`Dataset`¶

Wraps data with type information for structured data handling.

A dataset is queried with the same successive-level operator spec Query takes. An operator that yields a collection gives back a dataset, while one that yields an atom returns it bare.

$ wo 'Normal[Dataset[{<|"a" -> 1|>, <|"a" -> 2|>, <|"a" -> 3|>}][Select[#a > 1 &]]]'
{<|a -> 2|>, <|a -> 3|>}

$ wo 'Dataset[{<|"a" -> 1|>, <|"a" -> 2|>}][Total, "a"]'
3

An integer selects a row:

$ wo 'Normal[Dataset[{<|"a" -> 1|>, <|"a" -> 2|>}][2]]'
<|a -> 2|>

Statistics and list functions applied to a dataset see the data it wraps, not its internal Dataset[data, type, metadata] structure:

$ wo 'Total[Dataset[{1, 2, 3}]]'
6

$ wo 'Mean[Dataset[{1, 2, 3}]]'
2

A function that reduces to a single value returns it bare, while one that yields a collection gives back a dataset:

$ wo 'Head[Total[Dataset[{1, 2}]]]'
Integer

$ wo 'Head[Sort[Dataset[{3, 1, 2}]]]'
Dataset

$ wo 'Normal[Sort[Dataset[{3, 1, 2}]]]'
{1, 2, 3}

Part indexes the data as well:

$ wo 'Dataset[{{1, 2}, {3, 4}}][[2, 1]]'
3

$ wo 'Dataset[{<|"a" -> 1, "b" -> 2|>}][[1, "a"]]'
1

A string in a grouping or sorting operator extracts that key:

$ wo 'Normal[Dataset[{<|"a" -> 1, "b" -> 5|>, <|"a" -> 1, "b" -> 6|>}][GroupBy["a"]]]'
<|1 -> {<|a -> 1, b -> 5|>, <|a -> 1, b -> 6|>}|>

The operator form of Query applies the same spec to a dataset:

$ wo 'Query[Total][Dataset[{1, 2, 3}]]'
6

$ wo 'Normal[Query[All, "a"][Dataset[{<|"a" -> 1|>, <|"a" -> 2|>}]]]'
{1, 2}

A bare rule at an operator position is read as an option to Query, so the level is left untouched:

$ wo 'Normal[Dataset[{<|"a" -> 1|>, <|"a" -> 2|>}][All, "a" -> "b"]]'

OptionValue::nodef: Unknown option a for Query.
{<|a -> 1|>, <|a -> 2|>}

A dataset is an atom for traversal, while Length and Dimensions still describe the data it wraps:

$ wo 'AtomQ[Dataset[{1, 2}]]'
True

$ wo 'Depth[Dataset[{1, 2}]]'
1

$ wo 'Length[Dataset[{1, 2}]]'
2

$ wo 'Dimensions[Dataset[{{1, 2}, {3, 4}}]]'
{2, 2}

Dataset¶

`Dataset`¶