What belongs in the loop

Challenge 12 of 16

The rail is one long shape, so it is drawn once before the loop even starts. The posts are the part that repeats, and they are the only part that belongs inside.

Set the rail's length, the posts' top and the gap between them.

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Set the rail's length, the posts' top and the gap between them.

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Lesson Loops and Functions

You have been calling ready-made functions from inside loops for a while now. This topic is the other half of it: the function is yours, and the body is where you decide what one of the thing looks like.

That split is the whole idea. The body says what one tree looks like. The loop says how many trees and where. Neither one needs to know anything about the other:

function drawTree(x, y) {
shapeColour(BROWN);
rectangle(x - 6, y - 60, 12, 60);
shapeColour(GREEN);
circle(x, y - 60, 54);
}

for (var i = 0; i < 5; i = i + 1) {
drawTree(40 + i * 80, 320);
}

Five trees, and the drawing is written once. Change the head's size and all five change together, which is the real reason to put it in a function at all.

Now the part that catches everybody. The counter belongs to the loop. It does not exist inside the body, and if you reach for i in there you get "i is not defined" — the body's only knowledge of the outside world is the numbers it was handed. That is what the inputs are for, and one of the challenges here is built on exactly that mistake so you meet the message once on purpose.

Which means every number in the body has to be measured from an input. Write 200 where you meant x and every turn draws in the same place, one shape on top of the last, so a five-turn loop looks like it ran once.

A turn can do more than one thing. Two calls in the loop draw two things per turn; one function called from two loops draws two rows; and a function of yours can call another function of yours, so one line in the loop can put a whole pair of trees on the canvas.

And not everything belongs inside. A fence has one long rail and a row of posts: the rail is drawn once, before the loop, and only the posts repeat. Ask what there is one of and what there are many of.

Next topic a loop goes inside another loop, and a row becomes a grid.