module
5
The square in S4 took nine lines, and six of them were the same two instructions over and over. Repetition like that is worth getting rid of - not to save typing, but because a shape written as "do this four times" says what the shape is in a way that a list of moves does not.
repeat takes two things: a number, and a list of instructions in square
brackets.
repeat 4 [fd 50 rt 90]
Four times, do fd 50 and then rt 90. That is the whole square.
Remember the brackets are on the arrow keys - left arrow for [, right
arrow for ].
The instructions inside the brackets are an ordinary Logo line; there is nothing
special about being inside a repeat. You can put as many instructions in there
as you like, separated by spaces.
The square works because of a piece of arithmetic worth having in your head: the turtle turns 90 degrees four times, which is 360 degrees in total - one whole turn. It ends up facing exactly the way it started.
That is not a coincidence, and it is the rule for any shape of this kind. If you want a closed shape with a given number of sides, the turn at each corner has to be 360 divided by the number of sides. Five sides means turning 72 degrees. Ten sides means 36.
Get that wrong and the shape does not close, which you can see for yourself in the edits below.
ss
cs
repeat 4 [fd 50 rt 90]
Starting from
A fresh boot, at the ? prompt.
What you should see
The same square as S4 - fifty steps on a side, up and to the right of where the turtle started. The turtle finishes back at the corner it began from, and this time it is pointing up again, because four turns of 90 have brought it all the way round.
Three lines instead of nine, and the same picture pixel for pixel.
4 to a 3. You might expect a triangle. Run it and see what you
actually get, then work out why - the angle is the clue.90 to 60, leaving the 4 alone. Does the shape close? Use the
rule above to work out what count 60 would need to go with.50 to 100. The square gets bigger, but not four times bigger in
the way you might expect. What has happened to the top of it, and why? S8 is
about that.| Symptom | Cause |
|---|---|
Pressing [ does nothing |
There is no bracket key. Left arrow gives [, right arrow gives ]. |
repeat draws nothing and the line seems to be ignored |
Check there is a space between every instruction inside the brackets. Dr Logo needs them to tell the words apart. |
| The shape does not join up | The turns do not add up to 360 degrees. Number of sides times the angle should make a whole turn. |
| Part of the shape is missing off the top or the side | It has run off the screen. There are only about 95 steps above the middle and 128 to either side. |
repeat takes a number and a list of instructions in square brackets, and runs
the list that many times. A closed shape with n sides turns 360 divided by n
at each corner. Three lines now do what nine did.
S6, lifting the pen - moving the turtle without leaving a line behind it, so a drawing can have gaps in it.
