Expressions

Expressions define a quantity computed from the existing scalar columns, vector column elements and parameters of a catalogue using some algebraic or logical (boolean) expression. An expression adopts a value for every row in the catalogue. It is similar to a column, except that its value is computed `on the fly' from existing columns (and parameters), rather than being stored in the catalogue. Usually an expression will evaluate to a numeric value, but it may equally well evaluate to a LOGICAL or CHARACTER value. For example, if a catalogue contained columns x and y an expression might be `x + y'. The syntax for specifying expressions is described in Appendix .

An expression has a set of attributes which are identical to those for a scalar column (see Section  and Table ), but with the following exceptions.

• The GENUS attribute is always set to CAT__GVIRT.

• The EXPR attribute is set to the algebraic expression used to compute the value of the expression.

• The DIMS attribute is always CAT__SCALR; expressions are always scalars and vector expressions are forbidden.