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.

To show the numbers 1 to 10 with what you know so far, you would write ten SHOW lines. For 1 to 100, a hundred of them. Computers exist so that you never have to.

A loop does a piece of work over and over, as many times as you say, and you write that work once.

Count, without writing it out

FOR i = 1 TO 10
  SHOW i
END
Output
1
2
3
4
5
6
7
8
9
10

FOR i = 1 TO 10 starts a loop with a counter called i. END marks where the repeated work stops. The computer runs everything between them with i set to 1, then runs it again with i set to 2, and keeps going up to 10.

Three lines did the work of ten. Changing that 10 to 100 would do a hundred without another keystroke. That is the point of a loop: how much repetition you get is a number you set, not lines you type.

The counter is a value you can use

The counter is not only for keeping count. It is a variable, holding a different value each time round, and you can use it in the work:

FOR i = 1 TO 10
  SHOW i, " times 7 is ", i * 7
END
Passii * 7Shown
1171 times 7 is 7
22142 times 7 is 14
33213 times 7 is 21
10107010 times 7 is 70

Ten passes of the same two lines. Only i changes, and the computer works i * 7 out afresh from it each time round.

A loop that counts and hands you the count is how you fill a row, draw a grid 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 no times, because the start is already past the end.
  • The numbers are out by one. 1 TO 10 gives ten passes. 0 TO 10 gives eleven. Decide whether you are counting things or counting from zero, then set the range to match.
  • Only part of the work repeats. The repeated work is everything between FOR and END. A line outside them runs once, not once per pass.

What you’ve learnt

  • A loop repeats work without you writing it out. How many times is a value you set.
  • A counted loop runs a fixed number of times and keeps a counter that climbs as it goes.
  • The counter is a variable you can use in the repeated work. That is what makes a loop build things rather than only count.

What’s next

A counted loop is for when you know how many times. Often you do not: you repeat until something happens. In Unit 2 we meet that loop, and use it to turn the guessing game into something you can play.