# Re: [R] applying a function recursively

From: Prof Brian Ripley <ripley_at_stats.ox.ac.uk>
Date: Wed, 11 Jun 2008 17:12:01 +0100 (BST)

On Wed, 11 Jun 2008, Georg Otto wrote:

>
> Hi,
>
> I have a question about applying a function recursively through a
> list. Suppose I have a list where the different elements have
> different levels of recursion:
>
>
>> test.list<-list("I"=list("A"=c("a", "b", "c"), "B"=c("d", "e", "f"), "C"=c("g", "h", "i")),
> + "II"=list("A"=list("a"=c("a", "b", "c"), "b"=c("d", "e", "f"),
> + "c"=c("g", "h", "i")),
> + "B"=c("d", "e", "f"), "C"=c("g", "h", "i")))
>
>> test.list
> \$I
> \$I\$A
> [1] "a" "b" "c"
>
> \$I\$B
> [1] "d" "e" "f"
>
> \$I\$C
> [1] "g" "h" "i"
>
>
> \$II
> \$II\$A
> \$II\$A\$a
> [1] "a" "b" "c"
>
> \$II\$A\$b
> [1] "d" "e" "f"
>
> \$II\$A\$c
> [1] "g" "h" "i"
>
>
> \$II\$B
> [1] "d" "e" "f"
>
> \$II\$C
> [1] "g" "h" "i"
>
>
>
> I would like to apply a function recursively to that list, in a way
> that the function does someting with each vector (eg. rev()) and
> returns a list of modified vectors that has the same structure as the
> input list, in my example:
>
>
> \$I
> \$I\$A
> [1] "c" "b" "a"
>
> \$I\$B
> [1] "f" "e" "d"
>
> \$I\$C
> [1] "i" "h" "g"
>
>
> \$II
> \$II\$A
> \$II\$A\$a
> [1] "c" "b" "a"
>
> \$II\$A\$b
> [1] "f" "e" "d"
>
> \$II\$A\$c
> [1] "i" "h" "g"
>
>
> \$II\$B
> [1] "f" "e" "d"
>
> \$II\$C
> [1] "i" "h" "g"
>
>
>
> I understand that with a fixed number of recursion levels one can use
> lapply() in a nested way, but what if the numbers of recursion levels
> is not fixed or is different between the list elements as it is in my
> example?
>
> Any hint will be appreciated.
>
> Best,
>
> Georg
>
> ______________________________________________
> R-help_at_r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> and provide commented, minimal, self-contained, reproducible code.
>

```--
Brian D. Ripley,                  ripley_at_stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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