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https://issues.apache.org/jira/browse/ARROW-18265?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17637679#comment-17637679
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Joris Van den Bossche commented on ARROW-18265:
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Yes, but the square-bracket form is basically the same as a single number, that
is how it gets parsed (the top post could have been more explicit about this,
instead of only mentioning FromDotPath, but also the resulting FieldPath).
Using a small workaround through StructFieldOptions to show this in python:
{code}
>>> pc.StructFieldOptions(".x[0]")
StructFieldOptions(field_ref=FieldRef.Nested(FieldRef.Name(x)
FieldRef.FieldPath(0)))
>>> pc.StructFieldOptions(["x", 0])
StructFieldOptions(field_ref=FieldRef.Nested(FieldRef.Name(x)
FieldRef.FieldPath(0)))
{code}
So those two give currently the same result. Essentially, at the moment a "[0]"
element in a path gets interpreted as {{pc.field(0)}}.
Given the above, AFAIU, we either agree that {{field_ref(0)}} for a list type
means selecting the first element of each list (and thus not for selecting the
flat values), or we either have to introduce a new concept in
FieldRef/FieldPath to represent a list element selection (and thus also change
how a "[0]" in a string path gets parsed to use this new object).
> [C++] Allow FieldPath to work with ListElement
> ----------------------------------------------
>
> Key: ARROW-18265
> URL: https://issues.apache.org/jira/browse/ARROW-18265
> Project: Apache Arrow
> Issue Type: Improvement
> Components: C++
> Reporter: Miles Granger
> Assignee: Miles Granger
> Priority: Major
> Labels: pull-request-available
> Fix For: 11.0.0
>
> Time Spent: 1h 40m
> Remaining Estimate: 0h
>
> {{FieldRef::FromDotPath}} can parse a single list element field. ie.
> {{{}'path.to.list[0]`{}}}but does not work in practice. Failing with:
> _struct_field: cannot subscript field of type list<....>_
> Being able to add a slice or multiple list elements is not within the scope
> of this issue.
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