Class: SPARQL::Algebra::Operator::Sample

Inherits:
Operator
  • Object
show all
Includes:
Aggregate
Defined in:
lib/sparql/algebra/operator/sample.rb

Overview

The SPARQL sample set function.

[127] Aggregate::= ... | 'SAMPLE' '(' 'DISTINCT'? Expression ')'

Examples:

SPARQL Grammar

PREFIX : <http://example/>

SELECT ?w (SAMPLE(?v) AS ?S)
{
  ?s :p ?v .
  OPTIONAL { ?s :q ?w }
}
GROUP BY ?w

SSE

(prefix ((: <http://example/>))
 (project (?w ?S)
  (extend ((?S ??.0))
   (group (?w) ((??.0 (sample ?v)))
    (leftjoin
     (bgp (triple ?s :p ?v))
      (bgp (triple ?s :q ?w))))) ))

See Also:

Constant Summary collapse

NAME = 
:sample

Instance Method Summary collapse

Methods included from Aggregate

#aggregate, #replace_aggregate!, #replace_vars!

Constructor Details

#initialize(*operands, **options) ⇒ Sample

Returns a new instance of Sample.

Raises:

  • (ArgumentError) 


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# File 'lib/sparql/algebra/operator/sample.rb', line 33

def initialize(*operands, **options)
  raise ArgumentError,
    "sample operator accepts at most one argument with an optional :distinct" if
    (operands - %i{distinct}).length != 1
  super
end

Instance Method Details

#apply(enum, **options) ⇒ RDF::Term

Sample is a set function which returns an arbitrary value from the multiset passed to it.

Parameters:

Returns:

Raises:

  • (TypeError) —

    If enum is empty



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# File 'lib/sparql/algebra/operator/sample.rb', line 47

def apply(enum, **options)
  enum.detect(lambda {raise TypeError, "Sampling an empty multiset"}) {|e| e.first}.first
end

#to_sparql(**options) ⇒ String

Returns a partial SPARQL grammar for this operator.

Returns:

  • (String) 


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# File 'lib/sparql/algebra/operator/sample.rb', line 56

def to_sparql(**options)
  distinct = operands.first == :distinct
  args = distinct ? operands[1..-1] : operands
  "SAMPLE(#{'DISTINCT ' if distinct}#{args.to_sparql(**options)})"
end