10.2 Referencing Parameter Values

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 (not to be confused with parameters passed to STILTS commands) and can be thought of as extra columns which have the same value for every row. The values of such parameters can be referenced in STILTS 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 10.1 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 10.1 for more discussion.

There are also a couple of special values:

$ncol

The number of columns in the table.

$nrow

The number of rows in the table. Note in some cases this is not known (e.g. if the table is being streamed), in which case the value of this variable is null. Note also 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.