One rule, two speeds

Challenge 15 of 16

Two fish, two variables, and one rule doing the working out for both of them. nextX has no idea which fish it is answering for, so the speed has to come in as an input rather than live in the body.

Set the two speeds.

Your code JavaScript

Set the two speeds.

Console print() output

0 / 40 frames · 400×400

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Penalties: none this run

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Lesson Working Out an Answer

Every function you have written so far has drawn something. This topic is about the other kind: a function that works a number out and hands it back.

function halfOf(n) {
return n / 2;
}

There is nothing on the canvas when that runs. What it does is answer. Write halfOf(600) and that call becomes 300, so you can put it anywhere a number belongs:

circle(halfOf(600), 200, 120);

return is the word that hands the answer back. It means "this is my answer, and I am finished".

That gives you two kinds of function, and it is worth keeping them apart in your head and in your names. A drawing function is named for what it draws and you call it on a line of its own: drawTree(200, 320, 90). A working-out function is named for the answer it gives and you call it inside something else: drawTree(middleOf(60, 340), 320, 90).

The reason to bother is that a number worked out in one place can be read from many. Write groundLevel once and the grass, the trees and the houses can all ask it where the ground is. Change the one number it hands back and the whole scene moves together.

Two things go wrong, and both of them go wrong quietly.

If you work a number out and do nothing with it, nothing happens. A line that says middleOf(60, 340); on its own draws no post and reports no error. The answer came back and there was nobody to give it to.

And if a function has no return line at all, it still runs and still hands back nothing. Nothing is not a number, so a shape given it simply never appears. When something has vanished with no error, an empty answer is worth suspecting.

The last three challenges here animate, because the most useful thing to work out in an animation is the next number: hand a function where something is now, and let it answer with where it goes next.

Next topic the calls are given and the bodies are missing, and you write them.