Arrays

An array is one name holding many numbers, picked out by number instead of by name.

Making one

10 DIM S(4)

That's five boxes, not four: S(0), S(1), S(2), S(3) and S(4). Counting from zero is normal here, and DIM S(4) giving you five is the thing to remember.

Use them like any other variable:

20 S(0) = 10
30 S(3) = 40
40 PRINT S(0); S(3)

 10 40

OK

Everything starts at zero, so you can read a box you haven't filled in.

The point of them

The index can be a variable, which is what makes an array worth having:

10 DIM S(4)
20 FOR N = 0 TO 4
30 READ S(N)
40 NEXT N
50 T = 0
60 FOR N = 0 TO 4
70 PRINT "ROUND"; N + 1; " SCORED"; S(N)
80 T = T + S(N)
90 NEXT N
100 PRINT
110 PRINT "TOTAL"; T
120 PRINT "AVERAGE"; T / 5
130 DATA 12, 30, 7, 41, 20

RUN
ROUND 1 SCORED 12
ROUND 2 SCORED 30
ROUND 3 SCORED 7
ROUND 4 SCORED 41
ROUND 5 SCORED 20

TOTAL 110
AVERAGE 22

OK

Five scores, two loops, one name. Change DIM S(4) to DIM S(99) and the same program handles a hundred rounds.

Strings work too

10 DIM N$(3)
20 N$(0) = "ADA"
30 N$(1) = "GRACE"
40 PRINT N$(1)

An array of text, indexed exactly the same way.

One dimension only

This BASIC does not do grids. DIM G(3,3) is an error:

?BAD SUBSCRIPT ERROR

OK

Which doesn't stop you having a grid — you flatten it. A 4 × 4 board is sixteen boxes, and you work out which one you want:

10 DIM B(16)
20 REM SQUARE AT ROW R, COLUMN C:
30 P = (R - 1) * 4 + C
40 B(P) = 1

That line 30 is worth copying down. The treasure grid project is built on exactly it.

Two things that go wrong

?BAD SUBSCRIPT — you asked for a box outside the array. DIM S(4) then S(5) is five past the end of a five-box array, counting from zero.

?REDIM'D ARRAY — you DIMmed the same array twice. Usually it's a DIM inside a loop, or a DIM in a subroutine that gets called more than once. DIM each array exactly once, near the top.

What they cost

Every element is five bytes whether you use it or not, so DIM X(999) takes 5,000 bytes out of the roughly 30,000 you started with. PRINT FRE(0) tells you what's left:

DIM A(100)
PRINT FRE(0)

 30206

OK

CLR gives it all back, along with every other variable.

Next: doing the same sort of thing with words.