Game 0 Unit 4 of 15 1 hr learning time

LET, and the First Two Letters

Give the program a memory: LET names a box that holds a value. Meet the C64's naming rules — strings end in $, and the surprise that only the first two letters of a name count, so CASH and CARD are the same box.

27% of Meet C64 BASIC

So far the screen only shows what you typed into the program. A variable gives the program a memory: a named box that holds a value you can set, read, and change. LET makes one.

Milestone 1 — a named box

10 LET LIVES=3
20 PRINT "LIVES=";LIVES

Line 10 puts 3 in a box called LIVES; line 20 prints a label and then the box's value. RUN it: LIVES= 3. The name LIVES stands in for the number wherever you use it — that is the whole point of a variable.

The C64 screen showing LIVES= 3, then READY.
LET LIVES=3 fills a named box; PRINT reads it back. The name stands in for the value.

(The word LET is optional on the C64 — LIVES=3 works on its own. We'll write LET in the primer because it says out loud what's happening: let this name hold this value.)

Milestone 2 — the first-two-letters trap

Here is the C64's biggest naming surprise, and it bites everyone once. Watch:

10 LET CASH=10
20 LET CARD=99
30 PRINT "CASH=";CASH

You set CASH to 10, then CARD to 99, then print CASH. You'd expect 10. Run it:

The C64 screen showing CASH= 99, not the expected 10.
CASH prints 99 — because to the C64, CASH and CARD are the same box. Only the first two letters of a name count.

CASH came out as 99 — the value you put in CARD. The C64 only looks at the first two letters of a variable name. CASH and CARD both begin CA, so they are the same box; setting CARD overwrote CASH. You may type long, readable names — and you should, for yourself — but the machine only distinguishes them by the first two characters. Keep those two unique (SC for score, LV for lives) and you'll never be caught.

Milestone 3 — words need a dollar

A box can hold text instead of a number, but you must mark it: a string variable ends in $.

10 LET N$="ZARA"
20 PRINT "HELLO ";N$

N$ holds the word "ZARA"; the text goes in quotes. RUN it: HELLO ZARA. The $ is part of the name — N (a number) and N$ (a string) are two different boxes. Put a number into a $ name, or text into a plain one, and the C64 answers ?TYPE MISMATCH ERROR.

The C64 screen showing HELLO ZARA, then READY.
A string variable ends in $ and holds text. N and N$ are different boxes.

When it doesn't work

  • Two names that should differ share a value. Their first two letters match — the classic CASH/CARD collision. Rename one so the first two characters differ.
  • ?TYPE MISMATCH ERROR. You mixed kinds — text into a number box, or a number into a $ box. Numbers go in plain names; text goes in $ names.
  • ?SYNTAX ERROR on a sensible-looking name. A variable name may not contain a BASIC keyword — SCORE is fine, but TOTAL hides TO and trips it; and TI, TI$ and ST are reserved by the C64 (the clock and status), so you can't use them for your own boxes.

Before and after

You started with a program that could only print fixed words and finished with one that remembers — numbers in plain boxes, text in $ boxes. The idea underneath: LET names a box; the C64 tells boxes apart by their first two letters and by whether the name ends in $. Names are for you; keep the first two letters distinct and the machine agrees with you.

Try this

  • Two scores. Make SCORE and SCREEN and set them to different numbers, then print both. Watch them collide (SC), then rename one to break it.
  • Build a greeting. Ask nothing yet — just LET A$="..." with your name and print "HI ";A$.
  • Provoke the mismatch. Try LET N$=5 and read the ?TYPE MISMATCH ERROR.

What you've learnt

  • LET names a box that holds a value (and LET itself is optional).
  • The C64 distinguishes variable names by their first two characters only.
  • A string variable ends in $ and holds text; N and N$ are different boxes.
  • Names can't embed keywords, and TI/TI$/ST are reserved.

What's next

Your boxes are filled by the program itself. In Unit 5 we let the person at the keyboard fill them: INPUT, where the program stops, waits, and takes what's typed.