The trunk has to move too

Challenge 4 of 16

Your leaves move and your trunks do not, and both are drawn in the same body. Only the parts that mention x follow the number in the call; a plain number stays exactly where it is written.

Write what the trunk's across should be — ten to the left of x.

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Write what the trunk's across should be — ten to the left of x.

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Lesson Use It Again

Last topic ended on a problem. drawTree() drew a tree, but every call drew it in the same place, because nothing anywhere said where it should go. Saying it twice got you one tree.

The fix is a name in the definition's brackets:

function drawTree(x) {
shapeColour(BROWN);
rectangle(x - 10, 250, 20, 80);
shapeColour(GREEN);
circle(x, 200, 110);
}

That name — x here, though you can call it anything — is an empty box. It has no value of its own. The call fills it in:

drawTree(100);
drawTree(300);

The first call puts 100 into x and runs the body, so a tree is drawn at 100. The second puts 300 in and runs the same body again, so a tree is drawn at 300. One function, two trees, and the drawing lines written once.

The name is called an input, and the number a call hands over is called an argument.

Three things follow from that, and each one is worth watching for.

Everything in the body that should move has to mention the input. The trunk is written x - 10, not 190, or it stays where it is written while the leaves move off without it.

The names have to match. The body can only use the name the brackets introduce; use a different one and the sketch stops with that name is not defined.

The brackets have to be filled. A call that leaves them empty gives the input no value, and every shape that depends on it is drawn nowhere at all — with no error to tell you. If a shape has vanished, check what the call handed over.

An input does not have to be a position. It is just a number, so it can be a size instead, and one input can feed several parts of one drawing at once. The argument does not have to be typed out either: a variable works, a sum works, and width / 2 is the middle of the canvas whatever the canvas happens to be.

There is still something these functions cannot do. One input can only say one thing, so a row of trees is a row — everything ends up at the same height. The next topic adds a second input, and then you can put a drawing anywhere at all.