References can be made to individual elements within a table of like elements by specifying indexing for that reference. An index is assigned to that level of the table by using the INDEXED BY phrase in the definition of a table. A name given in the INDEXED BY phrase is known as an index-name and is used to refer to the assigned index. The value of an index corresponds to the occurrence number of an element in the associated table or any other table. An index-name must be initialized before it is used as a table reference. An index-name can be given an initial value by a SET statement.
Unlike a subscript, which is simply a numeric data item or literal, an index is a special type of item and holds a representation of an occurrence number. The form of this representation can vary; the contents of an index cannot be regarded as a numeric value.
Direct indexing is specified by using an index-name in the form of a subscript. Relative indexing is specified when the index-name is followed by the operator "+" or "-", followed by an unsigned integer numeric literal, all delimited by the balanced pair of separators left parenthesis and right parenthesis following the table element data-name. The occurrence number resulting from relative indexing is determined by incrementing (where the operator "+" is used) or decrementing (where the operator "-" is used), by the value of the literal, the occurrence number represented by the value of the index. When more than one index-name is required, they are written in the order of successively less inclusive dimensions of the data organization.
A table can be indexed only by the index described in its INDEXED BY phrase.
An index described for some other table can be used instead provided the two tables have elements of the same size.
At the time of execution of a statement which refers to an indexed table element, the value contained in the index used to index it must neither correspond to a value less than one nor to a value greater than the highest permissible occurrence number of an element of the associated table. This restriction also applies to the value resulting from relative indexing.
If the NOBOUND Compiler directive is specified, this rule is not applicable at run time.
Up to 3 index-names can be used with a data-name.
Up to 7 index-names can be used with a data-name.
Up to 16 index-names can be used with a data-name.
The general format for indexing is included in the general format for subscripting; see the section Subscripting.