DO / LOOP

Syntax

Syntax: DO [WHILE|UNTIL cond] : ... : LOOP [WHILE|UNTIL cond]
Source: pb_cg_emit_do (pb_codegen.c:5639)

Description

Creates a loop with flexible condition placement. Conditions can be tested at the top (DO WHILE / DO UNTIL) or at the bottom (LOOP WHILE / LOOP UNTIL). A bare DO...LOOP with no condition creates an infinite loop (use EXIT DO to break). WHILE continues while the condition is true; UNTIL exits once the condition becomes true. ITERATE DO jumps to the condition check at the bottom of the loop.

Parameters

Parameter	Description
`WHILE condition`	Loop continues while the condition is TRUE (non-zero). Can appear at DO (pre-test) or LOOP (post-test).
`UNTIL condition`	Loop exits when the condition becomes TRUE. Can appear at DO (pre-test) or LOOP (post-test).
(no condition)	A bare `DO...LOOP` loops forever. Must use `EXIT DO` to break out.

Example

FUNCTION PBMAIN() AS LONG
    LOCAL x AS LONG
    LOCAL input AS STRING
    
    ' Pre-test: DO WHILE x > 0
    x = 5
    DO WHILE x > 0
        PRINT "Countdown: "; x
        x = x - 1
    LOOP
    PRINT "Done!"
    
    ' Post-test: loop at least once
    x = 0
    DO
        PRINT "x = "; x
        x = x + 1
    LOOP UNTIL x > 3
    
    ' Infinite loop with EXIT
    x = 0
    DO
        x = x + 1
        IF x > 10 THEN EXIT DO
        ' Skip even numbers
        IF (x AND 1) = 0 THEN ITERATE DO
        PRINT "Odd: "; x
    LOOP
    
    FUNCTION = 0
END FUNCTION

Generated Assembly

DO WHILE (pre-test)

__pb_do_N:                            ; LOOP TOP
; Evaluate condition
; Condition-false-jump -> __pb_do_end_N
; ... body ...
jmp    __pb_do_N                       ; loop back
__pb_do_end_N:

DO UNTIL (pre-test, exits when true)

__pb_do_N:
; Evaluate condition
; Condition-true-jump -> __pb_do_end_N (inverted from WHILE)
; ... body ...
jmp    __pb_do_N
__pb_do_end_N:

LOOP WHILE (post-test)

__pb_do_N:
__pb_do_iterate_N:                    ; ITERATE target
; ... body ...
; LOOP WHILE condition:
; Evaluate condition -> RAX
test   rax, rax
jne    __pb_do_N                       ; loop if true
__pb_do_end_N:

LOOP UNTIL (post-test)

__pb_do_N:
__pb_do_iterate_N:
; ... body ...
; LOOP UNTIL condition:
test   rax, rax
je     __pb_do_N                       ; loop if false
__pb_do_end_N:

Infinite DO...LOOP

__pb_do_N:
; ... body ...
jmp    __pb_do_N                       ; unconditional loop
; (no __pb_do_end_N generated)

EXIT DO

jmp    __pb_do_end_N                   ; exits the loop

ITERATE DO

jmp    __pb_do_iterate_N               ; skips to condition check