Andrew Kesterson
cdaefcc941
Organised the way the C128 Programmer's Reference Guide is: the language first, then each hardware area, then the reference sections. One markdown file per chapter. The verb and function references are generated from the interpreter's own dispatch table, with an assertion that every row is described, so they cannot drift out of step with what the program accepts. 98 verbs and 30 functions. Every example was run before it was written down, which caught three claims that were wrong: a whole FOR loop on one line prints nothing rather than looping once, MID and INSTR count from zero where a C128 counts from one, and a multi-line DEF returns a value the caller has to assign away. Chapter 13 is the list a BASIC 7.0 programmer needs -- roughly sixty documented differences, including the two known FOR defects and the fact that drawing does not survive a frame. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
4.0 KiB
4. Control flow
IF ... THEN ... ELSE
10 A# = 5
20 IF A# = 5 THEN PRINT "FIVE" ELSE PRINT "NOT FIVE"
The condition is a whole expression, so AND, OR and NOT all work in one:
10 IF A# > 0 AND A# < 10 THEN PRINT "IN RANGE"
Everything after THEN on the line belongs to the condition, which matters as soon
as you put several statements on a line:
| Line | Condition | What runs |
|---|---|---|
IF C THEN A : B |
true | A, B |
IF C THEN A : B |
false | nothing |
IF C THEN A ELSE B : D |
true | A |
IF C THEN A ELSE B : D |
false | B, D |
The remainder always belongs to whichever arm was written last.
Blocks
BEGIN and BEND make an IF span lines:
10 IF A# = 5 THEN BEGIN
20 PRINT "IN THE BLOCK"
30 PRINT "STILL IN IT"
40 BEND
50 PRINT "AFTER"
When the condition is false every line up to the BEND is skipped.
FOR ... NEXT
10 FOR I# = 1 TO 5
20 PRINT I#
30 NEXT I#
STEP sets the stride, and a negative one counts down:
10 FOR I# = 10 TO 0 STEP -2
20 PRINT I#
30 NEXT I#
The counter is an ordinary variable and the body may assign to it. Two things to know:
- The counter does not survive the loop. It lives in the loop's own scope, so reading it afterwards gives zero. On a C128 it keeps its final value.
- A step that overshoots runs the body one extra time.
FOR I = 1 TO 9 STEP 3runs with 1, 4, 7 and 10. Both are recorded as known defects; see Chapter 13.
EXIT leaves the loop early:
10 FOR I# = 1 TO 100
20 IF I# = 5 THEN EXIT
30 NEXT I#
DO ... LOOP
The condition can go on either end, or neither:
10 I# = 0
20 DO WHILE I# < 3
30 PRINT I#
40 I# = I# + 1
50 LOOP
10 I# = 0
20 DO
30 PRINT I#
40 I# = I# + 1
50 LOOP UNTIL I# = 3
A condition on the DO is tested before the body, so the body may run zero times. A
condition on the LOOP is tested after, so it runs at least once. DO with no
condition at all loops forever until an EXIT or a GOTO leaves it.
EXIT works here too.
GOTO and GOSUB
10 GOSUB 100
20 PRINT "BACK"
30 END
100 PRINT "IN THE SUBROUTINE"
110 RETURN
RETURN goes back to the line after the GOSUB.
Labels
A label is a name with no type suffix. It marks a line, and anything that takes a line number takes a label instead:
10 GOTO SETUP
20 PRINT "SKIPPED"
100 LABEL SETUP
110 PRINT "ARRIVED"
Labels are filed before the program runs, so a forward GOTO works. This is worth
using for its own sake: a program written with labels is immune to RENUMBER, because
there is no number to rewrite.
ON
ON picks the nth target from a list, counting from one:
10 ON CHOICE# GOTO 100, 200, 300
20 PRINT "CHOICE WAS OUT OF RANGE"
Out of range is not an error — it falls through to the next statement, which is what
lets you write the check as the line after. ON ... GOSUB works the same way and
returns.
Trapping errors
TRAP sends an error to a handler instead of stopping the program:
10 TRAP HANDLER
20 DIM Q#(2)
30 PRINT Q#(9)
40 PRINT "CARRIED ON"
50 END
100 LABEL HANDLER
110 PRINT "CAUGHT " + ERR(ER#) + " ON LINE " + EL#
120 RESUME NEXT
Two variables are set when the trap fires: ER# is the error code and EL# is
the line it happened on. ERR(ER#) gives the message text. On a C128 these are called
ER and EL with no suffix; this dialect has no bare variable names.
RESUME comes in three forms:
| Form | Where it goes |
|---|---|
RESUME |
back to the line that failed, and tries again |
RESUME NEXT |
on to the line after the one that failed |
RESUME (line) |
to a line you name |
Bare RESUME only terminates if the handler fixed whatever was wrong — that is what
it is for.
TRAP with no argument turns trapping off. An error inside a handler is reported
normally rather than re-entering it, so a broken handler cannot loop forever.
STOP, END and CONT
STOP stops the program and returns to the prompt; CONT resumes from there. END
stops it without arming CONT. QUIT ends the interpreter itself.