8.1 Using the Stack

The stack is a list of indices that can be used to gather scan numbers or record numbers to be used in a later operation, such as averaging. The stack system is modeled closely after the UNIPOPS commands.

To add entries to the stack, use the addstack command or the appendstack command. The addstack command adds a sequence of entries using parameters that describe the first entry, last entry, and increment. The appendstack command appends an array of indices to the stack. For example:

emptystack             ; clears the contents of the stack
addstack, 15           ; Add only index 15
addstack, 18, 21       ; Add indices 18 through 21
addstack, 22, 26, 2    ; Add indices 22 through 26 with an increment of 2. (22, 24, 26)
appendstack, [29, 35]  ; Adds indices 29 and 35 to the stack

The tellstack command lists the indices currently contained in the stack. The GBTIDL global variable !g.acount contains the total number of entries in the stack. The power of the stack will become more evident in the discussion on averaging data. For now, here is a simple example of using the stack to show spectrum headers for scans 6, 8, 10 and 12:

emptystack 
addstack, 6, 12, 2 
for i=0,!g.acount-1 do getnod, astack(i) & header & end

The following procedure gives an example of one way the stack could be put to use. The procedure averages Nod data identified by scan numbers listed in the stack. To use a procedure like this one, first populate the stack with the appropriate scan numbers then call the procedure.

   pro myavg,_extra=extra
   freeze
   for i=0,!g.acount-1 do begin
       getnod,astack(i),plnum=0,units='Jy',_extra=extra
       accum
       getnod,astack(i),plnum=1,units='Jy',_extra=extra
       accum
   endfor
   ave
   unfreeze
   show
   end

The following stack commands are available.

Command	Purpose
addstack	Adds a sequential list of indices to the stack, using addstack,begin,end,increment syntax
appendstack	Adds a single index or array of indices to the stack
astack()	Returns the value of a specific stack entry, given an index,
avgstack	Averages the records associated with the stack entries
delete	Removes a stack entry from the list
deselect	Removes indices from stack based on criteria such as source, polarization, and integration number.
emptystack	Clears the stack
liststack	Runs a list on records identified by the stack.
select	Adds indices to stack based on criteria such as source, polarization, and integration number.
tellstack	Shows the indices in the stack
	or returns all of the stack entries if no index is specified