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LOOPS
There are essentially two forms of loop
operation
(1)
DO …
LOOP
(2)
DO …
+LOOP
The first loop structure is used as follow:
limit start DO
… forth-words
… +LOOP
The ‘forth-words’ within the loop
are executed until start equals to limit, incrementing the start (or index) by
one each time.
Type:
: TEST1 5 0 DO ." Forth " CR LOOP ;
<CR>
typing in
TEST <CR>
will print
Forth
Forth
Forth
Forth
Forth
The second loop structure is used as
follow:
limit start DO
… forth-words
… increment
+LOOP
The ‘forth-words’ within the loop
are executed from start to limit with the index being incremented or decremented
by the value increment.
Try:
: TEST2 5 0 DO ." Hello! " CR 2 +LOOP ;
<CR>
executing TEST2
will print
Hello!
Hello!
Hello!
Since the limit and index are held on the
return-stack, it would be useful if we could examine the index. Well, there are
words to do this:
I
This will copy the loop index from the
return stack onto the data stack.
J
This will push the value of the nested LOOP
index to the stack.
K
This will push the value of the double
nested LOOP index to the stack.
Type:
: TEST3
4 0 DO 4 0 DO 4 0 DO <CR>
I J K . . . CR <CR>
LOOP LOOP LOOP ; <CR>
executing
TEST3
will print
111
112
113
...
and so on.
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