis.edu <mailto:ghirson at ucdavis.edu>
>
>     Graduate Student
>     Agricultural and Environmental Chemistry
>
>     1106 Robert Mondavi Institute North
>     One Shields Avenue
>     Davis, CA 95616
>
>     ______________________________________________
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>     PLEASE do read the posting guide
>     http://www.R-project.org/posting-guide.html
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>     and provide commented, minimal, self-contained, reproducible code.
>
>
>
>
> -- 
> Jim Holtman
> Cincinnati, OH
> +1 513 646 9390
>
> What is the problem that you are trying to solve?

-- 
Greg Hirson
ghirson at ucdavis.edu

Graduate Student
Agricultural and Environmental Chemistry

1106 Robert Mondavi Institute North
One Shields Avenue
Davis, CA 95616