On Tuesday, May 6, 2014 4:07:11 PM UTC+1, Alex Miller wrote:
>
> I wrote this article long ago which hints about this at the end:
> https://www.ibm.com/developerworks/library/j-treevisit/
>
I started from that actually, very helpful article.
I have since noticed a bug in my previous skip function where it would loop
infinitely when skipping from the rightmost location.
The fix includes an end function, so I can no just iterate backwards using
that as you suggested.
Leaving this here for future reference, in case anybody comes across the
same problem:
(defn end
"returns the location loc where (end? (next loc)) is true."
[loc]
(loop [loc loc]
(let [loc (z/rightmost loc)]
(if (z/branch? loc)
(recur (z/down loc))
loc))))
(defn skip
"returns the next location that is not a child of this one"
[start-loc]
(loop [loc start-loc]
(cond
; can't skip, jump to end
(nil? loc) (z/next (end start-loc))
; at end
(z/end? loc) loc
; go to right/up
true (or (z/right loc)
(recur (z/up loc))))))
>
> The approach I have taken for editing trees with zippers is to do a
> post-walk from end to beginning - that way you're always done transforming
> and will not walk into your edited subtrees. The article does talk a little
> about how to separate navigation from transformation; it's not particularly
> hard. You want to start from your rightmost node, which you can get from a
> repeated application of zip/rightmost or last of zip/rights. Then
> repeatedly call prev till you reach a node without a parent at that point
> convert the loc to a node in the termination.
>
> I can dig up actual code for this later if you're interested.
>
> Alex
>
>
> On Monday, May 5, 2014 6:01:04 PM UTC-5, Pascal Germroth wrote:
>>
>> Hi,
>>
>> I'm using clojure.zip to edit a tree by visiting each location using
>> zip/next, possibly using zip/replace to alter the tree.
>> There are cases where I replace a part of the tree with another tree that
>> will/must not be visited, but I couldn't find a good way to skip nodes,
>> since
>> (zip/next (zip/replace loc new-subtree)) will walk right into my new
>> tree, and I can't use (zip/right (zip/replace loc new-subtree)) as the
>> replaced location might already be the rightmost.
>>
>> Is there a built-in function I missed, or a zip enhancement library I
>> could use?
>>
>> (defn skip
>> "returns the next location that is not a child of this one"
>> [loc]
>> (if (or (z/end? loc) (nil? loc))
>> loc
>> (loop [loc loc]
>> (or (z/right loc)
>> (recur (z/up loc))))))
>>
>> I came up with this replacement, does that seem like a good idea, or am I
>> using zip completely wrong (because what I really would like to do is
>> iterate backwards through the tree, starting at the end, using zip/prev;
>> but there's also no function to just jump to the end as far as I can tell)
>>
>>
>> Cheers,
>>
>> --
>> pascal
>>
>
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