Arrays

An array is an ordered set of values all having the same data type. Elements of an array are referenced by their position within the array. Each array has a specified number of dimensions and each dimension has a specified lower and upper bound. The default low bound of a dimension is 1.

Array elements are stored in row-major order. The successive elements of a row are stored in adjacent memory locations. In arrays with more than two dimensions, the net effect is that when the elements are accessed in their storage order, the rightmost subscript varies most rapidly.

Examples:

DECLARE A(1:4) FIXED BINARY(15); 
DECLARE B(0:3,0:5) FLOAT BINARY(21); 
DECLARE C(-2:10) CHARACTER(5);
DECLARE D(25,4,2) POINTER;

In these examples, A is a one-dimensional array capable of holding fixed-point binary values. The lower bound is 1, the upper bound is 4, and the number of elements is 4.

B is a two-dimensional array capable of holding floating-point binary values. Both lower bounds are zero, and the upper bounds are 3 and 5. Therefore, B has 4 rows of 6 columns each.

C is a one-dimensional array capable of holding character-string values each of which has 5 characters. Its lower bound is -2 and its upper bound is 10, giving it a total of 13 elements.

D is a three-dimensional array capable of holding pointer values. Because no lower bounds are given, they are assumed to be 1. The array has a total of 200 elements organized as 25 planes, each of which is 4 rows in length and 2 columns in width.

The maximum number of array dimensions is 8. An array bound may be any number in the range -2^31 to 2^31 -1. Array bounds are also subject to the following restrictions:

The lower bound must be less than or equal to the upper bound.
The value (upper bound — lower bound) must be less than 2^31 -1.
The total size of the array must be less than 2^31 bytes. For information on calculating the size of an individual array element, see your Open PL/I User's Guide.

Note: The particular implementation of PL/I or the underlying operating system may also impose restrictions on the size of an individual data item, or on the total sizes of static and/or automatic data regions. For more information, see your Open PL/I User's Guide.

The bounds of AUTOMATIC, DEFINED, CONTROLLED, or BASED arrays may be specified by integer-valued expressions, as explained in the chapter Storage Classes. The bounds of STATIC arrays must be integer constants. The bounds of parameter arrays can be either integer constants or an asterisk (*). If an asterisk is given as the bound of a parameter array, it represents both the lower and upper bound and means that the actual bounds of the corresponding array argument are to be used as the bounds of the array parameter. For example:

DECLARE A(0:5) FIXED BINARY; 
CALL P(A);
   .
   .
   .
P: PROCEDURE(X);
   DECLARE X(*) FIXED BINARY;
      .
      .
      .

During the call of P in the previous example, the bounds of X are (0:5), and any reference to X is a reference to A.

The elements of an array are referenced using as many subscripts as the array has dimensions. For example:

DECLARE A(-2:5,4,3) FIXED BINARY;

In this example, a reference to A (-2 ,1,1) is a reference to the element in the first column of the first row in the first plane. A reference to A (3,2,1) is a reference to the element in the first column of the second row in the sixth plane.

A subscript of an array must be an integer valued expression. The use of fixed-point fractions or floating-point values as subscripts results in an error message from the Compiler. However, such values can be converted to integer values by use of the TRUNC, CEIL, FLOOR, or ROUND built-in functions.

Each subscript must lie within the range specified by its corresponding lower and upper bound. Unless subscript range checking has been requested, the Compiler does not produce code to check the range of subscript values. If checking is requested, any subscript that exceeds its range results in a signal of the ERROR condition.

Subscripted references to array elements can be used in any context that permits a variable reference.

A cross-section of an array may be referenced by using an asterisk as a subscript. The asterisk refers to the entire range of the corresponding dimension (lower bound through upper bound). In the previous example, a reference to A (*,1,1), refers to the first-row, first-column elements in all eight planes. A reference to A ( *, *,*), refers to all 96 elements of the array.

Array cross-sections may be transmitted in stream or record I/O, passed as arguments, and assigned to other arrays of the same size and shape.