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subscript(+Term, +Subscript, ?Elem)

Accesses the subterm Elem of Term, as specified by Subscript.
+Term
Compound term (possibly nested), string, or external data handle.
+Subscript
A list of integers, ranges or integer arithmetic expressions.
?Elem
Prolog term.

Description

If term is a compound term, e.g. a vector represented as a structure, or a matrix represented as a structure of structures and so on, then subscript/3 provides access to the term's components. Subscript is a list of (sub)structure argument indices describing which element to access. subscript/3 could be implemented with a sequence of calls to arg/3.

The indices can be either an integer expression or a range in the form Lower..Upper where Lower and Upper are integer expressions. The expressions are evaluated and the corresponding components (or the components in the range specified) accessed.

If Term is a string, Subscript must be a list of the form [Index], and Elem is obtained via string_code(Index, Term, Elem).

If Term is an external data handle, Subscript must be a list of the form [Index], and Elem is obtained via xget(Term, Index, Elem).

The main use for this predicate is to provide array syntax in arithmetic expressions. Consider the arithmetic expression 

    X is Mat[I,J] + 1
which the ECLiPSe parser parses as 

    X is subscript(Mat,[I,J]) + 1
and the arithmetic evaluation mechanism turns that into 

    subscript(Mat,[I,J],T), +(T,1,X)
If Subscript contains a range of the form From..To, then this results in the retrieval of a list of elements with the indices from From to To.

Fail Conditions

None.

Resatisfiable

No.

Exceptions

(4) instantiation fault
Term or Subscript are not sufficiently instantiated.
(5) type error
Term not compound or Subscript not integer list.
(6) out of range
Subscript out of range.

Examples

    [eclipse 6]: subscript(s(t(a,b),t(c,d),t(e,f)), [3,2], X).
    X = f
    yes.

    [eclipse 11]: Vector = v(12,13,14,15), X is Vector[3].
    X = 14
    Vector = v(12, 13, 14, 15)
    yes.

    [eclipse 12]: Matrix = m(r(1,2,3),r(4,5,6),r(7,8,9)), X is Matrix[2,1].
    X = 4
    Matrix = m(r(1, 2, 3), r(4, 5, 6), r(7, 8, 9))
    yes.

    [eclipse 18]: Matrix = m(r(1,2,3),r(4,5,6),r(7,8,9)), Row is Matrix[2].
    Row = r(4, 5, 6)
    Matrix = m(r(1, 2, 3), r(4, 5, 6), r(7, 8, 9))
    yes.

    [eclipse 5]: Matrix = m(r(1,2,3),r(4,5,6),r(7,8,9)), 
                 subscript(Matrix, [2,1..3], Row),
                 subscript(Matrix, [1..3,2], Col),
                 subscript(Matrix, [2..3,1..2], Sub).
    Matrix = m(r(1, 2, 3), r(4, 5, 6), r(7, 8, 9))
    Row = [4, 5, 6]
    Col = [2, 5, 8]
    Sub = [[4, 5], [7, 8]]
    yes.

See Also

arg / 3, dim / 2, xget / 3