Next Previous Up Contents
Next: Null Values
Up: Algebraic Expression Syntax
Previous: Referencing Row Subset Flags

### 7.3 Referencing Table Parameters

Some tables have constant values associated with them; these may represent such things as the epoch at which observations were taken, the name of the catalogue, an angular resolution associated with all observations, or any number of other things. Such constants are known as table parameters and can be viewed and modified in the Parameter Window. The values of such parameters can be referenced in algebraic expressions as follows:

param$name
  If the parameter name has a suitable form (starting with a letter and continuing with letters or numbers) it can be referenced by prefixing that name with the string `param$`.
ucd$ucd-spec
  If the parameter has a Unified Content Descriptor it can be referenced by prefixing the UCD specifier with the string `ucd$`. Any punctuation marks in the UCD should be replaced by underscores, and a trailing underscore is interpreted as a wildcard. See Section 7.1 for more discussion.
utype$utype-spec
  If the parameter has a Utype, it can be referenced by prefixing the Utype specifier with the string `utype$`. Any punctuation marks in the Utype should be replaced by underscores. See Section 7.1 for more discussion.

Note that if a parameter has a name in an unsuitable form (e.g. containing spaces) and has no UCD then it cannot be referenced in an expression.

There are also a couple of special values:

$ncol
  The number of columns in the table. This figure refers to the underlying table, not the apparent table, so it is not affected by hiding columns.
$nrow
  The number of rows in the table. This figure refers to the underlying table, not the apparent table, so it is not affected by the value of the current subset. Note that this value is a `long` (8-byte integer); when using it in certain expressions you may find it necessary to convert it to an `int` (4-byte integer) using the `toInteger()` function.

Next Previous Up Contents
Next: Null Values
Up: Algebraic Expression Syntax
Previous: Referencing Row Subset Flags

TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@bristol.ac.uk