In general, the number of coordinates you feed into a Mapping to represent a single point need not be the same as the number that comes out. Often these numbers will be the same, and often they will both equal 2 (because 2-dimensional coordinate systems are common), but this needn't necessarily be the case.
The number of coordinates required to specify an input point is represented by the integer attribute Nin and the number required to specify an output point is represented by Nout. These are read-only attributes common to all Mappings. Generally, their values are fixed when a Mapping is created.
In , we saw how the Nin attribute for a ZoomMap was initialised by the call to the constructor function AST_ZOOMMAP which created it. In this case, the Nout attribute was not needed and it implicitly took the same value as Nout, but we could have enquired about its value had we wanted, as follows:
INCLUDE 'AST_PAR' INTEGER NOUT, STATUS, ZOOMMAP STATUS = 0 ... NOUT = AST_GETI( ZOOMMAP, 'Nout', STATUS )
AST A Library for Handling World Coordinate Systems in Astronomy