With many variables streaming past, there is a need to keep track on some particular variables. It would be rather inefficient and laborious to need to continually call uvprobvr, to check on particular variables. The uvtrack routine is used to instruct the uv routines to keep track of when certain variable changes its value, and to perform special processing on these variables at a later stage. Typically uvtrack would be called soon after uvopen, marking all the variables of particular interest. The special processing that the uv routines perform is dictated by the switches argument. This is a string, consisting of several characters, each character representing a particular processing step to be taken. Currently there are two switches -- u and c. The u switch is used by uvupdate, whereas the c switch is used by uvcopyvr.
The routine uvupdate returns a .true. value if one of the variables, marked with the u switch, has been updated ``recently'' (see Section 2.5.10).
The routine uvcopyvr copies variables marked with the c switch, from the input dataset (given by tin) to the output dataset (given by tout) if they have changed ``recently'' (see Section 2.5.10). You need only mark the variables in the input dataset.