Making decisions

IF is how a program stops doing the same thing every time.

The shape of it

10 A = 7
20 IF A > 5 THEN PRINT "BIG" ELSE PRINT "SMALL"

RUN
BIG

OK

IF a test holds, THEN do this, or ELSE do that. The ELSE half is optional.

Jumping instead of doing

THEN followed by a line number is short for "go there":

10 IF A > 5 THEN 100

That's the same as THEN GOTO 100, and you'll see it written both ways.

What you can test

=	equal to
<>	not equal to
< >	less than, greater than
<= >=	less than or equal, greater than or equal

They work on text too, in alphabetical order:

PRINT "A" < "B"

-1

OK

Which brings us to the thing everybody trips over.

True is −1

A comparison doesn't give you "true". It gives you −1 for true and 0 for false, and those are ordinary numbers you can print, store and add up:

PRINT 1 = 1
PRINT 1 = 2

-1
 0

OK

IF treats any non-zero value as true, which is why IF A THEN … works when A is 5.

Being able to count truths is genuinely useful:

10 H = 140 : A = 11
20 PRINT "TESTS PASSED:"; -((H >= 130) + (A >= 10))

Each passing test contributes −1, so negating the total counts them.

And, or, not — the trap

AND, OR and NOT do their work bit by bit on whole numbers. On the −1/0 values that comparisons produce, that behaves exactly like the words suggest:

PRINT (5 > 3) AND (2 > 1)

-1

OK

On anything else, it does arithmetic you probably didn't order:

PRINT 12 AND 10
PRINT NOT 1

 8
-2

OK

12 AND 10 is 8 because that's what those bits do. NOT 1 is −2 for the same reason — it flips every bit of 1, and the answer isn't 0.

The rule that keeps you out of trouble: use AND, OR and NOT on comparisons, not on counts. Write IF (N > 0) AND (N < 10), never IF N AND 10.

When you actually want the bits

Reading a joystick is exactly when you want them, because each direction is one bit. IF (JOY(1) AND 16) = 0 asks "is the up bit clear" and is the correct way to write it. Sticks and keys covers the whole layout.

Several things on the THEN

Everything after THEN on the line only happens when the test holds:

10 IF G < N THEN PRINT "TOO LOW" : GOTO 50

Both the PRINT and the GOTO are inside the IF. That's how a one-line IF gets to do two things, and it's the most common way to write a guard.

A worked example

10 H = 140
20 A = 11
30 IF H >= 130 AND A >= 10 THEN PRINT "YOU CAN RIDE"
40 IF H < 130 OR A < 10 THEN PRINT "NOT THIS TIME"
50 PRINT "HEIGHT TEST GIVES"; H >= 130
60 PRINT "AGE TEST GIVES"; A >= 10

RUN
YOU CAN RIDE
HEIGHT TEST GIVES-1
AGE TEST GIVES-1

OK

Lines 50 and 60 print the raw result of each comparison, so you can see the −1 that line 30 was really testing.

Next: doing something more than once.