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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

### Extracting sub-terms or array elements

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. In the basic case, the indices are expressions that evaluate to positive integers. These integers are then used to extract the corresponding sub-term of Term, e.g. an index list of [3,2] would extract the 2nd argument of the 3rd argument of Term.

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.

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)

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.

### Extracting sub-arrays

If Subscript contains a range expression of the form From..To, then this results in the retrieval of the sub-array of elements with the indices from From to To. A sub-array is always a term with functor []/N.

The special shorthand * is equivalent to 1..Arity and stands for all elements in the corresponding dimension. For instance, Mat[*,3] extracts the whole 3rd column of a matrix Mat.

If Subscript contains multiple such range expressions, a nested sub-array is retrieved.

### Operation on strings and handles

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).

### Modes and Determinism

- subscript(+, ++, -) is det

### Modules

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

### 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

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

        ?- Vector = v(11,12,13,14,15), X is Vector[4].
        X = 14
        yes.

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

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

        ?- Vector = [](11,12,13,14,15), X is Vector[2..4], Y is Vector[3..3].
        X = [](12, 13, 14)
        Y = [](13)
        yes.

        ?- Matrix = []([](1,2,3), [](4,5,6), [](7,8,9)),
    	 subscript(Matrix, [2,1..3], Row2),
    	 subscript(Matrix, [1..3,2], Col2),
    	 subscript(Matrix, [2..3,1..2], Sub).
        Row2 = [](4, 5, 6)
        Col2 = [](2, 5, 8)
        Sub = []([](4, 5), [](7, 8))
        yes.

        ?- Matrix = []([](1,2,3), [](4,5,6), [](7,8,9)),
    	 subscript(Matrix, [2,*], Row2),
    	 subscript(Matrix, [*,2], Col2).
        Row2 = [](4, 5, 6)
        Col2 = [](2, 5, 8)
        yes.

## See Also

arg / 3, dim / 2, string_code / 3, xget / 3, array_flat / 3