Going around again

A loop is how you get a lot done with a little typing.

The shape of it

10 FOR N = 1 TO 4
20 PRINT N; " SQUARED IS"; N * N
30 NEXT N

RUN
 1 SQUARED IS 1
 2 SQUARED IS 4
 3 SQUARED IS 9
 4 SQUARED IS 16

OK

FOR sets N to 1. Everything down to NEXT runs. NEXT adds one to N and sends it back round, until N passes 4.

Counting by something else

STEP says how much to add each time. Negative counts down:

10 PRINT "COUNTING DOWN"
20 FOR N = 10 TO 0 STEP -2
30 PRINT N;
40 NEXT N
50 PRINT
60 PRINT "LIFT OFF"

RUN
COUNTING DOWN
 10 8 6 4 2 0
LIFT OFF

OK

STEP can be a fraction too — STEP .5 counts in halves.

The body always runs at least once

This is the one surprising thing about FOR on this machine, and it's shared with every BASIC of this family:

10 FOR I = 5 TO 1
20 PRINT "BODY"; I
30 NEXT I
40 PRINT "AFTER"; I

RUN
BODY 5
AFTER 6

OK

FOR I = 5 TO 1 looks like it should do nothing at all. It runs once. The limit is checked at NEXT, not at FOR, so the body has always happened by the time anyone asks whether it should have.

If a loop might legitimately have nothing to do, test before you enter it:

10 IF C = 0 THEN 60
20 FOR I = 1 TO C

And note where I ends up: one step past the limit, not on it.

One variable per NEXT

Loops nest — a loop inside a loop, and a loop inside that:

10 FOR I = 1 TO 2
20 FOR J = 1 TO 2
30 PRINT I; J
40 NEXT J
50 NEXT I

RUN
 1 1
 1 2
 2 1
 2 2

OK

Each FOR gets its own NEXT, closed in the reverse order they were opened. NEXT J, I is not accepted here — the comma form produces ?NEXT WITHOUT FOR, so give each loop its own line. It costs a line and saves an evening.

You can leave the variable off entirely — a bare NEXT closes the innermost loop — but naming it is what makes a long program readable.

How deep can you go?

Fourteen. Every open FOR keeps eighteen bytes of bookkeeping — where the loop started, what it counts to, what it steps by — on a stack that is exactly 256 bytes, and fourteen of those fill it.

Fourteen is far more than any program you'd want to read, so this is a limit you will meet by accident or not at all. It's worth knowing what it looks like when you do: the fifteenth FOR doesn't complain. The complaint arrives later, at that loop's NEXT, as ?NEXT WITHOUT FOR — pointing at a line that is perfectly correct. If you ever see that on a NEXT you're sure of, count your open loops.

Getting out early

GOTO a line outside the loop and you're out:

10 FOR N = 1 TO 100
20 IF N * N > 50 THEN 50
30 NEXT N
50 PRINT "FIRST SQUARE OVER 50 IS"; N * N

Leaving a loop this way is normal and fine. Doing it thousands of times in one program, without ever letting the loops finish, is what eventually earns you an ?OUT OF MEMORY — see when it goes wrong.

Stopping a loop that won't stop

Press Esc. The program stops, tells you which line it was on, and gives you the prompt back:

BREAK IN 20

OK

Nothing is lost — LIST still shows the program, and CONT starts it going again.

Next: giving a piece of program a name.