Unit 5 of 11 1 hr learning time

Asking the Player

Until now you filled the boxes when you wrote the program. Now the program asks whoever is using it — and starts doing something different depending on the answer.

45% of General Programming

So far, you decided what went in every box — back when you wrote the program. That means the program does the same thing every single time it runs. A real program does something more interesting: it asks the person using it, and puts their answer in a box. That is the moment a program stops reciting and starts responding.

Read an answer into a box

The instruction we want is "ask a question, wait for the person to type something, and keep their answer." In pseudocode:

ASK "What is your name? " INTO name
SHOW "Hello, ", name

In BASIC the instruction is INPUT. One catch from the last unit: the answer is text, so its box needs a single-letter name ending in $ — we'll use n$:

  10 INPUT "What is your name? "; n$
  20 PRINT "Hello, "; n$

Line 10 shows the question and waits for someone to type a reply and press Enter; whatever they type is dropped into the box n$. Line 20 then greets them by looking in that box. Run it and type Sam:

A Spectrum screen showing: Hello, Sam.
Typed `Sam` at the question, and the program greeted Sam. The answer went into `n$`; line 20 read it back.

Now run the same program again and type something else — Ada:

A Spectrum screen showing: Hello, Ada.
Same program, a different answer, a different result. Nothing in the code changed — only what was typed.

That is the whole point. The program didn't change between those two runs — you did. A program that asks can give a different result every time, depending on who's at the keyboard and what they say. Every game you'll build leans on this: it is how the player plays instead of just watching.

When it's wrong, see why

  • Variable not found, or the line won't run. You named the text box with more than one letter — name$, say. BASIC only allows single-letter names for text boxes (Unit 4), so name$ is rejected. Use n$. (Other languages are happy with name; Sinclair BASIC isn't — the rules are local.)
  • It greets you with an empty space. You pressed Enter without typing anything, so the box holds nothing. Run it again and type a reply before Enter.
  • The greeting runs into the name, like Hello,Ada. The fixed text needs its space inside the quotes — "Hello, " — from Unit 3. The computer joins on exactly what you wrote.

What you've learnt

  • Input lets a program read what someone types and keep it in a box — the program responds to its user.
  • A program that asks gives a different result for a different answer, without a line of its code changing. That's the difference between watching and playing.
  • The naming rules are local: BASIC needs a single-letter $ name for a text box; the idea — ask, and remember the answer — is the same in every language.

What's next

You can ask for a name and greet it. But the answer to a question is often a number — and numbers can be added, compared, counted. In Unit 6 we ask for numbers and do arithmetic with them, the first time the program works something out from what the player gave it.