Control Flow

IF Statements

Block IF

Multi-line conditional with THEN on its own:

IF score$ >= 90 THEN
    PRINT "Grade: A"
    PRINT "Excellent!"
END IF

IF/ELSE

IF age$ >= 18 THEN
    PRINT "Adult"
ELSE
    PRINT "Minor"
END IF

IF/ELSEIF/ELSE

IF score$ >= 90 THEN
    PRINT "A"
ELSEIF score$ >= 80 THEN
    PRINT "B"
ELSEIF score$ >= 70 THEN
    PRINT "C"
ELSEIF score$ >= 60 THEN
    PRINT "D"
ELSE
    PRINT "F"
END IF

One-line IF

Jump to label based on condition:

IF lives$ = 0 THEN GOTO [game_over]
IF key$ = KEY_ESC# THEN GOTO [menu] ELSE GOTO [continue]

With GOSUB:

IF ready$ = 1 THEN GOSUB [start_game]

Comparison Operators

Operator	Meaning
= or ==	Equal
<> or !=	Not equal
<	Less than
>	Greater than
<=	Less than or equal
>=	Greater than or equal

Logical Operators

IF age$ >= 18 AND age$ <= 65 THEN
    PRINT "Working age"
END IF

IF day$ = "Saturday" OR day$ = "Sunday" THEN
    PRINT "Weekend!"
END IF

IF NOT gameOver$ THEN
    PRINT "Keep playing"
END IF

FOR Loops

Note: FOR can initialize variable itself, without LET

Basic FOR

FOR i$ = 1 TO 10
    PRINT i$
NEXT

Output: 1 2 3 4 5 6 7 8 9 10

FOR with STEP

FOR i$ = 0 TO 100 STEP 10
    PRINT i$
NEXT

Output: 0 10 20 30 40 50 60 70 80 90 100

Counting Down

FOR i$ = 10 TO 1 STEP -1
    PRINT i$
NEXT
PRINT "Liftoff!"

Nested FOR

FOR row$ = 1 TO 3
    FOR col$ = 1 TO 3
        PRINT row$; ","; col$; " ";
    NEXT
    PRINT ""
NEXT

WHILE Loops

Repeat while condition is true:

LET count$ = 0
WHILE count$ < 5
    PRINT count$
    count$ = count$ + 1
WEND

Output: 0 1 2 3 4

Infinite Loop with Exit Condition

WHILE 1
    LET key$ = INKEY
    IF key$ = KEY_ESC# THEN GOTO [exit]
    ' Game logic here
WEND

[exit]
PRINT "Goodbye"

Labels and GOTO

Labels are marked with square brackets:

[start]
    PRINT "Starting..."
    GOTO [main]

[main]
    PRINT "Main section"
    GOTO [end]

[end]
    PRINT "Done"
    END

GOSUB and RETURN

Call a subroutine and return:

PRINT "Before subroutine"
GOSUB [greet]
PRINT "After subroutine"
END

[greet]
    PRINT "Hello!"
    PRINT "Nice to meet you!"
    RETURN

Output:

Before subroutine
Hello!
Nice to meet you!
After subroutine

Nested GOSUB

GOSUB [level1]
END

[level1]
    PRINT "Level 1"
    GOSUB [level2]
    PRINT "Back to Level 1"
    RETURN

[level2]
    PRINT "Level 2"
    RETURN
    
' Output:
' Level 1
' Level 2
' Back to Level 1

Variables as GOTO/GOSUB pointers

Note: A variable can be used to indicate the location of the jump.

LET foo$ = "[start]"
LET bar# = "[jump]"
GOTO foo$

PRINT "Line we never see".

[start]
PRINT "[start]"
GOSUB bar#

PRINT "Ending line"
END 

[jump]
PRINT "[jump]"
RETURN

[start]
[jump]
Ending line

Function Scope Isolation

Inside user-defined functions, GOTO and GOSUB can only jump to labels within the same function:

' GOTO cannot jump outside function
DEF FN test$(x$)
    [local_label]
    IF x$ < 10 THEN
        x$ = x$ + 1
        GOTO [local_label]    ' OK - within function
    END IF
    RETURN x$
END DEF

[outside] ' GOTO [outside] from inside function would cause ERROR:
PRINT "Outside"

Using Key Constants

[game_loop]
    LET key$ = INKEY
    
    IF key$ = KEY_ESC# THEN END
    IF key$ = KEY_UP# THEN y$ = y$ - 1
    IF key$ = KEY_DOWN# THEN y$ = y$ + 1
    IF key$ = KEY_LEFT# THEN x$ = x$ - 1
    IF key$ = KEY_RIGHT# THEN x$ = x$ + 1
    IF key$ = KEY_SPACE# THEN GOSUB [fire]
    
    GOTO [game_loop]

END Statement

Terminates program execution:

IF error$ THEN
    PRINT "Error occurred"
    END
END IF
PRINT "This runs if no error"