Appending and Prepending columns to a dataset in 10.0.2.0

Question

fixed in 10.1 (windows)

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I just experience a problem in prepending columns to a dataset. I did this a hundred times in 10.0.1.0 - no problems. Now it seems as if it is only possible to append/prepend one column at a time.

dataset = Dataset[{
   <|"a" -> 1, "b" -> "x", "c" -> {1}|>,
   <|"a" -> 2, "b" -> "y", "c" -> {2, 3}|>,
   <|"a" -> 3, "b" -> "z", "c" -> {3}|>,
   <|"a" -> 4, "b" -> "x", "c" -> {4, 5}|>,
   <|"a" -> 5, "b" -> "y", "c" -> {5, 6, 7}|>,
   <|"a" -> 6, "b" -> "z", "c" -> {}|>}]

Then:

Prepend[#, "First" -> "one"] & /@ dataset

works fine, but

Prepend[#, {"First" -> "one", "Second" -> "two"}] & /@ dataset

gives an error message:

Expression of the form Prepend[SkeletonForm[Struct[{a, b, c},
{Atom[Integer],  Atom[String], Vector[Atom[Integer], AnyLength]}],
{weakEval2[First -> one],  weakEval2[Second -> two]}]] cannot be
evaluated.

Can anyone give me a hint, if I´m doing wrong (but it worked fine ´til yesterday wen 10.0.2.0 came....)

Answer 1

This is a bug in 10.0.2 that will be fixed in 10.0.3 (if not earlier, and pushed out via a paclet update), but for now you can use this 'indirection' workaround:

prepender[x_] := Prepend[x, {"First" -> "one", "Second" -> "two"}];
prepender /@ dataset

Answer 2

Fixed in 10.1 (windows):

---

code

dataset = Dataset[{<|"a" -> 1, "b" -> "x", "c" -> {1}|>, <|"a" -> 2, "b" -> "y", "c" -> {2, 3}|>, <|"a" -> 3, "b" -> "z", 
    "c" -> {3}|>, <|"a" -> 4, "b" -> "x", "c" -> {4, 5}|>, <|"a" -> 5, "b" -> "y", "c" -> {5, 6, 7}|>, <|"a" -> 6, "b" -> "z", 
    "c" -> {}|>}]
Prepend[#, {"First" -> "one", "Second" -> "two"}] & /@ dataset

Answer 3

Workaround: Use Normal to unpack the dataset before prepending:

Prepend[#,{"First"->"one","Second"->"two"}]& /@ Normal@dataset // Dataset