Mapping over trees #236

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1. lionel- commented on Sep 1, 2016

   

   lionel-

   on Sep 1, 2016

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   On that note I wonder if the following would be good UI:

   array_tree(x, ~last)
   array_tree(x, list(1, 2, ~last)
   
   x %>% at_depth(~last, f)
2. lionel- commented on Sep 15, 2016

   

   lionel-

   on Sep 15, 2016

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   Other useful idiom for tree structures: visit each node twice. Once on the way in, the second time on the way back. See https://stlab.adobe.com/group__asl__tutorials__forest.html

   The mapped function would get a boolean indicating if this is the second time we see the node.
3. alistaire47 commented on Feb 10, 2017

   

   alistaire47

   on Feb 10, 2017

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   Would this map over the deepest level or all leaf nodes? The latter (or some other intelligible version of rapply) could be really useful for parsing deeply nested data coming from JSON or the like.
4. lionel- commented on Feb 26, 2017

   

   lionel-

   on Feb 26, 2017

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   http://okmij.org/ftp/continuations/zipper.html
5. lionel- commented on Feb 26, 2017

   

   lionel-

   on Feb 26, 2017

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   @alistaire47 yes I was thinking at_bottom() would apply a function to leaves. We could have map_tree() to apply a function to all nodes, then at_bottom() would be implemented as:

   x %>% map_tree_if(is_atomic, fn)

   And maybe have map_tree_until(), which would not inspect further the nodes for which the predicate returns FALSE, which could be useful for applying a function to list-like S3 objects.
6. lionel-

   changed the title [-]at_bottom()[/-] [+]Mapping over trees[/+] on Feb 26, 2017
7. hadley

   added

   featurea feature request or enhancementa feature request or enhancement

   on Mar 3, 2017
8. hadley commented on Mar 5, 2017

   

   hadley

   on Mar 5, 2017

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   What if instead of at_bottom(.x, .f) you simply did at_depth(.x, -1, .f)?

   And what if at_depth() was instead called modify_depth()? And to match the other modify() functions was called modify_depth()?
9. hadley commented on Mar 5, 2017

   

   hadley

   on Mar 5, 2017

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   I also wonder about an option to at_depth() that gives you this behaviour:

     recurse <- function(x, depth) {
       if (depth > 1) {
         if (is.atomic(x)) {
           x
         } else {
           lapply(x, recurse, depth = depth - 1)
         }
       } else {
         lapply(x, .f, ...)
       }
     }

   So that if the list is ragged, you can choose to only operate on lists at a certain level, ignoring branches that aren't that deep.
10. hadley

    closed this as completedin 3a38083on Mar 7, 2017
11. jayhesselberth

    added a commit that references this issue on Jan 10, 2025

    Add .ragged & support negative depths

    ...

    This issue will close once commit 3a38083 is merged into the 'master' branch.

    3a38083