LET, and What You May Name

You met the idea of a variable in General Programming — a named box that remembers a value. In Sinclair BASIC the keyword is LET, and the Spectrum has its own rule about what you may call a box. That rule trips nearly everyone once; we'll trip it on purpose so it never surprises you.

Milestone 1 — store a value, then use it

  10 LET score = 100
  20 LET n$ = "Sam"
  30 PRINT n$; " scored "; score

LET score = 100 puts 100 in a box named score. LET n$ = "Sam" puts the text Sam in a box named n$. Line 30 reads both back. RUN it: Sam scored 100.

The = in LET stores: "put this value in the box." It isn't asking whether two things are equal — that's a different job, for Unit 6.

Milestone 2 — change what's in the box

A box remembers until you change it. Add two lines that update score:

LET score = score + 50 reads the box, adds 50, and puts the result back in the same box. Run it: score goes from 100 to 150.

Milestone 3 — the naming rule, shown failing

Here's the rule. Numeric names can be long — score, lives, hits are all fine. But a text variable must be a single letter plus $ — n$, a$, x$. A longer text name like name$ is not allowed. Watch what happens when you try:

  10 LET name$ = "Sam"
  20 PRINT name$

That report is worth understanding. The Spectrum read name as a numeric variable name — long numeric names are fine — and then hit the $, which makes no sense stuck on the end of a number's name. So it stops with Nonsense in BASIC. The fix is to make the text box a single letter: n$, not name$. (Other home computers have their own rules — a C64 keeps only the first two letters of any name — which is the point: the idea of a named box is universal, but the naming rules are local.)

When it doesn't work

• Nonsense in BASIC on a LET line. Usually a text variable with a long name. Shorten it to a single letter plus $ — n$.
• Variable not found. You read a box you never stored into — often a typo, so the name you read doesn't match the name you set. Check the spelling on both lines.
• A name did nothing useful. You may have used a BASIC keyword as a name. Keywords are reserved; pick a plain word the Spectrum doesn't already own.

Before and after

You started knowing what a variable is and finished writing them in Sinclair BASIC — storing, using, and changing a box — and you met the rule that catches everyone: text names are a single letter plus $. The idea underneath: LET stores a value in a named box; numeric names can be long, text names cannot.

Try this

• A second number. Add LET lives = 3 and print it beside the score.
• Fix the broken one. Change name$ to n$ in Step 3 and run it — the same program works the moment the name is legal.
• Change twice. Add 50 to score a second time. Predict the total before you run it.

What you've learnt

• LET stores a value in a named box; the = means "store", not "is equal to".
• Numeric names can be long (score); text names must be a single letter plus $ (n$).
• LET score = score + 50 reads a box and stores a new value back in it.
• A too-long text name stops with Nonsense in BASIC — the Spectrum read the word as a number's name, then the $ made no sense.

What's next

Your boxes hold values you typed into the program. In Unit 5 the player fills them: INPUT reads what someone types at the keyboard into a variable — the first time the program waits for you.