The middle of any canvas

Challenge 11 of 16

This canvas is neither 400 wide nor 400 tall, so the middle is not 200 across and it is not 200 down either. width and height are whatever the canvas happens to be, and half of each is the exact middle.

Write the two sums that put the flower there.

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Write the two sums that put the flower there.

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Lesson Two Inputs

Last topic's functions could put a drawing anywhere along one line. drawTree(100) and drawTree(300) gave you two trees, but both at the same height, because one input can only say one thing.

Put two names in the brackets and you can say both:

function drawTree(x, y) {
shapeColour(BROWN);
rectangle(x - 10, y - 80, 20, 80);
shapeColour(GREEN);
circle(x, y - 130, 110);
}

Now a call hands over two numbers:

drawTree(110, 330);

The first number goes into the first name and the second into the second, so x is 110 and y is 330, and the tree stands at 110 across, 330 down.

That is the whole idea, and it has a name: the inputs are positional. It is the place in the brackets that decides where a number lands — not what the name means, and not what you meant when you typed it. Write drawTree(330, 110) and the tree goes to 330 across and 110 down, high up and off to the right. Both numbers were right and the picture is still wrong.

The same rule works on the definition. The names are yours to choose, and across and down read better than x and y for some drawings, but their order is what matters. The first name in the brackets is the one the first number lands in, whatever it happens to be called.

Everything from last topic still applies, twice over now. Every part of the body that should move needs to mention the input that moves it: a trunk written y - 80 follows the tree down, and a trunk written 250 stays at 250 while the leaves go without it. And the brackets still have to be filled. Hand over one number where the body wants two and the second name has no value at all, so the shapes are drawn nowhere and vanish without a word of complaint.

The arguments do not have to be typed out either. A variable works, a sum works, and width / 2 and height / 2 are the exact middle of the canvas whatever size the canvas happens to be.

There is one thing left that these functions cannot do. Every tree comes out the same size however cleverly you place it, so a tree meant to be far away looks like a tree standing close. The next topic hands the brackets a third number.