AIPS HELP file for BY in 31DEC18
As of Mon Oct 15 23:21:56 2018
Use: In the FOR loop construction BY is used to signal the value by
which to increment the incrementing variable. (See HELP FOR).
This value may be negative. The inclusion of BY is optional. If
omitted, the increment will default to 1.
Grammar: FOR variable = start TO fini BY increment
statement1; statement2;... ; END
where 'variable' is a scalar variable that will change each
time through the loop, beginning at 'start', have 'increment'
added to it each time through the loop (the increment may be
positive or negative) and will end when the next loop would be
executed with 'variable' greater than (increment > 0; less than
with increment < 0) 'fini'. Note that the loop will be
executed at least once no matter what the start, fini and
increment parameters are. If the BY section is omitted, the
increment will default to 1.
Statementi are AIPS statements. These statements may not
have an omitted optional immediate numeric argument. Such
statements depend on the POPS stack being empty in the absence
of the argument. But the FOR loop uses the stack for the
'fini' and 'increment' values and so the stack is not empty.
Thus TVON with no argument will see a non-empty stack and
assume that it is getting an immediate argument from the stack.
Use in this case TVON(2**(TVCHAN-1)) inside your loop.
Verbs of this sort include EHEX, GROFF, GRON, HUEWEDGE,
IMWEDGE, REHEX, TVOFF, TVON, TVWEDGE.
Example: FOR I = K+3 TO K BY -1; A(I+1) = A(I); END
This example will shift the section of array A between
A(K) and A(K+3) forward 1 element.