DSSP Loops

DSSP provides two types of loops, as well as several words that can be used to terminate a loop from within.

DO loops

The DSSP DO loop is used when the number of repetitions (or the maximum number) is known in advance.

Example for DO:

* : HELLO ."Hello world!" CR ;
* 5 DO HELLO
Hello world!
Hello world!
Hello world!
Hello world!
Hello world!
* 

RP Loops

The DSSP RP loop is used when a loop must continue indefinitely, or until some condition is met.

Example for RP:

* : HELLO ."Hello world!" CR ;
* RP HELLO
Hello world!
Hello world!
Hello world!
Hello world!
Hello world!
Hello world!
Hello world!
Hello world!
Hello world!
Hello world!
(...and so on...)

Loop Exits

DSSP provides the following words to exit from a loop: EX, EX-, EX0, EX+, EXT

The simplest loop exit is EX. If executed, EX will unconditionally exit from a loop. EX is normally combined with one of the conditional words, as in the example below:

* LOOP  : LOOP  RP BODY ;
* BODY  : BODY  1- C BR- DONE NOTDONE ;
* DONE  : DONE  ."Exiting loop!" CR EX ;
* NOTDONE  : NOTDONE  ."The value is " . CR ;
* 5 LOOP
The value is 4
The value is 3
The value is 2
The value is 1
The value is 0
Exiting loop!
* 

The words EX-, EX0, and EX+ all work similarly to EX, except that the loop exit is done conditionally without relying on any other conditionals.

EX- will consume the top stack cell and exit the loop if the value was negative. EX0 will consume the top stack cell and exit the loop if the value was zero. EX+ will consume the top stack cell and exit the loop if the value was positive.

For example:

* : BODY 1- C EX- ."The value is " . ;
* : LOOP RP BODY ."Loop terminated!" CR ;
* 5 LOOP
The value is 4
The value is 3
The value is 2
The value is 1
The value is 0
Loop terminated!

Finally, EXT allows the program to (unconditionally) exit multiple layers of nested loops. EXT consumes the top of the stack and exits that many layers of loops (they can be either RP loops, DO loops, or some interleaving of the two). EXT is normally combined with one of the conditional words, as in the (admittedly complicated) example below:

* : FOOLOOP ."FOO" CR BAR ;
* : BAR RP BARLOOP ;
* : BARLOOP ."BAR" CR RP BAZ ;
* : BAZ ."BAZ" CR 1- C BR0 DONE EX ;
* : DONE ."Doing a nested exit." CR 3 EXT ;
* 3 RP FOOLOOP
FOO
BAR
BAZ
BAR
BAZ
BAR
BAZ
Doing a nested exit.