Lists in the program

When a program needs twenty numbers to start with, you don't want twenty assignments. You want a list.

DATA and READ

10 DATA 262, 294, 330
20 READ A, B, C
30 PRINT A; B; C

RUN
 262 294 330

OK

DATA lines hold values. READ takes the next one each time it's asked. The DATA lines themselves do nothing when the program runs past them — they're storage, not instructions.

Where you put them doesn't matter. Bottom of the program is conventional, and next to the loop that reads them is kinder to whoever reads the listing.

Reading a list you didn't count

Put a value on the end that can't be real, and stop when you see it:

10 PRINT "SHOPPING LIST"
20 PRINT
30 READ I$
40 IF I$ = "END" THEN 80
50 N = N + 1
60 PRINT N; ". "; I$
70 GOTO 30
80 PRINT
90 PRINT N; " THINGS TO BUY"
100 DATA "BREAD", "MILK", "SOLDER", "COFFEE", "END"

RUN
SHOPPING LIST

 1. BREAD
 2. MILK
 3. SOLDER
 4. COFFEE

 4 THINGS TO BUY

OK

That end marker is called a sentinel. "END" for text, 0 or -1 for numbers — anything that can't turn up in the real data. Now you can add items to line 100 without touching anything else in the program.

The alternative is to count them yourself and put the count first:

10 DATA 4
20 DATA "BREAD", "MILK", "SOLDER", "COFFEE"
30 READ N
40 FOR I = 1 TO N

Which is fine right up until you add a fifth item and forget to change the 4.

Reading past the end

Ask for one more than there is and the program stops:

?OUT OF DATA ERROR IN 30

That's usually a sentinel you forgot, or a READ inside a loop that goes round once too often.

Going back to the start

RESTORE puts the read position back at the first DATA item:

10 DATA 5, 6
20 READ A
30 RESTORE
40 READ B
50 PRINT A; B

RUN
 5 5

OK

Both READs got the same value. RESTORE is what you call at the start of a new game so the same DATA can be dealt out again.

Mixing numbers and text

One DATA line can hold both, as long as the READ asks for them in the same order:

10 DATA "TWINKLE", 262, "TWINKLE", 294
20 READ W$, F

Ask for a number where the next item is text and you get ?TYPE MISMATCH.

Where this really earns its keep

Tunes, maps, levels, sprite shapes, question-and-answer pairs. Anything you'd otherwise type out as a hundred assignments. The tune player is nothing but a READ loop with a DATA melody, and adding a verse means adding a line.

Next: naming a formula so you only write it once.