Numbers and variables

A variable is a name that remembers a value. Type this:

A = 5
PRINT A * A

 25

OK

A held the 5 until you asked for it. It will hold it until you change it, RUN something, or switch the machine off.

Two kinds of variable

A plain name holds a number. A name with a $ on the end holds text — a string:

N$ = "ADA"
PRINT "HELLO, "; N$

HELLO, ADA

OK

A and A$ are two different variables and can both exist at once.

Naming them

A name starts with a letter and can carry on with letters and digits: A, X2, HITS, TOTAL. There's one rule to actually remember and one trap.

The rule: only the first two characters count. COUNT and COURSE are both CO as far as BASIC is concerned, so they're the same variable:

COUNT = 7
COURSE = 9
PRINT COUNT

 9

OK

Writing them out in full is fine — helpful, even — as long as no two names in your program start with the same two letters.

The trap: a name can't contain a keyword. SCORE has OR inside it, so:

SCORE = 5

?SYNTAX ERROR

OK

BASIC spotted OR in the middle of your name and gave up. The usual keywords to watch for are OR, AND, TO, IF, ON and FN. Call it SC and move on.

Why it does that

BASIC squashes every keyword down to a single byte the moment you press Enter, before it has any idea you meant a variable name. SCORE becomes SC, the byte for OR, E — which is not something it can assign to.

What it can count to

Numbers are held in five bytes, which buys you a range of about ±1.7 × 10³⁸ and nine digits of precision:

PRINT 1 / 3

 .333333333

OK

Very large and very small numbers switch to scientific notation on their own:

PRINT 1234567890123
PRINT .000123

 1.23456789E+12
 1.23E-04

OK

There is one kind of number, not two — no separate integers. 7 / 2 is 3.5, and if you want 3 you ask for it with INT:

PRINT 7 / 2; INT(7 / 2)

 3.5 3

OK

That leading space

Every number prints with a space in front of it. That space is where a minus sign goes:

PRINT 1; -1; 2

 1-1 2

OK

Positive numbers look indented and negative ones don't, so columns of figures line up. It's deliberate, and it means that when you glue a number to a word you usually want to write the space yourself:

PRINT "SCORE:"; 100

SCORE: 100

OK

Doing math

The usual five, in the usual order — powers first, then multiply and divide, then add and subtract:

+	add
-	subtract
*	multiply
/	divide
^	to the power of

PRINT 2 + 3 * 4
PRINT (2 + 3) * 4
PRINT 2 ^ 10

 14
 20
 1024

OK

Parentheses work the way you'd hope. When in doubt, put them in.

Try this

A program that turns Celsius into Fahrenheit:

10 DEF FN F(C) = C * 9 / 5 + 32
20 PRINT "C", "F"
30 FOR C = 0 TO 100 STEP 25
40 PRINT C, FN F(C)
50 NEXT C

RUN
C             F
 0             32
 25            77
 50            122
 75            167
 100           212

OK

Three things in there you haven't met yet: DEF FN makes a formula into your own function, FOR counts from one number to another, and the comma lines the results up in columns. All three get their own chapter. The columns are next.