
subscript(+Term, ++Subscript, -Elem)

   Accesses the subterm Elem of Term, as specified by Subscript

Arguments
   Term                Compound term (possibly nested), string, or external data handle.
   Subscript           A list of integers, ranges or integer arithmetic expressions.
   Elem                Prolog term.

Type
   Term Manipulation

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.

   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.

   NOTE: subscript/3 implements a superset of the functionality of arg/3.
   So arg/3 is likely to be faster than subscript/3 in cases where they
   implement the same functionality, i.e. structure argument lookup or
   one/multi-dimensional array element lookup.



Modes and Determinism
   subscript(+, ++, -) is det

Modules
   This predicate is sensitive to its module context (tool predicate, see @/2).

Exceptions
     4 --- Term or Subscript are not sufficiently instantiated.
     5 --- Term not compound or Subscript not integer list.
     6 --- 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, string_code / 3, xget / 3, flatten_array / 2
