Arrays and The Array Subscript Notation
In B-Prolog, the maximum arity of a structure is 65535. This entails that a structure can be used as a one-dimensional array, and a multi-dimensional array can be represented as a structure of structures. To facilitate creating arrays, B-Prolog provides a built-in, called new_array(X,Dims), where X must be an uninstantiated variable and Dims a list of positive integers that specifies the dimensions of the array. For example, the call new_array(X,) binds X to a two dimensional array whose first dimension has 10 elements and second dimension has 20 elements. All the array elements are initialized to be free variables.
The built-in predicate arg/3 can be used to access array elements, but it requires a temporary variable to store the result, and a chain of calls to access an element of a multi-dimensional array. To facilitate accessing array elements, B-Prolog supports the array subscript notation X, where X is a structure and each Ii is an integer expression. This common notation for accessing arrays is, however, not part of the standard Prolog syntax. To accommodate this notation, the parser is modified to insert a token ^ between a variable token and is just a shorthand for X^. This notation is interpreted as an array access when it occurs in an arithmetic expression, a constraint, or as an argument of a call to @=/2. In any other context, it is treated as the term itself. The array subscript notation can also be used to access elements of lists. For example, the nth/3 predicate can be defined as follows:
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