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Why does taking a Part fail in a Dataset query function when there is an operation in the index argument?
dataset = Dataset[Table[<|"a" -> n|>, {n, 3}]];
dataset[All, Range[4][[#a]]&]
dataset[All, Range[4][[#a + 1]]&]
To get it to work I need to use Extract instead:
dataset[All, Extract[Range[4], {#a + 1}]&]
Are there other workarounds?
This behavior is concerned with Association's HoldAllComplete attribute.
Attributes[Association]
{HoldAllComplete, Protected}
I am not completely certain of the internal workings but it appears that the a# + 1 expression is being held unevaluated when passed to Part. Wrapping the expression in Evaluate to force evaluation does not appear to help.
dataset[All, Range[4][[Evaluate[#a + 1]]] &]
Message: Plus[1, Atom[Integer]] is not a valid part specification.
However, you there is a workaround by passing the expression as a parameter to a pure function.
dataset[All, Range[4][[#]] &@(#a + 1) &]
Hope this helps.
How about
ds = Dataset[Table[<|"a" -> n|>, {n, 3}]];
ds[All, "a", # + 1 &]
BTW, you 1st query can be simplified to
ds[All, "a"]
