One you wrote, used by another

Challenge 13 of 20

A line you write can call a function you also wrote. The wheels are missing the circle that draws them, and each cart is missing its second wheel altogether.

Write the circle line and the second call.

Your code JavaScript

Write the circle line and the second call.

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Lesson Write the Functions Yourself

Every function in this course was written for you. You read its body, worked out what it needed, and decided what to hand it. This topic turns that round: the calls are there, the bodies are empty, and the lines are yours to write.

There is nothing new to learn. A body is the drawing lines you have used since your very first sketch, with the function's inputs standing in for the numbers:

function drawPost(x) {
shapeColour(BROWN);
rectangle(x - 10, 250, 20, 70);
}

That is the whole idea. Write 200 where x goes and the post only ever stands in one place; write x and the same line puts a post wherever the call says.

Each missing line has a comment directly above it saying what belongs there. Read it as the instruction it is. When it says rectangle(across, down, wide, tall) it is telling you the shape and the order of its numbers, and when it says "at x, size above y" it is telling you to write x and y - size rather than two numbers of your own.

Four things are worth keeping in mind.

An empty body is not an error. The call still runs, it just draws nothing, so a blank line shows up as a missing part of the picture rather than as a message in the results panel.

The pen is part of a body. If a body does not set its own colour, whatever colour was set last is still loaded, and your shape comes out in it.

The order of the lines is the order things are painted. Whatever you write second is drawn on top of whatever you wrote first.

And the names in the brackets have to match the names in the body. Call them what you like, but call them the same thing in both places.

The last five challenges move, so a variable changes between frames and a call hands the new number over. That is last topic again, with the body missing as well. Then the course is done.