Game 0 Unit 5 of 15 1 hr learning time

INPUT — the Program Listens

Let the person at the keyboard fill a variable. INPUT stops the program, prints a prompt and a ?, waits for what's typed, and stores it — text into a $ box, numbers into a plain one.

33% of Meet C64 BASIC

In Unit 4 the program filled its own boxes with LET. INPUT hands that job to the person at the keyboard: it stops, shows a prompt, waits for a line to be typed, and stores it in a variable. That one statement is what turns a program from a printout into something you can use.

Milestone 1 — ask for a word

10 INPUT "WHAT IS YOUR NAME";N$
20 PRINT "HELLO ";N$

Line 10 prints WHAT IS YOUR NAME and then waits. RUN it: the C64 shows your prompt, adds a ? of its own, and parks the cursor — nothing else happens until you type.

The C64 screen showing WHAT IS YOUR NAME? with the cursor waiting.
INPUT prints your prompt, adds its own ?, and stops. The program waits here until a line is typed and RETURN is pressed.

Type a name and press RETURN. The word goes into N$, and line 20 greets it:

The C64 screen showing WHAT IS YOUR NAME? ZARA and then HELLO ZARA.
What you type lands in N$; the next line reads it straight back. The program listened.

The ; after the prompt string is what keeps the prompt and the ? on the same line. The box is a string box (N$) because a name is text — the $ you met in Unit 4.

Milestone 2 — ask for a number

Drop the $ and INPUT collects a number instead, ready for arithmetic:

10 INPUT "HOW MANY LIVES";L
20 PRINT "YOU HAVE";L;"LIVES LEFT"

RUN it, type 3, and the value lands in L as a number — not the text "3". Line 20 prints it back inside a sentence:

The C64 screen showing HOW MANY LIVES? 3 and then YOU HAVE 3 LIVES LEFT.
A plain name collects a number you can compute with. The spaces around the 3 are the leading and trailing spaces the C64 gives a number.

When it doesn't work

  • ?REDO FROM START. You typed letters into a number INPUT. A plain name wants a number; type one, or change the variable to a $ name to accept text.
  • The prompt and ? landed on different lines. You used a comma instead of the ; after the prompt string. Use ; to keep them together.
  • ?EXTRA IGNORED. You typed a comma in your answer — the C64 treats a comma as the gap between two inputs and drops the rest. For one plain answer, leave commas out.

Before and after

You started with a program that only knew what you wrote into it, and finished with one that asks and listens — a name into a $ box, a number into a plain one. The idea underneath: INPUT stops, prompts, waits, and stores what's typed. Output plus input is everything a conversation needs; from here the program can react.

Try this

  • Two questions. Ask for a first name and a count of lives, then print one sentence using both.
  • Greet by length. After INPUT N$, print "YOUR NAME HAS";LEN(N$);"LETTERS".
  • Provoke the redo. Answer a number INPUT with a word and read the ?REDO FROM START.

What you've learnt

  • INPUT stops the program, prints your prompt plus a ?, and stores what's typed.
  • Use ; after the prompt string to keep the prompt and ? on one line.
  • A $ name collects text; a plain name collects a number you can compute with.
  • Wrong-kind answers raise ?REDO FROM START; stray commas raise ?EXTRA IGNORED.

What's next

The program can ask a question — but it does the same thing no matter what the answer is. In Unit 6 we let it decide: IF / THEN, the test that makes one line run only when a condition holds.