Unit 8 of 11 1 hr learning time

Doing It Again

A loop repeats work without writing it out over and over — a set number of times, with a counter you can use as it goes.

73% of General Programming

To print the numbers 1 to 10 with what you know so far, you'd write ten PRINT lines. To print 1 to 100, a hundred. That's absurd, and computers exist precisely so you never have to. A loop does a piece of work over and over, as many times as you say, without you writing it out more than once.

Count, without writing it out

A counted loop runs a set number of times, keeping a counter that climbs as it goes. In pseudocode:

REPEAT FOR count = 1 TO 10
  SHOW count
END

In BASIC:

  10 FOR i = 1 TO 10
  20 PRINT i
  30 NEXT i

FOR i = 1 TO 10 starts a loop with a counter called i; NEXT i marks the end of the work to repeat. The computer runs everything between them with i set to 1, then again with i set to 2, and so on up to 10 — then stops. Run it:

A Spectrum screen showing the numbers 1 to 10, each on its own line.
One `PRINT` line, run ten times — the numbers 1 to 10. The loop did the repeating; you wrote the work once.

Three lines did what ten PRINTs would have, and changing 10 to 100 would do a hundred without another keystroke. That is the loop's whole point: the amount of repetition is a number you set, not lines you type.

The counter is a value you can use

The counter isn't only for keeping count — it's a variable, holding a different value each time round, and you can use it in the work. Put it to work in a times table:

Use the counter in the work it repeats
+1-1
11 10 FOR i = 1 TO 10
2- 20 PRINT i
2+ 20 PRINT i; " times 7 is "; i * 7
33 30 NEXT i
44
A Spectrum screen showing the seven times table: 1 times 7 is 7, up to 10 times 7 is 70.
The same loop, but now each pass uses its counter — `i` is `1`, then `2`, then `3`… and `i * 7` is worked out fresh each time. Ten lines of a times table from three lines of program.

Each time round, i holds the next number, and i * 7 is worked out from it. A loop that counts and hands you the count is how you fill a row, draw a grid, step through a list, or lay out a level — anywhere the work is "the same thing, once per number".

When it's wrong, see why

  • It runs once, or not at all. Check the range. FOR i = 1 TO 1 runs once; FOR i = 1 TO 0 runs not at all, because the start is already past the end.
  • The numbers are off by one. 1 TO 10 gives ten passes (1,2,…,10); 0 TO 10 gives eleven (0,1,…,10). Decide whether you're counting things or counting from zero, and set the range to match.
  • Only the last line repeats, or nothing lines up. The work to repeat is everything between the loop's start and its end marker (FOR … NEXT). A line outside them runs once, not each time round. Check what's inside the loop and what's outside it.

What you've learnt

  • A loop repeats work without writing it out — the number of repeats is a value you set, not lines you type.
  • A counted loop runs a fixed number of times and keeps a counter that climbs each pass.
  • The counter is a variable you can use in the repeated work — that's what makes a loop build things, not just count.

What's next

A counted loop is for when you know how many times. But often you don't — you repeat until something happens: until the player wins, loses, or quits. In Unit 9 we meet that loop, and use it to turn the guessing game from Unit 7 into something you can play.