Components of a CAT catalogue

This section describes the components of a CAT catalogue. It is necessary to understand the structure of a CAT catalogue in order to use the CAT library effectively. An idealized computer-readable version of an astronomical catalogue, or similar tabular dataset, might comprise the following elements:

1. the table of values which comprise the catalogue,

2. a description of this table; the details of all the columns that it contains, the number of rows etc,

3. textual information about the catalogue; perhaps a short description of the catalogue or a copy of a published paper describing it. This information is intended to be read by a human rather than interpreted by a computer.

The CAT library is mostly concerned with the first two items. However, it also provides some simple facilities to retrieve and write the textual information of the third item. These latter facilities are provided so that the textual information in a catalogue can be displayed to a user or copied when a new catalogue is created from an old one. The routines for manipulating textual information are described in Section . They do not interact with any other items in a CAT catalogue and they are not mentioned again in this section.

The table in a CAT catalogue is very similar to a relation in the theory of relational databases, and has many of the same properties. Each row in the table must contain the same number of fields. Corresponding fields in different rows must be of the same type. The table may contain an arbitrary number of rows. In the formal theory of relational databases, no two rows may be identical. CAT relaxes this rule by permitting identical rows, though it is difficult to see what purpose such rows might serve.

The internal organization of a CAT catalogue (the way it is formatted on disk) is unknown to an application using the CAT library. The values in the catalogue are accessed purely through the subroutine interface to the CAT library.