15.1 
Repeating
operations with a loop
There are many situations where you will a macro to perform an
action repeatedly, either a certain number of times or until a
certain condition exists. For
example, you might want to perform the same operations at each
occurrence of a specific string in a document, or you might want
to print numbered labels. This is made possible by creating a
"loop": the commands within the loop are executed over and over
again, until the macro exits from the loop.
Caution: It is very easy to forget to
provide for an exit
from a loop. Such an "endless loop" in a macro will
keep executing forever until you manually terminate the macro by
pressing the Escape key (or until you run out of memory, or some
other external constraint stops the macro). Remember that you
must provide for some occurrence that will satisfy the condition
that ends the loop.
15.2 Loops with
the
Go() command
The simplest loop consists of a Label() and a
Go() command to jump back to that label. You
must provide some test for the macro to exit the loop when that
test is met.
Examples:
|
x
:=1
Label(StartOfLoop@)
Type("This is line number ")
Type(x)
HardReturn
x:=x+1
// increment
x
If(x>10) Quit EndIf
// quit after 10 cycles
Go(StartOfLoop@)
// return to start of
loop
|
|
SearchString("foo")
Label(LoopStart@)
SearchNext()
Type("/bar")
Go(LoopStart@)
// macro will end when search fails
|
These examples are given only for educational purposes; their
form is not recommended because Go() commands
should be avoided whenever possible, and the condition is better
tested within the command than within the loop. The
While() and Repeat() commands
should be used instead of GO() loops.
15.3 Loops with
the
While() command
It is easier and more efficient to write loops using the
While() command. The
While() command tests the condition before the
loop is entered, and if the condition is true the commands in the
loop are executed. Execution then returns to the
While() command, and the condition is tested
again. The cycle continues as long as the condition is true.
The form of the While() loop is:
|
While(condition)
|
| |
[commands to be executed] |
| |
EndWhile |
You can see the similarity to
If() EndIf.
In order to avoid an endless loop, something must occur in the
loop that eventually will cause the condition to be false; the
most common examples are moving the cursor, and changing the
value of a counter. For
example, the following loop could be used to create numbered
labels from 1 to 100:
|
x:=1
While(x<=100)
Type("Label No. ")
Type(x)
HardPageBreak
x:=x+1
EndWhile |
If variable x were not
incremented, it would never reach 100, and the condition would
never become untrue, so the loop would never end.
Sometimes a search is used to position the cursor, and then
the cursor is moved to a position before the found text.
If this is performed in a loop (to catch each occurrence of the
text), the cursor must be moved to a position after the
found text before the next iteration through the loop, otherwise
the cursor will be positioned at the same occurrence of the text
in each pass through the loop, resulting in an endless loop.
This bears repeating: It is very easy to forget to include a
step that will finally
result in the condition being untrue to end the loop. If a macro
seems to run forever, check to make sure you have not
inadvertently created an endless loop.
15.4 Loops with
the
Repeat command
When using the While() command, the condition is
tested before the first pass through the loop. This means that
if the condition is false at the start, the commands in the loop
will not be executed even once. If you want to make certain that
the loop is entered at least once, you can have the test
performed at the end of the loop by using the
Repeat and Until() commands, in
the following form:
|
Repeat |
| |
[commands to be executed] |
| |
Until(condition)
|
The commands in the loop will be executed at least once, and
will keep being executed until the condition is true. Note that
this is the opposite of the While() test, where
the loop continues as long as the condition is true,; with
Repeat-Until, the loop continues as long as the
condition is false.
Other commands that set up loops are
For/EndFor, ForNext/EndFor, and
ForEach/EndFor. These commands are beyond the
scope of this tutorial.
