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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